gas atmosphere analyses during debinding and - EMC

gas atmosphere analyses during debinding and - EMC

FEDERAL UNIVERSITY OF SANTA CATARINA
DEPARTMENT OF MECHANICAL ENGINEERING
MATERIALS ENGINEERING
Diploma Thesis from
RENAN MULLER SCHROEDER
Fraunhofer-Gesellschaft
IFAM - Institut für Fertigungstechnik und Angewandte Materialforschung
GAS ATMOSPHERE ANALYSES DURING
DEBINDING AND SINTERING OF POWDER
INJECTION MOLDING COMPONENTS
Scientific Advisor:
Dr. Thomas Hartwig
Bremen, Germany, 2009.
FEDERAL UNIVERSITY OF SANTA CATARINA
GRADUATION COURSE OF MATERIALS ENGINEERING
Renan Muller Schroeder
Gas Atmosphere Analyses during Debinding and
Sintering of Powder Injection Molding Components
Diploma thesis submitted to the Graduation
Course of Materials Engineering from the
Federal University of Santa Catarina in
partial fulfillment of the requirements for the
degree of Materials Engineer.
Supervisor: Dr. Thomas Hartwig
Bremen, Germany, 2009.
Renan Muller Schroeder
Gas Atmosphere Analyses during Debinding and
Sintering of Powder Injection Molding Components
This Diploma thesis was assessed proper to obtain the degree of Materials Engineer and
was approved by the Graduation Course of Materials Engineering from the Federal
University of Santa Catarina.
______________________________
__________________________________
Prof. Fernando Cabral, PhD.
Prof. Dylton do Valle Pereira Filho, M.sc.
Graduation Course Coordinator
Diploma Thesis Coordinator
Assessment Committee:
___________________________________
Prof. Dr.-Ing. Matthias Busse
___________________________________
Dr.-Ing. Frank Petzoldt
__________________________________
Dr. Thomas Hartwig
SCHROEDER, Renan Muller, 1987 -
Gas Atmosphere Analyses during Debinding and Sintering of Powder Injection
Molding Components / Renan Muller Schroeder. – 2009
71 f.: il. color. 30cm.
Supervisor: Dr. Thomas Hartwig
Trabalho de Conclusão de Curso – Universidade Federal de Santa Catarina, Curso de
Engenharia de Materiais, 2009.
1. Powder Injection Molding 2. Debinding and sintering atmosphere. 3. Mass
Spectrometry. I. Hartwig, Thomas. II Universidade Federal de Santa Catarina, Curso de
Graduação de Engenharia de Materiais. III. Gas Atmosphere Analyses during
Debinding and Sintering of Powder Injection Molding Components
ACKNOWLEDGMENTS
First I would like to thank my supervisor Dr. Thomas Hartwig, for the opportunity
to complete my internships and to write my diploma thesis in Germany. I am sincerely
grateful for the hours spent with me and the support received.
Thanks also to Prof. Dr.–Ing. Matthias Busse and Dr.–Ing. Frank Petzoldt to
accept the presentation of my thesis at IFAM (Institut für Fertigungstechnik und
Angewandte Materialforschung). I thank them also for allowing the use of all facilities of
IFAM to perform this work.
I want to thank Prof. Dr.–Ing. Aloisio N. Klein, for his recommending me to
IFAM.
I would like to say thank you to Mrs. Dagmar Fischer, who helped me with all
documentation in Germany. To Gabriel Dutra and Stefan Lösch who were great
colleagues during the entire year in Bremen. To Vera Friederici, my office colleague,
who helped me to improve my knowledge in the German language. To all the other
colleagues of IFAM that were working with me daily.
I express thanks to my other fellows Mateus Carlesso, Vinicius Bossoni, Diego
Portaluppi, Felipe Darabas, Gabriel Beltrame, Christoffer Rahner and David Solf for the
nice moments achieved together during this whole year away from Brazil.
Finally, I thank my family which understood the importance to stay in another
country during such time and very far away from them.
I
ABSTRACT
Sintering is the major cost factor in Powder Injection Molding. Companies are
interested in reducing the cost while keeping on improving quality of sintering. It is well
known that the atmosphere surrounding the samples in the furnace has pronounced
effects on the sintered parts and thereby, know-how about it may allow streamlining the
process.
In this study, gas atmosphere analyses were carried out in order to identify the gas
species created during debinding and sintering of PIM components. The intervals of
occurrence and the thermochemical reactions leading to the degassed substances were
determined, while varying powder, binder and process gas. The experiments were carried
out with a mass spectrometer connected on-line to an industrial batch PIM furnace.
Powder and process gas had a clear effect on the outgassing and final properties of
the sintered parts. On the other hand, the exact nature of binder decomposition was not
easily identified via mass spectrometry. It was found that methane production controls
the final carbon content of the sintered parts and that this can be influenced by powder,
gas atmosphere and sintering program.
In general, mass spectrometry allowed learning much about sintering and will
certainly help to improve processing of PIM parts in batch and continuous furnaces.
II
RESUMO
A maior parte dos custos no processo de Moldagem de Pós por Injeção estão
relacionados a etapa de sinterização. Em períodos de turbulência econômica, empresas
necessitam aliar redução de despesas mantendo a qualidade de seus produtos, fato que
espera-se atingir a partir do aperfeiçoamento de processo e dos equipamentos disponíveis
no mercado. A atmosfera de um forno de sinterização é um fator culminante na qualidade
dos componentes obtidos e portanto, aumento de know-how nesta variável contribuirá
para o crescimento estável desta tecnologia.
Partindo deste racíocinio, foram realizadas análises de espectrometria de massa na
atmosfera de ciclos de extração de ligantes e sinterização. O objetivo deste estudo foi
determinar as espécies formadas durante o processo, seus respectivos intervalos de
ocorrência e as reações termo-químicas envolvidas. A influência de diferentes pós,
sistemas ligantes e gases utilizados no processo também foram avaliadas.
Em termos de resultados, pós e gases de processo demonstraram efeitos
significativos na atmosfera do forno. Infelizmente, através das análises realizadas, nao
foram observadas mudanças significativas de acordo com diferentes sistemas ligantes.
Foi demonstrado que a produção de metano controla o teor de carbono na peça final e que
isto pode ser influenciado pelas características iniciais dos pós, gases do processo e ciclos
de sinterização.
De um modo geral, espectrometria de massa demonstrou ser uma tecnologia estado
da arte no controle do processo de sinterização em fornos continuos ou de bateladas e
certamente, auxiliará aperfeiçoamentos na área industrial de Moldagem de Pós por
Injeção.
III
LIST OF FIGURES
Figure 1 – Five factors impact on the selection of PIM for any application [12]............... 3
Figure 2 – Outline of PIM process...................................................................................... 4
Figure 3 – Boudouard equilibrium [3]. ............................................................................... 9
Figure 5 – Basics of a quadrupole mass spectrometer [23]. ............................................. 12
Figure 6 – Schematic drawing of the connection between MS and furnace..................... 13
Figure 7 – Example of a mass spectrum obtained by MID measurements....................... 14
Figure 8 – Example of a mass spectrum obtained by Scan Analog measurements.......... 15
Figure 9 – Mass spectrum 44(C3H8+ / CO2) of grade OM under H2................................. 21
Figure 10 – Mass spectrum 27(C2H3+) and 28(CO / C2H4+) of grade OM under H2........ 21
Figure 11 – Mass spectrum 16(CH4+ / NH2 /O) and 17(NH3 /OH-) of grade OM under H2.
................................................................................................................................... 21
Figure 12– Mass spectrum 18(H2O) of grade OM under H2. ........................................... 21
Figure 13 – Mass spectrum 44(C3H8+ / CO2) of grade CC under H2. ............................... 26
Figure 14 – Mass spectrum 27(C2H3+) and 28(CO /C2H4+) of grade CC under H2.......... 26
Figure 15 – Mass spectrum 16(CH4+ / NH2 / O) of grade CC under H2........................... 26
Figure 16 – Mass spectrum 18(H2O) of grade OM under H2. .......................................... 26
Figure 17 – Mass spectrum 14(CH2+ /N), 16(CH4+ /NH2 /O) and 17(NH3 /OH-) of grade
OM under Ar............................................................................................................. 28
Figure 18 – Mass spectrum 18(H2O) of grade OM under Ar. ......................................... 28
Figure 19 – Mass spectrum 27(C2H3+) and 28(CO / C2H4+) of grade OM under Ar........ 28
Figure 20 – Mass spectrum 44(C3H8+ / CO2) of grade OM processed under Ar. ............. 28
Figure 21 – DSC/TGA analyses of grade OM under Argon. ........................................... 30
Figure 22 – Mass spectrum 16(CH4+ /NH2 /O), 17(NH3 / OH-) and 18(H2O) of grade CC
under Ar. ................................................................................................................... 31
Figure 23 – Mass spectrum 27(C2H3+) and 28(CO /C2H4+) of grade OM under Ar......... 31
Figure 24 – Mass spectrum 44(C3H8+ /CO2) of grade OM under Ar................................ 31
Figure 25 – DSC/TGA analyses of grade CC................................................................... 32
Figure 26 – Mass Spectrum 16 (NH2 / CH4+/ O) of grade OM and CC under H2............ 36
Figure 27 – Mass Spectrum 27 (C2H3+) of grade OM and CC under H2.......................... 36
Figure 28 – Mass Spectrum 28 (C2H4+ / CO) of grade OM and CC under Ar. ................ 37
IV
Figure 29 – PA molecule. ................................................................................................. 39
Figure 30 – Analog scan during Polyamide extraction under H2. .................................... 40
Figure 31 – Analog scan during Polyethylene extraction under H2.................................. 41
Figure 32 – Mass spectrum 27 (C2H3+) during extraction of PA and PE under H2.......... 42
Figure 33 – Mass spectrum 27 (C2H3+) during extraction of PA and PE under Ar.......... 42
Figure 34 – Mass spectrum 27 (C2H3+) during processing of Fe2Ni under different gas
compositions. ............................................................................................................ 45
Figure 35 – Mass spectrum 16 (NH2 / CH4+/ O) during processing of Fe2Ni under
different gas compositions. ....................................................................................... 45
V
TABLE OF CONTENTS
ACKNOWLEDGMENTS ........................................................................... I
ABSTRACT.................................................................................................II
RESUMO................................................................................................... III
LIST OF FIGURES .................................................................................. IV
1. INTRODUCTION....................................................................................1
2. LITERATURE OVERVIEW..................................................................3
2.1 POWDER INJECTION MOLDING ........................................................................ 3
2.1.1 Thermal Debinding Mechanisms....................................................................... 5
2.1.2 Sintering Mechanisms........................................................................................ 5
2.1.3 Sintering Atmospheres....................................................................................... 6
2.2 THERMOCHEMICAL ASPECTS OF POWDER METALLURGY ...................... 7
2.2.1 Reduction of Iron Oxides by Hydrogen............................................................. 7
2.2.2 Carbothermal Reduction of Oxides ................................................................... 8
2.2.3 Carburizing and Decarburizing.......................................................................... 9
2.3 INTRODUCTION TO MASS SPECTROMETRY ............................................... 10
2.3.1 Electron Ionization........................................................................................... 11
2.3.2 Quadrupole Mass Analyzer.............................................................................. 12
2.3.3 Electron Multiplier Detection .......................................................................... 12
3. EXPERIMENTAL PROCEDURE.......................................................13
3.1 MASS SPECTROMETER DETAILS.................................................................... 13
3.1.1 Equipment description ..................................................................................... 13
3.1.2 Multiple Ion Detection..................................................................................... 14
3.1.3 Scan Analog ..................................................................................................... 15
3.1.4 Analysis Method Applied ................................................................................ 15
3.2 MATERIALS CHARACTERIZATION ................................................................ 16
3.2.1 DSC/TGA Analysis ......................................................................................... 16
3.2.2 Carbon Analysis............................................................................................... 16
3.3 SAMPLES PREPARATION AND EXPERIMENTS............................................ 17
3.3.1 Materials .......................................................................................................... 17
VI
3.3.2 MIM Process.................................................................................................... 17
3.3.2.1 Feedstock Preparation.............................................................................. 17
3.3.2.2 Injection Molding...................................................................................... 18
3.3.2.3 Debinding and Sintering ........................................................................... 18
3.3.3 Experiments ..................................................................................................... 19
4. RESULTS AND DISCUSSIONS ..........................................................21
4.1 INFLUENCE OF POWDER ON DEBINDING AND SINTERING..................... 21
4.1.1 Thermal Behavior under Hydrogen – Grade OM ............................................ 21
4.1.2 Thermal Behavior under Hydrogen – Grade CC ............................................. 26
4.1.3 Thermal Behavior under Argon – Grade OM.................................................. 28
4.1.4 Thermal Behavior under Argon – Grade CC................................................... 31
4.1.5 Carbon Analyses .............................................................................................. 33
4.1.6 Contrasts during Processing of Grade OM and Grade CC .............................. 34
4.1.7 Conclusions...................................................................................................... 37
4.2 INFLUENCE OF BINDER ON DEBINDING AND SINTERING....................... 39
4.2.1 Polyamide System............................................................................................ 39
4.2.2 Polyethylene System........................................................................................ 41
4.2.3 Debinding of Polyamide and Polyethylene System......................................... 42
4.2.4 Conclusions...................................................................................................... 44
4.3 INFLUENCE OF PROCESS GAS ON DEBINDING AND SINTERING ........... 45
4.3.1 Thermal Analyses ............................................................................................ 45
4.3.2 Carbon Analyses .............................................................................................. 47
4.3.3 Conclusions...................................................................................................... 48
5. SUMMARY AND OUTLOOK .............................................................49
6. REFERENCES.......................................................................................50
APPENDIX A – TYPICAL MASS SPECTRA........................................54
VII
1. INTRODUCTION
Industry has been sponsoring for several decades research that may allow
reduction of processing costs in the high-priced sintering step of Powder Injection
Molding. Apart from novel equipments that bring out faster processing time and high
reproducibility, monitoring closely and understanding the variables in PIM furnaces may
also contribute towards processing of high quality parts at low operational outlay.
Furthermore, build up of know-how about furnaces will improve sintering quality,
process parameters selection, materials development, furnace design, process control and
many other topics that enable to run an efficient and profitable process.
One important parameter in furnace controlling is the gas atmosphere composition
throughout the entire heat treatment. For manufacturing of powdered precision parts
many atmospheres are used, including air, inert gas, endo gas, hydrogen mixtures,
vacuum, etc. For all those conditions, a main concern is the concentration of reactive
species that are carried into or produced in the furnace. The evolution of these products
can greatly change the atmosphere composition and consequently the process
effectiveness and material chemistry.
Studies about the influence of atmosphere composition in powder metallurgical
processes have been done during the last 30 years, as e.g. [1, 2, 3]. Most of these studies
were performed aiming at recognizing the effects of sintering atmospheres on the final
component characteristics.
Some work related to traditional powder metallurgy were done in order to
understand
the
products
formed
during
delubrication
and
to
elucidate
the
thermodynamics of the reactions. Saha [4] and Gateaud [5] studied the kinetics of
delubrication and the residual gas formed during this process. Other studies [6, 7, 8]
aimed at analyzing the degassing and the reactions occurring during sintering of pressed
components.
Up to date, there are only some non conclusive studies related to powder injection
molding. Streicher et al. [9] studied the effect of the gas composition in the debinding of
metallic parts, but without describing the gas specimens formed. More recently, in a
specific study, da Silva Sobrinho et al. [10] analyzed the residual gas during plasma
1
debinding of polypropylene binder. Studies about in situ gas analyses of PIM materials
debinding and sintering were not found in the literature.
Due to this lack of information, on the products and reactions occurring during
debinding and sintering of powder injection molding parts, a group of researchers from
Fraunhofer IFAM Institute in Bremen - Germany decided to start activities related to this
topic.
The purpose of this diploma thesis was to analyze the evolution of gas and its
composition during the process in order to understand what happens in a MIM furnace.
The studies were performed using different powders, binders and process gas. This
investigation aimed at determining the influence of these three process parameters on the
residual specimens formed in the furnace.
The atmosphere of the furnace was analyzed by a mass spectrometer connected in
situ to a single step debinding and sintering industrial batch furnace. DSC/TGA thermal
analyses were carried out to identify the kinetics of debinding and thermodynamics of
reactions.
2
2. LITERATURE OVERVIEW
2.1 POWDER INJECTION MOLDING
Powder Injection Molding (PIM) is a powder metallurgical process which allows
good mechanical properties of metallic, ceramic and composite materials using the shape
advantages of polymers. The process can be divided in MIM – Metal Injection Molding,
CIM – Ceramic Injection Molding and CCIM – Cemented Carbide Injection Molding.
Today, the market is around 72% exclusively in metal production, 19% is covered by
firms that do only ceramics, 5% made up by companies that do only carbides and 6% by
those that mix all the techniques [11].
This technology has been established as a mass production process due to the
possibility of processing near-net-shape parts with complicated geometry of high
performance materials. As identified in Figure 1, PIM has five key features – low
production costs, shape complexity, tight tolerances, applicability to several materials and
good final properties. Most successful applications rely on particular combinations of
these attributes [12].
Figure 1 – Five factors impact on the selection of PIM for any application [12].
3
As part of its evolution, the process has many variants, reflecting different
combinations of powders, binders, molding techniques, debinding routes and sintering
cycles. An outline of PIM production is given in Figure 2.
The process consists of mixing powders with a binder system in order to obtain
the feedstock which has rheological properties to be molded in an injection molding
machine, similar to those used for plastics injection molding. After shaping, the parts
contain powder and binder, which is the so-called “green part”. The binder has to be
removed in sequence to retain only the powder, which has the desired properties for the
final component. Nowadays, there are many techniques of debinding, but the most
popular alternative is a stepwise combination of chemical immersion process followed by
a thermal debinding cycle. The next step is sintering, which consists of bonding particles
together due to thermal diffusion, leading to densification. After sintering, it is expected
to reach already the desired features for the PIM component. If needed, post sintering
processes like drilling, machining, plating and heat treatments can be applied to MIM
parts [12, 13].
Metal powder
Binder
Feedstock preparation
Moulding
Debinding
Sintering
Figure 2 – Outline of PIM process.
As debinding and sintering are the steps of PIM studied in this investigation, some
features are presented in order to understand better the process.
4
2.1.1 Thermal Debinding Mechanisms
Mechanics of thermal debinding are summarized in melting reaction followed by
gas formation from backbone polymer degradation. Gases formed are purged from the
sintering zone by a continuous heated gas flow trough the furnace. The rate of
degradation depends basically on atmosphere, wall thickness and temperature, but in
specific debinding furnaces, parameters like pressure, gas flow, etc can influence the
debinding conditions. The initial powder/binder features and powder loading are also
influents during debinding [12, 14].
Three models describe the pyrolytical removal of binder from a green body:
I-
Evaporation;
II-
Oxidative degradation;
III- Thermal degradation.
Evaporation is only possible for binder with low molecular weight, which easily
undergoes evaporation without chain scission. Oxidation originates from an initial bond
breaking reaction due the presence of oxygen and continues with reactions until the
formation of inactive products. This kind of debinding is only possible with oxidative
atmospheres and easily reducible materials, such as iron-nickel steels. Thermal
degradation of a polymer occurs either by random scission through the polymer chains.
Debinding route will be determined by atmosphere and heating rate basically [9].
2.1.2 Sintering Mechanisms
German [15] describes the sintering process as a thermal treatment for bonding
particles together into a coherent solid structure. The mechanics of this treatment are
mass transport events, which occur often at the atomic levels. In other words, sintering is
a diffusional reaction that occurs due thermal activation and consequently surface energy
reduction of particles through the arrangements of necks between such particles.
5
The mechanisms of mass transport during normal sintering are [15]:
a. Volumetric diffusion through vacancies;
b. Grain Boundary diffusion;
c. Surface diffusion;
d. Viscous and Plastic flow;
e. Vapor transport.
Sintering is summarized in three steps [15]:
a. Creation of necks;
b. Grain growth and pores reduction;
c. Closing and spherical shaping of pores.
2.1.3 Sintering Atmospheres
The sintering atmosphere is very important to achieve the best final properties of
the materials. The principal roles of the atmospheres are to provide heat transfer, oxides
reduction, particles bonding and densification, prevent oxidation, stoichiometry and
microstructure control, removal of contaminants produced from the samples, etc [1].
Basically, there are four kinds of sintering atmosphere [15]:
I- Neutral: inert gases such as nitrogen, argon and helium are useful just to
protect the materials against reactions and also to minimize explosive dangers. Sintering
in a vacuum is also a neutral system used to avoid unwanted reactions in reactive
materials. Sometimes nitrogen can nitride, for example during sintering of Titanium
components.
II- Oxidation: this kind of atmosphere inhibits the diffusion process due to
the high stability of oxides, like chromium and titanium oxides. Because of this, oxidative
atmosphere are used widely for ceramics, where the raw materials are originally oxides.
III- Reduction: special gases induce reactions like stable phases reduction,
which ensure good particle bonding. A good practical situation of this phenomenon is the
oxides reduction during hydrogen sintering.
IV- Decarburization: some gases with decarburization characteristics like
hydrogen and oxygen can be used to reduce the carbon content and equilibrate the
chemical composition of some alloys such carbon alloys.
6
The composition of the furnace atmosphere may have to be changed during the
course of the heat treatment in order to enable the execution of different metallurgical
functions. Thereby, the behavior of the atmosphere and the products outgassing can be
deeply affected, as well.
Therefore furnace atmosphere optimization becomes a very important aspect for
achieving components of the highest quality. Costs are also affected by fast
troubleshooting and closely monitoring of furnaces atmospheres.
2.2 THERMOCHEMICAL ASPECTS OF POWDER METALLURGY
Metal powders used for manufacturing of sintered parts are generally covered
on the surface by layers of contaminants, especially oxygen, nitrogen, hydrogen, water,
carbon monoxide, carbon dioxide. The amount of contaminants is related to the specific
surface of powders, storage conditions, technological pretreatments, powder production
mechanisms, etc. This chemical aspect can be important for the MIM process, since these
layers frequently inhibit the formation of metallic bonds between powder particles during
sintering. Final properties related to performance, e.g. hardness and corrosion resistance,
are also linked to the elimination of these undesirable contaminants [15, 16].
These reactions, taking place prior to and during sintering, are requisite for a
better understanding of the gases produced during PIM processing. To some extend, they
change the atmosphere surrounding the parts into the furnace which may lead to variation
in quality of the sintered part. Recognizing these specimens and their outgassing intervals
may help to troubleshoot problems of reproducibility and also to set proper parameters
for PIM production. Some of these reactions between atmosphere, contaminants and
powders are presented below.
2.2.1 Reduction of Iron Oxides by Hydrogen
The chemical reactions for the reduction of iron oxides in H2 atmosphere are
thermally controlled.
7
Under reducing atmospheres, at temperatures soon after 300ºC, there is the
reduction of Fe2O3 to Fe3O4 according to the reaction 1. Close to 500ºC, there is the
reduction of Fe3O4 to metallic iron (Fe) according to the reaction 2. These temperatures
are shifted up or down with the dew point of H2, showed in the Richardson-Ellingham
diagram. In both reactions there is production of water vapor and hydrogen consumption
[18, 19].
3Fe2O3 (s) + H2 (g) ↔ 2Fe3O4 (s) + H2O (g)
(1)
Fe3O4 (s) + 4H2 (g) ↔ 3Fe (s) + 4H2O (g)
(2)
2.2.2 Carbothermal Reduction of Oxides
Carbon containing sintered steels are used extensively for PM precision parts that
are subjected to mechanical loads in service. It may be included as admixed material
(graphite) or in solid solution phase. Moreover, carbon can be detected as contaminants,
since graphite is used as lubricant for pressed parts and carbon based binders are used for
PIM. An incomplete extraction of these materials can shift up the carbon content in the
parts.
The interaction between carbon and metals (oxides) has been a subject for
research during the last 100 years. Several mechanisms have been proposed and till now
they are not completely understood. The oldest and most widespread is the oxide
reduction through gaseous intermediates CO and CO2 in accordance with the following
reactions [20]:
MO (s) + CO ↔ M (s) + CO2
(3)
CO2 + C (s) ↔ 2CO
(4)
Equation 4 represents the Boudouard equilibrium. The temperature dependency of
this reaction is shown in figure 3. At high temperature, the equilibrium is very much on
the side of CO while at low temperatures CO2 is predominant.
8
Figure 3 – Boudouard equilibrium [3].
Other mechanism proposed for interpretation of carbothermal reduction was using
thermal dissociation, in accordance with the following reactions [8, 20]:
MO (s) + C (s) ↔ M (s) + CO
(5)
MO (s) + 2C (s) ↔ MC (s) + CO
(6)
In this mechanism, two solids react to one gas and a new solid. Leitner et al [8]
described the reaction 6 as precursor for production of carbide powders.
2.2.3 Carburizing and Decarburizing
In addition to carbothermal reduction, there are similar concerns with other carbon
reactions. Carbides can be formed or decomposed during sintering, based on reactions
involving carbon monoxide or methane production. In the same way, carbon in solid
solution can react with the atmosphere. The reaction equations can be summarised as
follows [13]:
Carburisation (→) and Decarburisation (←)
M(s) + 2CO ↔ MC(s) + CO2
(7)
M(s) + 2CO ↔ MC(s) + CO2
{MC(s) = carbide}
(8)
M(s) + CH4 ↔ MC(s) + 2H2
{MC(s) = carbon in solid solution}
(9)
M(s) + CH4 ↔ MC(s) + 2H2
(10)
9
Control of the water vapour content is very important to many sintered products.
Consider the reaction between steel (solid solution of carbon in iron) and water vapour,
(Fe + C in solution) (s) + H2O (g) → Fe(s) + CO (g) + H2 (g)
(11)
As the water content increases, decarburizing effects are enhanced. Carbon
monoxide and hydrogen are the predominant gases formed. Therefore, atmospheres with
low water content are characterized by an effective way to controlling the carbon removal
[15].
Since there are several possible reactions in such systems, it is important to
consider the five main products formed or reacting (CO, CO2, H2, H2O, CH4). Some
other reactions involving these species are summarised below,
CH4 + CO2 ↔ 2H2 + 2CO
(12)
CH4 + 3CO2 ↔ 2H2O + 4CO
(13)
CH4 + 2CO2 ↔ H2 + H2O + 3CO
(14)
CH4 + H2O ↔ 3H2 + CO
(15)
CO2 + H2 ↔ H2O + CO
(16)
The calculation of probability for each reaction during sintering requires a full
control of the partial pressures of these gases. Thermodynamic knowledge is required to
predict theoretically these values.
2.3 INTRODUCTION TO MASS SPECTROMETRY
The primary developments of mass spectrometry were presented by J. J. Thomson
in the beginning of the last century. In this pioneering demonstration, two isotopes with
mass 20 and 22, of noble gas Neon, were determined.
The fundamentals of mass spectrometry are basically the ionization of certain
organic or inorganic substance in order to obtain ions. These ions have a particular mass
measured in atomic units (a.m.u.) and charges which allows separation according to the
10
mass-charge ratio (m/z). Therefore, these mass-charge ratios are detected and the
composition of certain gas is defined.
The equipment is divided in three zones, as schematized in figure 4. The ion
source is responsible for sample ionization. The mass analyzer separates the ions
according to the m/z ratio. The detector measures the intensities of these ratios and
distributes the signals for computer processing. The data is presented, usually, by
graphics of ion current intensity vs. atomic mass units. The highest intensities are socalled peaks and these determine which substances are presented in higher or lower
concentration. If gas calibrations are available, a quantitative measurement of gas
composition can be performed [21, 22].
Ion Source
Mass
Analyzer
Detectors
Figure 4 – Three zones describing a mass spectrometer.
There are several classes of mass spectrometry techniques which differ in the
instrumentation applied in the three zones described in figure 4. In this thesis only
electron ionization, quadrupole mass analyzer and electron multiplier detection are
described below.
2.3.1 Electron Ionization
Electron ionization is the most widespread method to obtain ions for mass
spectrometry. The electrons are, usually, formed by heating a tungsten filament. These
electrons are accelerated to the ionization chamber by voltages between 5 and 100 V. The
bombardment of specimens by electrons leads to ion formation, according to reaction 17
Furthermore, the ions can react and fragment in new specimens (ion and radical),
as demonstrated in equation 18 [21].
M + e- → M+ + 2e-
(17)
M + → A+ + B .
(18)
11
Its stability, ease of control of beam intensity, lack of contamination problems and
high sensitivity, contributed to this technique becoming the most popular ionization
method in spectrometry techniques.
2.3.2 Quadrupole Mass Analyzer
Figure 5 illustrates the concept of a basic quadrupole mass spectrometer with an
ionization source, a quadrupole filter and ion detectors. The quadrupole mass filter
consists of four parallel electrodes accurately positioned in a radial array at 90º intervals.
Opposing electrodes are usually connected to form two electrode pairs. The ions formed
in the ionization zone are accelerated into the quadrupole filter with a very small voltage,
typically 10-20V. This way, due to the differences in the propagation frequency, the ions
can be filtered according to the m/z ratio [21].
Figure 5 – Basics of a quadrupole mass spectrometer [23].
2.3.3 Electron Multiplier Detection
The electron multiplier detection is the mostly used system for identification of
ions in mass spectrometers. It consists of discrete or continuous dynodes that convert the
ion impact to an electrical signal. A collector plate, placed in the last dynode, is
connected to a pre-amplifier. In this point, the output current is transformed into voltage.
Thus, this voltage can be recorded and displayed as graphics of intensity vs. m/z ratio
[21].
12
3. EXPERIMENTAL PROCEDURE
3.1 MASS SPECTROMETER DETAILS
3.1.1 Equipment description
The equipment connected to the furnace was a Quadrupole Mass Spectrometer
GAM 200 manufactured by IPI – InProcess Instruments. The advantage of the GAM 200
is its specific and sensitive analysis of multicomponent gases in a short time and with
high reproducibility. This mass spectrometer is a computer controlled system using
InProcess-Quadstar software. Qualitative and quantitative methods are available, but the
second one requires specific gas calibration [23].
Figure 6 – Schematic drawing of the connection between MS and furnace.
Figure 6 illustrates the schematic drawing of the connection between mass
spectrometer and furnace. The gas produced inside the furnace is conducted through a
glass capillary into the analysis zone of the spectrometer. Polymer condensation is
avoided by heating all the system. The heating is carried out by heating bands at 300ºC.
The highest working temperature of the capillary is 600ºC. As the sintering temperature
in the furnace is much higher, the equipment was attached directly below the chamber.
13
3.1.2 Multiple Ion Detection
In Multiple Ion Detection mode (MID) the measurements are performed almost
continuously, concentrated on certain critical masses. The measured intensities of the
chosen masses and temperature are displayed as a function of time, e.g. figure 7. This
type of graphic allows determining qualitatively some chosen products contained in the
Intensity (A)
2,50E-11
1400
1200
1000
800
600
400
200
0
Mass 12
2,00E-11
Temperature
1,50E-11
1,00E-11
5,00E-12
0,00E+00
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
Temperature (ºC)
atmosphere.
7
Time (h)
Figure 7 – Example of a mass spectrum obtained by MID measurements.
Table 1 – Expected gases specimens and their supposed mass units.
Products
Hydrogen (H2)
Nitrogen (N2)
Argon (Ar)
Oxygen (O2)
Mass Unit (amu)
02
28, 14
40, 20
32,16
Products
Ammonia (NH3)
Carbon Monoxide (CO)
Carbon Dioxide (CO2)
Methane (CH4)
Water (H2O)
18, 17, 16
Hydrocarbons (CxHy)
Mass Unit (amu)
17, 16, 15, 14
28, 12
44, 16, 12
16, 15, 14, 13, 12
26, 27, 28, 29, 41, 42,
43, 44, 50, 55, 56
Table 1 describes some expected species with their supposed mass units. The
choice of measuring these masses was based on the NIST database, Quadstar software
library, literature [21, 22] and previous experiments performed at IFAM. Appendix A
exhibits the spectra for these substances.
The advantage of this technique is to provide a profile containing the evolution of
certain masses during a defined time. By this, it is possible to determine the interval
where a defined product is degassed.
14
3.1.3 Scan Analog
The Scan Analog is a continuous scan of all masses in a given range. When the
scan is finished it starts again almost immediately. In each scan the ion current is
exhibited as a function of the mass range, e.g. figure 8.
When the products yielded are not known previously, this technique needs to be
applied. Thereby it is possible to have an overview of all masses and thus, to determine
which one deserves to be looked at in depth using the MID mode.
The scans in this investigation were in the range of 0 to 100 amu (atomic mass
unit). This choice was based on previous investigation conducted at IFAM, where it was
found that polyethylene and polyamide binders exhibited almost no signals for masses
higher than 100 amu.
Figure 8 – Example of a mass spectrum obtained by Scan Analog measurements.
3.1.4 Analysis Method Applied
The measurement sequence was programmed using MID technique with an
intermediate Scan Analog, every 250 seconds. The data was acquired during the entire
15
heating cycle and the information was saved on a computer. The data acquisition was
stopped before the cooling time.
Before starting the experiments, the residual gas background in the set up was
calibrated. It can falsify the results if it is not handled properly. The partial pressures and
the composition of the residual gas in the analysis chamber are essentially depending on
the type of vacuum pump and history of the system. The calibration was performed at the
ultimate pressure (0,001 mbar) in an empty cycle (without flowing gas and MIM
samples), according to the mass spectrometer specification. The ion currents determined
this way were subtracted in all subsequent measurements.
Before each test, the pressure inside of the spectrometer was monitored. This
procedure was adopted to certify that no polymer had blocked the capillary in the
previous cycle. With the doors of the furnace opened, the normal pressure inside of the
analyses chamber of the mass spectrometer was around 2xE-5 mbar.
3.2 MATERIALS CHARACTERIZATION
3.2.1 DSC/TGA Analysis
The machine used for DSC/TGA experiments was a NETZSCH STA 409 C/CD.
Pure argon was used as purge gas to flow through the analysis zone of the equipment.
The heating rate applied was 5ºC/min up to 1400ºC, with a holding of 60 min. at 600ºC.
This dwell time was necessary to allow the complete binder removal. The other
parameters were adopted according the specification of the equipment. Unfortunately,
these tests were not carried out in Hydrogen due to safety procedures of IFAM.
3.2.2 Carbon Analysis
The technique employed to control the carbon content in sintered components was
the Carbon Combustion Method. The analyses were performed according to the standard
ASTM E1019 and the equipment used was a LECO CS 444. For each samples, three
measurements were performed.
16
3.3 SAMPLES PREPARATION AND EXPERIMENTS
3.3.1 Materials
This investigation was carried out using four powders and three binder systems,
summarized in table 2. The ferrous MIM components were produced with three grades of
carbonyl iron powder. The first one was Fe HQ powder, which is a super fine specialty
grade for molding of micro parts with basically 0.8%C, 0.8%N, 0.3%O and avg. particle
size of 1μm. The grade Fe OM (<0.9%C, <0.9%N, <0.3%O and avg. particle size of
4μm) is the standard MIM powder commonly used in industrial batches of steels. The Fe
CC is hydrogen-reduced silicated grade with extremely low content in carbon and
nitrogen (<0.05%C, <0.01%N, <0.3%O and particle size of 6μm). [24]
Table 2 – List of powders and binders.
POWDERS
Fe HQ – BASF
Fe OM – BASF
Fe CC – BASF
Fe2Ni
BINDER SYSTEMS
Polyamide based
Polyethylene based
Polyamide based
The alloy Fe2Ni was provided by a MIM producer. Due to confidentiality not all
of the composition was revealed. This powder was a mixture of 2/3 Fe CC, 1/3 Fe OM
and 2%Ni with a polyamide based binder system.
Fe HQ had the same limitation with a comparable binder. The others feedstocks
were prepared with Fraunhofer binder system B31 which is a polyethylene based system.
3.3.2 MIM Process
3.3.2.1 Feedstock Preparation
Carbonyl iron based feedstocks were prepared using a powder:binder ratio of
55%vol. of powder. Fe2Ni and Fe HQ samples were prepared by MIM producers and this
way, the mixing characteristics were confidential.
17
The feedstocks prepared at IFAM were mixed in a turbomixer Lödige in 45 min.
at 100ºC maximum temperature.
3.3.2.2 Injection Molding
The injection molding was performed on a Klöckner Ferromatik FM 40 using
standard injection parameters for MIM materials. Polyamide based and B31 feedstocks
have different rheological behaviour. Thus, different injection parameters, described in
table 3 and 4, were applied.
Fe2Ni feedstock was molded by the client company and for that reason the
injection parameters are confidential.
The samples were molded in the shape of standard tensile test components, in a
four cavity tooling. 500 components of each feedstock were molded.
Table 3 – Injection Molding parameters for
PE feedstock.
Temperature
Nozzle
Temperature
Tooling
Injection Pressure
Injection Speed
Packing Pressure
115ºC
40ºC
300 bar
90 mm/s
180 bar
Table 4 – Injection Molding parameters for
PA feedstock.
Temperature
Nozzle
Temperature
Tooling
Injection Pressure
Injection Speed
Packing Pressure
145ºC
35ºC
950 bar
90 mm/s
650 bar
3.3.2.3 Debinding and Sintering
Debinding cycles were carried out in two stages, one chemical extraction followed
by the thermal treatment in the furnace
The chemical debinding of B31 feedstocks was executed by immersion in liquid
hexane at 35ºC for 12 hours. Fe2Ni and Fe HQ feedstocks were extracted in acetone.
All tests of thermal debinding and sintering were performed in a MIM furnace Elnik 3001
T-50, which has laminar gas flow as debinding technology. The standard cycle for all
components is summarized in table 5.
18
The samples were debound and sintered at 800 mbar. For reaching this quickly in
the beginning of the cycle, a high gas flow in the first step was applied. The second and
third program step corresponded to debinding ramp and debinding holding. This first
holding was performed at 600ºC for 90 minutes. The sintering ramp and sintering holding
concluded the heating stage of PIM process. The sintering temperature was hold at
1200ºC for 60 minutes. The process gases used during these heat treatments are presented
below in the topic Experiments. Steps 6 and 7 of the sintering program corresponded to
cooling. In each cycle 150 tensile test components (~ 2250g of feedstock) were sintered.
This quantity was chosen to obtain a good signal in the mass spectra.
Table 5 – Standard program for debinding and sintering of PIM samples.
Conditions
Segment
Type
Temperature
(ºC)
Heating Rate
(ºC/min)
Time Holding
(min)
Pressure
(mbar)
Gas Flow
(l/min)
1
2
3
4
5
6
7
Ramp
Ramp
Holding
Ramp
Holding
Ramp
Holding
50.0
600.0
600.0
1200
1200
600.0
50.0
10.0
3.0
-
8.0
-
15.0
-
-
-
90.0
-
60.0
-
-
800.0
800
800
800
800
800
800
70.0
20.0
20.0
20.0
20.0
12.0
-
3.3.3 Experiments
The experiments were conducted in order to investigate the effects of powder,
binder and process gas on the gaseous species formed during debinding and sintering.
The tests were carried out with the mass spectrometer connected to the furnace. These
were compared to thermal and carbon analysis, which were described earlier. Table 6
summarizes the experiments.
Test A was a comparative study on the effects of different carbonyl iron powders
on the gases produced during debinding and sintering. From these experiments, the
influence of powder chemistry and powder production conditions were evaluated. These
tests were conducted in two atmospheres – pure H2, pure Ar – using the standard cycle.
19
Table 6 – Summary of carried out experiments.
TEST
A
B
C
POWDER
Fe CC
Fe OM
Fe OM
FE HQ
BINDER
Polyethylene
Polyamide
Fe2Ni
Polyamide
Polyethylene
H2 / Ar (%)
100 / 0
0 / 100
100 / 0
0 / 100
100 / 0
75 / 25
50 / 50
25 / 75
0 / 100
Test B was performed to identify the effects of different binders on the products
yielded during debinding and sintering. As polyamides have Nitrogen and Oxygen in
their chemistry, different products during debinding were expected. Powders used in this
test had almost the same starting composition. Therefore, different behaviors about the
products formed should come out only due to binder effects.
The objective of test C was to verify the effect of reductive (Hydrogen) and
neutral (argon) atmospheres on the outgassing products formed during MIM Fe2Ni steel
production. The purity of these two gases is shown in table 7. As argon was purer than
hydrogen, a longer background was expected when applying hydrogen. The intermediate
products composing the atmosphere of debinding and sintering and the predominant
reactions for them were determined.
Table 7 – Purity of hydrogen and argon used as process gases.
Purity (%)
O2
H2O
N2
Argon
≥ 99,998
≤ 3 ppm
≤ 5 ppm
≤ 10 ppm
Hydrogen
≥ 99,9
≤ 50 vpm
≤ 100 vpm
≤ 500 vpm
An extra cycle was conducted in order to evaluate the carbon removal during
manufacture of iron carbon steels under flowing hydrogen. The cycle was the same of the
standard one, but finishing with the isothermal debinding. By this, the mechanisms that
were influencing carbon control were determined. As far the other tests, carbon analyses
were carried out.
20
4. RESULTS AND DISCUSSIONS
4.1 INFLUENCE OF POWDER ON DEBINDING AND SINTERING
4.1.1 Thermal Behavior under Hydrogen – Grade OM
Intensity (A)
1,20E-11
9,00E-12
Mass 44
1200
Temperature
1000
800
6,00E-12
600
400
3,00E-12
200
0,00E+00
Temperature (ºC)
Figures 9 to 12 exhibit the MS investigation for iron grade OM processed in H2.
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
Time (h)
Figure 9 – Mass spectrum 44(C3H8+ / CO2) of grade OM under H2.
Mass 27
Mass 28
Temperature
Intensity (A)
1,60E-10
1200
1000
800
1,20E-10
600
8,00E-11
400
4,00E-11
200
0,00E+00
Temperature (ºC)
2,00E-10
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
Time (h)
Mass 16
Mass 17
Temperature
4,00E-09
Intensity (A)
and 28(CO / C2H4+) of grade OM under H2.
3,00E-09
1200
1000
800
600
2,00E-09
400
1,00E-09
200
0,00E+00
Temperature (ºC)
Figure 10 – Mass spectrum
27(C2H3+)
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
Time (h)
+
Figure 11 – Mass spectrum 16(CH4 / NH2 /O) and 17(NH3 /OH-) of grade OM under H2.
Intensity (A)
5,00E-10
Mass 18
1200
Temperature
1000
4,00E-10
800
3,00E-10
600
2,00E-10
400
1,00E-10
200
0,00E+00
Temperature (ºC)
6,00E-10
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
Time (h)
Figure 12– Mass spectrum 18(H2O) of grade OM under H2.
21
The first gas evolution was measured in the 200-300ºC temperature range. This
outgassing was observed by mass 44, illustrated in figure 9, which can be attributed to
carbon dioxide (CO2) and hydrocarbons (CxHy). Due to the simultaneous presence of
m12 and m16, this first product can be identified as carbon dioxide. These two mass units
are linked to the atomic carbon and atomic oxygen, respectively, as shown in the
spectrum of pure carbon dioxide in Appendix A.
Carbon dioxide was related to desorption of adsorbed gases on the powder
surfaces. As already reviewed, powders are generally covered by loosely bonded
specimens provided from several sources like storage conditions, production features, etc
[7]. Equation 19 represents the chemical reaction for degassing of CO2.
CO2 (adsorbed) ↔ CO2 (g)
(19)
The subsequent peak in m44 spectrum corresponded to the binder removal (CxHy),
in the 300-470ºC temperature range. Debinding was confirmed by m27 – figure 10 – that
corresponds inherently to hydrocarbons [22].
Mass 16 – figure 11 – was found in similar temperature range of debinding (250420ºC). It can be linked usually to several products like methane, carbon dioxide, carbon
monoxide, ammonia, water, etc. Due to this overlapping, an evaluation of the coupled
masses is required. In this interval, m16 was taking place at similar time and intensity of
m17 + m18. These three atomic numbers are correlated normally to water, but as m16
and m17 showed higher intensity than m18, an overlapping of NH3 + H2O was assumed.
For pure water, the spectrum of m18 should be much higher than for m17. For pure NH3,
m18 should appear in a very low intensity (base line). For CH4, m16 is typically coupled
closely to m15. Appendix A presents the standard mass spectra of water, ammonia and
methane.
Given that Fe OM contains nitrogen in the powder, it was assumed that the peak,
between 250-420ºC temperature ranges, arose from hydrogenation of nitrogen, according
to reaction 20.
[Fe + N2 (contained in the powder)] + H2 (g) ↔ Fe (s) + NH3 (g)
(20)
22
The commercial production of ammonia by direct combination of nitrogen and
hydrogen is performed around 300ºC, using iron oxides as catalyst and under pressures
with magnitude of 200-400 atmospheres [25]. Nevertheless, the low pressure applied in
PIM furnaces does not prevent NH3 formation, merely restrain the reaction.
The possibility of having, to some extent, pure nitrogen evolution could be
considered. However, this cannot be measured by mass 28, since nitrogen occurred at the
same time as binder degradation (C2H4 = 28 amu).
The water found, overlapping ammonia production, illustrated in figure 12,
derived from reduction of loosely bonded hydroxides (equation 21) and from removal of
the bonded water, contained in the binder and on the powder (equation 22). Some initial
reduction of iron oxides by hydrogen has to be considered, as well. Reaction 23
represents the oxides reductions.
(Fe)-H2O (bonded water) ↔ Fe (s) + H2O (g)
(21)
(Fe)-OH (hydroxides) + H2 (g) ↔ Fe (s) + H2O (g)
(22)
(Fe)-O (oxides) + H2 (g) ↔ Fe (s) + H2O (g)
(23)
Afterwards in the 500-600ºC temperature range, a narrow shift in the intensities of
mass 18 and 28 – figures 12 and 10 – determined an overlapping of water and carbon
monoxide. At this point, CO production was proved by the combination of m28 + m12 +
m16. This overlay of gases was associated to the reduction of iron oxides present on the
surface of powder particles. Water occurred by means of reaction 23 and CO due to the
following possibilities:
1 - Hydrogen reduction (Equation 23) and carbothermal reduction (Equation 24) were
taking place concurrently. In this manner, H2O and CO would be brought on by different
reactions. At this temperature, the carbon source supporting the carbothermal reduction
should be binder residues or carbon-graphite, since the carbon contained in the powder
was still present in stable phases (e.g. carbides). The intensities obtained by MS were
higher for H2O than for CO. According to the Ellingham diagram, the free energy of
formation is more negative for water than CO, which fits to the results. In other words, at
23
this temperature the production of water is more stable than for carbon monoxide. The
reaction involving carbon becomes favored around 700ºC.
(Fe)-O (oxides) + C (s) ↔ Fe (s) + CO (g)
(24)
2 - The second possibility suitable for the applied conditions could be the reaction
involving water and iron containing carbon. This reaction was presented in the literature
overview (Equation 11). As the water content increases, decarburizing effects are
enhanced. Carbon monoxide and hydrogen are the predominant gases formed. Working
in H2-atmosphere, the equilibrium is shifted towards the water, though, which makes this
possibility less probable.
The reaction involving reduction of carbon dioxide by hydrogen to form carbon
monoxide and water (equation 16 in the literature overview) was rejected as there was no
peak on m44 in this temperature range. Thermodynamic calculations could determine
which reactions are favorable to happen during the PIM process.
Moreover, figure 11 exhibits a large peak of m16 in the middle of the debinding
holding (600ºC) going up to around 720ºC. This peak was combined to m15, m14, m13,
m12 which easily identified it as methane.
The methane formation was explained by the decarburizing behavior of the
flowing hydrogen atmosphere, according to equation 25 below:
(Fe)-C(s) + 2H2 (g) ↔ Fe(s) + CH4 (g)
(25)
Production of methane is evaluated in depth when comparing the specimens
produced from both powders (see Chapter 4.1.6).
At high temperatures (600-800ºC) a new overlapping of m18 and m28 was
revealed. This combined production of water and carbon monoxide characterized the
reduction of stable iron oxides situated in the particles core. Danninger et al [6] has been
identifying these dual deoxidation point for different materials, as well.
There is the interesting and surprising phenomenon that the products started to
come out slightly below the onset of α ↔ γ transition (720ºC) and finishing subsequent to
24
eutectoid transformation. The solubility of carbon is much higher in austenite than in
ferrite, thus the starting of reduction concurring with austenite formation could be
expected. There was however a large decarburizing reaction (methane formation). To
some extent, it might allow the dissolution of carbides below the usual temperature.
Danninger et al. [26, 27] also studied this effect under neutral atmospheres and his
contribution is further described in the topic about processing grade OM under argon.
Table 8 summarizes the products obtained during processing of grade OM under
hydrogen.
Table 8 – Summary of obtained gas products from grade OM sintered under H2.
Temperature Range (ºC)
Gas Products
Reactions
200 – 300
CO2
Desorption Reactions
NH3
Nitrogen Evolution
H 2O
Reduction Processes
CxHy
Binder decomposition
250 – 470
300 – 470
500 – 600
600 - 720
600 – 800
H 2O
CO
CH4
H 2O
CO
Reduction Processes
Methanation
Reduction Processes
25
4.1.2 Thermal Behavior under Hydrogen – Grade CC
Figures 13 to 16 exhibit the MS investigation for iron grade CC processed in H2.
1200
Temperature
1,50E-11
Intensity (A)
1400
Mass 44
Temperature (ºC)
2,00E-11
1000
800
1,00E-11
600
400
5,00E-12
200
0,00E+00
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
Time (h)
Figure 13 – Mass spectrum 44(C3H8+ / CO2) of grade CC under H2.
1400
Mass 27
Mass 28
Temperature
8,00E-11
1200
1000
6,00E-11
800
4,00E-11
600
400
2,00E-11
200
0,00E+00
Temperature (ºC)
Intensity (A)
1,00E-10
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
Time (h)
Intensity (A)
8,00E-09
and 28(CO /C2H4+) of grade CC under H2.
1400
Mass 16
1200
1000
Temperature
6,00E-09
800
4,00E-09
600
400
200
2,00E-09
0,00E+00
Temperature (ºC)
Figure 14 – Mass spectrum
27(C2H3+)
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
Time (h)
+
Figure 15 – Mass spectrum 16(CH4 / NH2 / O) of grade CC under H2.
Intensity (A)
1400
Mass 18
5,00E-10
1200
Temperature
1000
4,00E-10
800
3,00E-10
600
2,00E-10
400
1,00E-10
200
0,00E+00
Temperature (ºC)
6,00E-10
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
Time (h)
Figure 16 – Mass spectrum 18(H2O) of grade OM under H2.
The initial product degassed from Fe CC samples was detected by m44 – fig. 5 –
in the 300-420ºC temperature range. Binder removal (m27 – fig. 6) and water production
26
(m18 – fig. 8) were evident in the same interval. Thereby, the first gas evolution of this
powder was marked by an overlapping of CO2 + CxHy + H2O. Here, the presence of CO2
may be controversial since m44 can represent hydrocarbons, as well. However, when
analyzing the intervals of occurrence for m27 and m44, it was noticeable that the second
one finishes to some extent earlier than the other. In addition, the shapes of the peaks are
quite different, which suggests the possibility of CO2 concurring to binder removal. This
substance was linked in the same way to desorption of adsorbed gases on the particle
surfaces (Equation 1).
The water was created by reduction of loosely bonded hydroxides and bonded
water contained in the binder and on the powder. Some initial reduction of iron oxides by
hydrogen has to be considered, as well.
Soon after debinding, a huge peak in mass spectrum 16 was observed. This one,
given in the 410-600ºC temperature range, was coupled to m15 + m14 + m13 + m12
which characterized it as methane. The low temperature range of occurrence for CH4 is
one of the most interesting behaviors of this investigation and it is discussed deeply in the
topic Contrasts during Processing of Grade OM and Grade CC.
The last gases produced by grade CC were the dual emission of water and carbon
monoxide, linked to the reduction of oxides (Equation 4 and 5). Here, the outgassing
began to some extent after α ↔ γ transition (Plain iron has it around 910ºC). Generally it
was unexpected that in carbonyl iron CC, deoxidation by reaction with carbon was thus
pronounced. Apparently, the very low natural carbon content in the powder and some
carbon residues diffused during heating were sufficient to provide reduction.
One important feature to keep in mind was that Fe CC exhibited low densification
after sintering in hydrogen atmosphere (samples were easily broken by hand). This
behavior is discussed in chapter 4.1.6.
Table 9 summarizes the gases obtained during processing of grade CC under H2.
Table 9 – Summary of obtained gas products from grade CC sintered under H2.
Temperature Range (ºC)
300 – 420
420 - 600
910 – 1200
Gas Products
CO2
H 2O
CxHy
CH4
H2O + CO
Reactions
Desorption Reactions
Reduction Processes
Binder decomposition
Methanation
Reduction Processes
27
4.1.3 Thermal Behavior under Argon – Grade OM
Figures 17 to 20 exhibit the MS investigation for grade OM processed under Ar.
1200
1000
Mass 17
Temperature
8,00E-11
800
6,00E-11
600
4,00E-11
400
2,00E-11
200
0,00E+00
Temperature (ºC)
1,00E-10
Intensity (A)
1400
Mass 14
Mass 16
1,20E-10
0
0
0,5
1
1,5
2
2,5
3
3,5
Time (h)
4
4,5
5
5,5
6
6,5
7
Figure 17 – Mass spectrum 14(CH2+ /N), 16(CH4+ /NH2 /O) and 17(NH3 /OH-) of grade
OM under Ar.
8,00E-11
1400
6,00E-11
1200
Temperature
1000
800
4,00E-11
600
400
2,00E-11
Temperature (ºC)
Intensity (A)
Mass 18
200
0,00E+00
0
0
0,5
1
1,5
2
2,5
3
3,5
Time (h)
4
4,5
5
5,5
6
6,5
7
Intensity (A)
8,00E-10
6,00E-10
Mass 27
1400
Mass 28
1200
Temperature
1000
800
4,00E-10
600
400
2,00E-10
Temperature (ºC)
Figure 18 – Mass spectrum 18(H2O) of grade OM under Ar.
200
0,00E+00
0
0
0,5
1
1,5
2
2,5
3
3,5
Time (h)
4
4,5
5
5,5
6
6,5
7
Figure 19 – Mass spectrum 27(C2H3+) and 28(CO / C2H4+) of grade OM under Ar.
1400
1,00E-10
Mass 44
1200
Temperature
1000
6,00E-11
800
4,00E-11
600
400
2,00E-11
Temperature (ºC)
Intensity (A)
8,00E-11
200
0,00E+00
0
0
0,5
1
1,5
2
2,5
3
3,5
Time (h)
4
4,5
5
5,5
6
6,5
7
Figure 20 – Mass spectrum 44(C3H8+ / CO2) of grade OM processed under Ar.
28
The first outgassing peak was measured via mass 16, figure 17. This sharp peak,
going from 320ºC to around 420ºC, was combined to m17 in similar intensity range,
which suggested the presence of Ammonia (spectra of pure NH3 in Appendix A shows
the combination of m17 + m16 for NH3). The peak of ammonia was overlapping the
debinding range, which explained its formation. As long as the feedstock has some
concentration of hydrogen, this one could work as a H2 source for ammonia production,
according to equation 20.
Debinding happened up to 520ºC as shown by m27 (fig. 19). The production of
water (m18) was identified in the same interval as debinding, as illustrated in figure 18.
Water was supposed to come from reduction of hydroxides and desorption of bonded
water contained in the binder and on the powder.
Following, the spectrum of m28 shown in figure 19 exhibits initially the peaks of
debinding and afterwards – between 500ºC to 600ºC – a large one that is combined with
m14 + m16 (fig. 17). These overlapping of secondary masses made clear the production
of more than one substance. Mass 14 determined the outgassing of nitrogen, whereas
mass 16 the production of carbon monoxide or carbon dioxide.
The high temperature and absence of hydrogen source prevented ammonia
formation and thus only nitrogen was found. This event confirmed the assumption about
ammonia reaction postulated in the beginning of the current topic. At this interval,
CO/CO2 represented the starting of carbothermal reduction of loosely bonded oxides,
driven by reaction 24.
In spite of the clear overlapping of products, the peak of m28 given in figure 11
represents almost exclusively the evolution of N2, with a very little quantity of CO+. The
latter ion is a slight contribution of CO2, which also shows a small peak m28 (see
Appendix A). This corresponds well with the Boudouard Equilibrium, given in figure X,
which says that at 500ºC the equilibrium is well on the side of CO2.
After everything else, the spectrum of m28 (fig. 19) exhibited the production of
CO in the temperature range closely below the onset of α ↔ γ transformation, going up to
800ºC. Here, deoxidation effects in Fe OM were surprisingly found again before phase
transition (720ºC). Danninger et al. [26] concluded that reduction of oxides not
necessarily requires dissolution of carbon in austenite, but already the low solubility in
ferrite is sufficient to react with the oxides layer covering powder particles.
29
Figure 21 gives the DSC/TGA analysis carried out for processing of grade OM
under argon. By this it is possible to identify and confirm some of the phenomena
described based on mass spectrometric results.
Figure 21 – DSC/TGA analyses of grade OM under argon.
The endothermic peak around 100ºC was associated with binder melting. This
behavior cannot be detected via MS, since only gaseous compounds are identified by this
technique. The NH3 formation occurs by an exothermal reaction which can be seen also
in the DSC/TGA test, given in figure 13. In the 300-370ºC temperature range there was a
slightly exothermic peak, which fits to the ranges of occurrence for ammonia.
Debinding (CxHy) was also confirmed by DSC/TGA analysis around 380ºC where
a large mass loss and energy consumption was observed. Soon after debinding, another
mass loss was computed. This one could be linked to the carbothermal reduction and
evolution of nitrogen occurring in this temperature range. Two small endothermic peaks
were observed in the beginning and in the end of the isothermal holding, which were
artificially produced by the DSC/TGA equipment.
Close to 720ºC, an endothermic peak representing the phase transition of this
eutectoid steel was observed. The last shift up in energy variation was related to that one
necessary for sintering and melting.
Table 10 summarizes the products obtained during processing of grade OM under
Ar.
30
Table 10 – Summary of obtained gas products from grade OM sintered under Ar.
Temperature Range (ºC)
320 – 420
Gas Products
NH3
CxHy
H 2O
N2
CO + CO2
CO
400 – 520
500 – 600
600 – 800
Reactions
Nitrogen Evolution
Binder decomposition
Reduction Processes
Nitrogen Evolution
Reduction Processes
Reduction Processes
4.1.4 Thermal Behavior under Argon – Grade CC
Figures 22 to 24 exhibit the MS investigation for grade CC processed under Ar.
1400
Mass 16
Mass 17
Mass 18
Temperature
6,00E-11
1200
1000
800
600
3,00E-11
400
200
0,00E+00
Temperature (ºC)
Intensity (A)
9,00E-11
0
0
0,5
1
1,5
2
2,5
3
3,5
Time (h)
4
4,5
5
5,5
6
6,5
7
Figure 22 – Mass spectrum 16(CH4+ /NH2 /O), 17(NH3 / OH-) and 18(H2O) of grade CC
under Ar.
3,00E-10
Intensity (A)
2,00E-10
1200
Mass 28
1000
Temperature
800
1,50E-10
600
1,00E-10
400
5,00E-11
200
0,00E+00
Temperature (ºC)
1400
Mass 27
2,50E-10
0
0
0,5
1
1,5
2
2,5
3
3,5
Time (h)
4
4,5
5
5,5
6
6,5
7
Figure 23 – Mass spectrum 27(C2H3+) and 28(CO /C2H4+) of grade OM under Ar.
5,00E-11
Intensity (A)
1200
Temperature
1000
3,00E-11
800
2,00E-11
600
400
1,00E-11
200
0,00E+00
Temperature (ºC)
1400
Mass 44
4,00E-11
0
0
0,5
1
1,5
2
2,5
3
3,5
Time (h)
4
4,5
5
5,5
6
6,5
7
Figure 24 – Mass spectrum 44(C3H8+ /CO2) of grade OM under Ar.
31
The first product identified during processing of grade CC under argon was water
and hydrocarbons in the 400-520ºC temperature range. Again, water was formed due to
the reduction of hydroxides and desorption of bonded water contained in the polymers
and on the powder.
Between 500-600ºC, an overlapping of mass 28 + m44 was observed. These two
products were outcomes from carbothermal reduction of oxides that were covering the
powder particles. Comparing directly mass 28 (fig. 23) with mass 44 (fig. 24) it was clear
that the first one has the highest peak intensity. This behavior was unexpected, since
production of CO2 is favored more than CO in this temperature range, according to the
Boudouard equilibrium given in figure 3. The presence of retained carbon-binder could,
to some extent, shift the Boudouard Equilibrium to the right side of equation 4, thus
producing more CO.
At high temperatures, carbon monoxide (fig. 23) was found again. This peak close
to 910ºC was related to the carbothermal reduction of oxides from internal pores, which
are the latest to be reduced. At this temperature CO2 was not observed, which fits to
Boudouard equilibrium. It is interesting to record that this deoxidation occurs only after
phase transition in the steel, which can be linked to dissolution of carbon in austenite in
the same way explained previously in the topic of processing grade CC under hydrogen.
The results of MS were confirmed via DSC/TGA experiments which are
presented in figure 25.
Figure 25 – DSC/TGA analyses of grade CC.
32
The first energy variation was related with binder melting, as already explained in
the preceding chapter. Following, there was a huge mass loss at around 400ºC that
represented binder extraction. The small mass variation close to the isothermal holding at
600ºC may represent the slight reduction of superficial oxides with respective elimination
of CO and CO2.
At 720ºC there is the peak associated to the beginning of austenite formation. It
was surprising to observe an intense energy reduction after starting the phase transition.
By now it is not sure if this came up as reduction of baseline or any reaction occurring in
this powder.
The DSC/TGA profile finishes with an increase of energy consumption in order to
sinter and melt.
Table 11 summarizes the products obtained during processing of grade CC under
Ar.
Table 11 – Summary of obtained gas products from grade CC sintered under Ar.
Temperature Range (ºC)
400 – 520
500 – 600
600 – 800
Gas Products
CxHy
H 2O
CO
CO2
CO
Reactions
Binder decomposition
Reduction Processes
Reduction Processes
Reduction Processes
4.1.5 Carbon Analyses
Table 12 gives the average results of carbon analysis carried out in the sintered
parts. The ferrous parts processed under hydrogen obtained similarly low values of
carbon content. Under argon, the results of Fe OM were in agreement with those
described by BASF (≤ 0.9%C), while Fe CC revealed some “pick up” of carbon during
the process, as the natural carbon content in the powder was ≤ 0.05%C.
Table 12 – Carbon analyses of sintered parts.
Fe OM – H2
Fe CC – H2
Fe OM – Ar
Fe CC – Ar
0.005
0.005
0.840
0.104
33
Overall, sintering under pure hydrogen is unsuitable for carbonyl iron powders
grade OM and CC, if carbon level has to be controlled. Production of methane on a large
scale was the basis for such intense decarburizing effect. Table 13 exhibits the carbon
content of an extra cycle carried out up to the end of the isothermal debinding (600ºC),
where CH4 production has almost ceased.
Table 13 – Carbon analyses after isothermal debinding at 600ºC.
Fe OM – H2 – 600ºC
Fe CC – H2 – 600ºC
0.016
0.008
By these values, a strong decarburization was measured already at 600ºC. For
grade OM, the carbon loss was still 3 times higher than the final content. For grade CC,
the carbon was already similar to those acquired in the sintered components.
In contrast, processing of Fe OM under argon led to similar carbon content as of
the initial powder level. For Fe CC processed under argon, some carburizing effect was
revealed. The decarburizing potential, created by solid solution reactions of oxides with
carbon, was not sufficient to avoid diffusion of residual binder (carbon) into the matrix.
This carburizing behavior of grade CC is explained in depth in the next chapter, where
the differences during processing of both powders are discussed.
4.1.6 Contrasts during Processing of Grade OM and Grade CC
The marked differences in the degassing behavior of both powders were basically
related to mass spectrum 16 and 28.
Mass 16 (fig. 26) revealed an interesting phenomenon, if processed under
hydrogen. First, Ammonia was observed during processing of samples prepared with
grade OM. As explained earlier, NH3 arose from evolution of nitrogen of the powder
reacting with the stream gas in the furnace (H2). As grade CC is nitrogen reduced, this
kind of behavior was not observed.
In sequence, the intervals of occurrence for methane (mass 16) were different.
Grade CC exhibited this peak earlier in the 410-600ºC range, whereas grade OM revealed
34
CH4 production only after half an hour at isothermal holding (600ºC) going up to 720ºC.
The divergences are explained if the spectra from mass 27 (fig. 27) and mass 16 (fig. 26)
are compared. The intensity of debinding was lower for grade CC than for grade OM. On
the other hand, the intensity of methane was higher for grade CC than for grade OM.
Thereby, the possibility of grade CC exhibiting some “pick up” of carbon-binder during
polymers decomposition was assumed, which reduced m27 and then increased m16 later
on.
Carburization via binder pick up was possibly enhanced in grade CC due to the
higher reactivity of this powder, provided by its chemistry and also production route. The
low quantity of carbon and the applied hydrogen reduction, during powder production,
might favor a fresh particles surface, which contains more energy for keeping admixed
carbon-binder at the surface. As carbon-binder is quickly reacted to hydrogen, methane is
formed directly after debinding interval. This way, grade CC under H2 has the interesting
behavior of carburization followed by decarburization in a small temperature interval.
On the other hand, grade OM has low reactivity with the binder, which allowed an
almost complete polymer extraction in the expected interval. In this manner, production
of methane arose only when the carbon contained in the powder became available for
reaction with the stream gas. In other words, the carbon from grade OM is much more
stable (carbides) and requires much more time to be able to react with H2, if compared to
the carbon-binder picked up by grade CC.
This “carbon pick up” behavior explains why under neutral atmospheres (argon),
the sintered part is carburized in case of grade CC and unchanged for grade OM. Since
the carbon diffuses into grade CC, it does not have any other possibility to be removed if
not as CO or CO2, but given that the oxygen content is very low, the carbon-binder
becomes trapped in the sintered part. For grade OM, binder is almost completely
removed and thereby, the final carbon content was unchanged compared to the initial
powder condition. Some reduction by carbothermal reaction should remove any possible
small amount of retained binder. Under hydrogen, it does not matter if there is carbon or
not, since all carbon is removed as methane.
This retained carbon-binder may also explain the major occurrence of CO instead
of CO2 at around 500ºC during processing of Fe CC under argon, as commented in
chapter 4.1.2.
35
Furthermore, the very early carbon loss in grade CC (~ 500ºC) might explain the
low densification of the samples, commented also in chapter 4.1.2. It is well known that
sintering is activated by chemical potential (here created by carbon). At 600ºC, grade CC
was already carbon free (plain iron), which shows a low sinterability. In addition, the
isothermal sintering temperature was low for plain iron, which contributes also to low
densities. Even more, this grade CC is silicated during powder production, which to some
extent, may affect the sintering behavior. Fe OM in comparison achieved much higher
densities. The carbon is removed via CH4 only in the end of the debinding holding, which
may favor the start of necks formation already at low temperatures (600ºC) due to the
higher carbon activity.
Mass 16
1400
8,00E-09
FeOM-H2
1200
NH3
FeCC-H2
CH4
1000
800
Temperature
4,00E-09
600
CH4
2,00E-09
400
Temperature (ºC)
Intensity (A)
6,00E-09
200
0,00E+00
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
Time (h)
Figure 26 – Mass Spectrum 16 (NH2 / CH4+/ O) of grade OM and CC under H2.
1000
FeCC-H2
1,50E-10
800
Temperature
600
1,00E-10
400
5,00E-11
Temperature (ºC)
1200
FeOM-H2
2,00E-10
Intensity (A)
1400
Mass 27
2,50E-10
200
0,00E+00
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
Time (h)
Figure 27 – Mass Spectrum 27 (C2H3+) of grade OM and CC under H2.
For mass 28 (fig. 28), three distinct outgassing zones were observed if processed
under argon. The first zone was related only to binder removal, as hydrocarbons.
36
The second range revealed a much more intense peak for grade OM than grade
CC in the 500-600ºC temperature range. The difference in the intensity was linked to an
overlapping of nitrogen in grade OM. This N2 was measured and identified by m14
which is the atomic mass unit for nitrogen. Therefore, in this interval the concentration of
nitrogen was much higher than CO and CO2 which induced these dissimilarities in
degassing between the powders.
The third range of outgassing was mostly probably linked to deoxidation of
internal oxides placed in the core of the particles and it was very clear, that they occurred
in different intervals for each material. These ranges were directly associated to the phase
transition in each steel. Danninger et al [27] studied these effects and concluded that the
degassing peak is shifted according to the α ↔ γ transition and also that shifting of
degassing zones is only caused by dissolved alloy metals in pre-alloyed powders, not by
admixed or diffusion bonded elements.
Mass 28
1400
7,00E-10
FeOM-Ar
6,00E-10
FeCC-Ar
C 2 H4
CO
CO2
1000
N2
5,00E-10
Temperature
4,00E-10
1200
CO
800
600
3,00E-10
400
2,00E-10
Temperature (ºC)
Intensity (A)
8,00E-10
200
1,00E-10
0
0,00E+00
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
Time (h)
Figure 28 – Mass Spectrum 28 (C2H4+ / CO) of grade OM and CC under Ar.
4.1.7 Conclusions

Under hydrogen the gases produced during debinding and sintering were carbon
dioxide, hydrocarbons, ammonia, water, methane and carbon monoxide.

Under argon the gases eliminated were carbon dioxide, hydrocarbons, ammonia,
nitrogen, water and carbon monoxide. Water was found only during debinding.
37

Fe CC did not reveal NH3 evolution, as it is a nitrogen-reduced powder. NH3 was
linked to the N2 evolution from Fe OM reacting with the H2 from process gas and
also from binder decomposition.

Grade CC revealed a “pick up” of binder during debinding which shifts the
carbon content in the sintered part according to the process gas. Under hydrogen,
this carbon-binder reacted soon after debinding with H2 to form CH4, which
decarburized the sintered part. Under argon this retained binder is not completely
removed, since the oxygen levels are low and insufficient to remove all binder via
CO/CO2 production.

Grade OM did not reveal much binder “pick up”, but even with this the carbon
levels were affected by the process gas. Under hydrogen, the carbon dissolved in
the powder was removed mainly via methane formation. In argon atmosphere,
this carbon was kept unchanged since argon is a neutral gas.

The CO peaks found in high temperature had distinct behaviors. Fe OM showed it
between 600-800ºC, while grade CC between 930-1200ºC. In Fe OM, deoxidation
by carbon was surprisingly detected slightly below α ↔ γ transformation (720ºC),
whereas for iron CC it was found just after transition (910ºC). The solubility of
carbon is much higher in austenite than in ferrite, and it might have been expected
the starting of reduction concurring with austenite formation. According to the
Austrian researchers [26, 27] the very low solubility of carbon in ferrite could be
sufficient for reduction of iron oxides.
38
4.2 INFLUENCE OF BINDER ON DEBINDING AND SINTERING
4.2.1 Polyamide System
Over 100 different types of polyamides (PA) are available for engineering
applications, but the most usual formulations are PA6 and PA6,6. The molecule of
polyamide is characterized by containing a repeating amide group (-CONH-) and the
different grades are identified by the number of carbon atoms in each chemical unit [28].
Table 14 gives the energies of the chemical bonds exhibited in PA molecules. It can be
noted from that, the decomposition of PA molecules tends to cleave at the weakest bond,
when submitted to heat (in this case C-N bond). Furthermore, the presence of an
electronegative group (C=O) tends to keep away the nitrogen by inductive effect, as it is
demonstrated in figure 29. [4]
Table 14 – Bond energies in PA chains.
Bond
Figure 29 – PA molecule.
C-C
C-H
N-H
C-N
C=O
Bond Energy
(kcal/mol)
80
98.2
84
62
178
It has been reported that the predominant volatile pyrolysis of PA6 is εcaprolactam. Cyclic oligomers and products with nitrile end-groups have also been
reported. On the other hand, the major substances of PA6,6 pyrolysis are cyclopentanone,
cyclic oligomers and products with nitrile and isocyanate end-groups. The evolution of
water and carbon dioxide after 200ºC are also features PA decomposition [28, 29].
Based on this previous introduction about PA degradation, figure 30 illustrates the
mass spectrum acquired during the initial stage of debinding under hydrogen. This
spectrum is compared to the typical mass spectra of ε-caprolactam, cyclopentanone and
2-propeamide given in Appendix A.
39
The first peak of MS profile was associated to the process gas (hydrogen).
Further, m16 + m17 + m18 described the presence of water and ammonia. These
overlapping substances were produced by desorption of bonded water and evolved effects
coming out of the powder. Additionally, ammonia could be eliminated from binder
during breakdown of unstable structures containing nitrogen, as the bond C-N has the
lowest energy.
The spectra of m28 + m44 in high intensity confirmed the feature of CO2
production during pyrolysis of PA groups. Based on the omnipresence of mass 30 and the
high intensities of m39 + m41, it was assumed that this polyamide was not PA6
(comparison of MS acquired and Caprolactam graphic). In contrast, the spectra of
cyclopentanone and 2-propenamide fit more or less to that one acquired in the
experimental investigation. As these presented substances are not the only produced
during polyamide pyrolysis, the overlapping between the experimental results and library
is not intrinsically coincident. Furthermore, the effects of powder on the binder
degradation are not considered in this comparison.
Figure 30 – Analog scan during Polyamide extraction under H2.
40
4.2.2 Polyethylene System
It has been reported [30] that polyethylene decomposes into a large number of
paraffinic and olefinic compounds without a residue. The predominant mechanism of
degradation is a radical chain reaction which starts by random scission of the polymer
chain into primary radicals. The specimens formed are based in diene, alkene and alkane.
Aromatics and cyclic compound are also generally identified. In Appendix A is given the
representative spectra of the pyrolysis products of PE at 480ºC.
Figure 31 illustrates the mass spectra acquired during initial stage (310ºC) of
polyethylene debinding. From this, it is visible the peaks divided in groups according to
the number of carbon in each molecule (C1 – C7). Group C1 has a complex interpretation
since parallel reactions were taking place with the same mass units. The group containing
two carbons has the broadest peaks in m27 + m28 + m29, which were associated to
Ethane and Ethene molecules. Subsequently, groups C3 + C4 represented the compounds
Propane + Propene + Butane + Butene. This way, all further groups were based on these
three classes of hydrocarbons, without discarding possible aromatics and cyclic groups,
e.g. cyclobutane and cyclopentane.
Figure 31 – Analog scan during Polyethylene extraction under H2.
41
4.2.3 Debinding of Polyamide and Polyethylene System
Figure 32 - 33 exhibit the mass spectrum 27 (CxHy) representing polyamide and
polyethylene removal under hydrogen and argon, respectively.
Debinding of polyamide happened around 250-370ºC under hydrogen, whereas in
argon at 330-450ºC. Debinding of polyethylene under hydrogen occurred in the 320480ºC temperature range, while when conducted in argon the range was 380-500ºC. The
intervals of occurrence were directly linked to the stream gas in the furnace and this
behavior was described in the topic Influence of Process Gas on Debinding and Sintering.
It is interesting to record that both binder extractions finished before isothermal
holding at 600ºC. It is well known that dwell times are not attractive when speaking
about processing time and by these results it would be possible to discuss about their
necessity. However, when introducing the influences of process gas on quality of the
sintered part, the importance of setting correctly this holding time will be demonstrated.
Mass 27
2,50E-10
Intensity (A)
1400
PE - H2
PA - H2
Temperature
2,00E-10
1200
1000
800
1,50E-10
600
1,00E-10
400
5,00E-11
Temperature (ºC)
3,00E-10
200
0,00E+00
0
0
0,5
1
1,5
2
2,5
3
3,5
Time (h)
4
4,5
5
5,5
6
6,5
7
Figure 32 – Mass spectrum 27 (C2H3+) during extraction of PA and PE under H2.
Mass 27
2,50E-10
1200
1000
1,50E-10
800
1,00E-10
600
400
5,00E-11
Temperature (ºC)
2,00E-10
Intensity (A)
1400
PE - Ar
PA - Ar
Temperature
200
0,00E+00
0
0
0,5
1
1,5
2
2,5
3
3,5
Time (h)
4
4,5
5
5,5
6
6,5
7
Figure 33 – Mass spectrum 27 (C2H3+) during extraction of PA and PE under Ar.
42
Table 15 exhibits the average results of carbon content in the sintered parts.
The carbon level was extremely low for both feedstocks processed under
hydrogen. It might be considered that debinding was well conducted, however the desired
carbon of the powder was not satisfactorily upheld.
Table 15 – Carbon analyses in the sintered parts.
Material
Binder = Polyamide
Powder = Fe HQ
Binder = Polyethylene
Powder = Fe OM
Data Sheet
Hydrogen
Argon
0.8%C
0.006%C
0.985%C
≤ 0.9%C
0.005%C
0.840%C
A carburizing effect on samples manufactured with grade HQ and PA was
identified, if processed under argon. Comparing the intensities of mass 27 – figures 32
and 33 – it was noticeable that debinding of polyamide was not fully achieved under
argon, since the intensities were not in similar range of those acquired in H2-cycles.
It looks like that grade HQ has analogous performance of grade CC, described in
the topic Influence of Powder on Debinding and Sintering. Here, the very low particle
size (≤ 1.0μm) could provide enough energy to gather carbon-binder admixed over the
powder particles. As sintering is enhanced in finer powders, some undesired diffusion of
this carbon binder in the steel might have been expected already at low temperatures,
which could shift up the carbon levels. Furthermore, the oxygen ratio in this powder is
very low, which could be not sufficient to provide control of carbon via carbothermal
reactions. Possibly, increasing the dew point, e.g. addition of water, could induce
decarburization according to equation 11 given in the literature overview.
Polyethylene was suitably debound under argon and as consequence the carbon
content was upheld.
43
4.2.4 Conclusions

The degradation products of polyethylene and polyamide were not easily
distinguished. The presence of oxygen and nitrogen in PA structure did not
change much the profiles compared to those obtained in PE trials. The evolution
of N2 and H2O (both reactions provided by powder) might have overspread the
results from binder. Possibly, the same investigation using Fe CC could make out
these features, as it is a nitrogen-reduced powder.

CO2 production, mass 30 in low intensity and high peaks for m39 + m41 were
found in polyamide cycles and are characteristics that might be arising from
PA6,6 decomposition. Furthermore, some similarities with mass spectra from
typical PA6,6 pyrolysis products were also identified.

The energy of chemical bonds in PA structure explained its early removal. C-N
bonds exhibits 62 kcal/mol while C-C has 80kcal/mol. In other words, it is easier
to break the chains containing nitrogen than those between carbons only. This
way, degradation is achieved earlier in PA then PE.

Debinding of polyamide under argon was not completely accomplished, since
some carbon “pick up” was observed. This carburizing effect was similar to that
exhibited by Fe CC powder and it has a direct relation with initial powder
features. Atmosphere of pure H2 demonstrated to be inadequate for manufacturing
of both powders, if maintaining carbon is desired.
44
4.3 INFLUENCE OF PROCESS GAS ON DEBINDING AND SINTERING
4.3.1 Thermal Analyses
The influence of process gas on debinding and sintering was evaluated with Fe2Ni
samples. The most important effects are illustrated in figures 34 – 35.
mass 27
8,00E-11
4,00E-11
1000
800
600
400
2,00E-11
Temperature (ºC)
1200
100% H2
75% H2
50% H2
25% H2
100% Ar
Temperature
6,00E-11
Intensity (A)
1400
200
0,00E+00
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
Time (h)
+
Figure 34 – Mass spectrum 27 (C2H3 ) during processing of Fe2Ni under different gas
compositions.
mass 16
1,20E-09
8,00E-10
6,00E-10
1200
1000
800
600
4,00E-10
400
2,00E-10
Temperature (ºC)
1,00E-09
Intensity (A)
1400
100% H2
75% H2
50% H2
25% H2
100% Ar
Temperature
200
0,00E+00
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
Time (h)
+
Figure 35 – Mass spectrum 16 (NH2 / CH4 / O) during processing of Fe2Ni under
different gas compositions.
It was noticeable that reducing the fraction of hydrogen delayed the start of
hydrocarbons removal (m27 – fig.34). Furthermore, the peak intensities were roughly
changed if mixing hydrogen and argon. Under pure gases the intensities were similar,
which may suggest that debinding was probably accomplished completely.
The shift in the debinding interval can be correlated to the high heat capacity and
conductivity of H2. As long as hydrogen was introduced in higher amounts, an
accelerated heating rate was identified. In other words, hydrogen speeds up the reactions.
This behavior was described also by Philips [1].
45
Figure 35 illustrates the profile of mass spectra 16. The first peaks for all
conditions (300-500ºC) were associated to the polymer extraction and also ammonia
production (reaction with the nitrogen contained in Fe OM).
The subsequent peaks of m16, starting around 2.5 hours (500ºC), represented an
overlapping of methane and carbon monoxide, with the first one in much higher
concentration. The intensity of this peak under pure hydrogen was higher than under
mixed stream gases. Additionally, the range of occurrence was shorter compared to
mixed gases. This behavior was linked to the higher decarburizing potential obtained in a
pure hydrogen atmosphere. Basically, as the H2 content was increased, higher was the
methane production. This reaction between carbon and hydrogen was previously
explained (equation 9 and 10 in the Literature Overview).
Another feature of the spectrum measured under 100%H2 was a dual methane
peak. The first one represented methane production via carbon-binder. The second was
associated to CH4 reaction using the carbon from grade OM. Considering the methane
production during processing of individual powders, grade OM and CC, the methane
peak occurred in different ways (see chapter 4.1). Grade OM revealed this peak only in
the end of the isothermal holding for debinding, whereas grade CC exhibited already
soon after binder removal. By now, the reasons for the earlier removal of carbon from
grade OM, if mixed with grade CC, could not be determined. Possibly, some reactions
with grade CC could speed up the methane production.
The cycles carried out in mixed atmospheres provided a wide interval of methane
formation, which was explained by the low decarburizing potential of these atmospheres.
Carbon of binder and powder remains longer in the parts with less hydrogen flowing in
the furnace and thereby, methane production happens evenly while carbon is available
and methane reaction is thermodynamically stable. A higher amount of carbon in these
sintered components might be expected due to an incomplete decarburizing effect.
Carbon analyses are discussed in the end of this chapter.
The experiment conducted under argon did not exhibit CH4, exactly like observed
in the topic about individual powders processed under argon (chapter 4.1.3 and 4.1.4).
Mass spectra 18, 28 and 44 revealed an overlapping of individual behaviors of the
two powders (grade CC + grade OM) and thereby, they are not presented here again
46
The influence of Nickel was not evaluated in this investigation due to its low
content. However, it is known that increasing Ni fraction, the onset of phase transition is
shifted to low temperatures. With α ↔ γ temperature reduced, carbon starts dissolution
earlier which makes to expect that carbothermal reaction is dropped to a lower interval,
as well.
4.3.2 Carbon Analyses
Carbon content – given in table 16 – was the parameter used to qualify the diverse
process gas compositions investigated. Under pure hydrogen a large decarburizing
behavior was obtained. The neutral results were reached under pure argon and with
75%H2. Performing with 50% and 25% of hydrogen the cycles were carburizing.
Table 16 – Carbon content in the sintered part processed under different gas process.
Condition
100% H2
%C
0.006
Behavior
Decarburizing
75%H2
25%Ar
0.293
%C in
starting
powder
≤ 0.35%
50%H2
50%Ar
0.552
25%H2
75%Ar
0.647
Carburizing
Carburizing
100% Ar
0.321
%C in
starting
powder
≤ 0.35%
Methane in large intensity (fig. 34) was found to be the main decarburizing agent
in Fe2Ni processing. Decarburizing by methane under pure H2 atmosphere was already
discussed in the topic Influence of Powder on Debinding and Sintering. The high
intensity and early end of methane peak, under pure hydrogen, make obvious that carbon
would be in very low levels after sintering.
If conducted the process under 25% and 50% of H2, the acquired results were
higher than the expected. The carburizing behavior was linked to the equilibrium of
reductive atmosphere and debinding hold. It was suggested that the reductive potentials
of these atmospheres were not sufficient to give the estimated carbon content and it might
be considered that increasing the holding time, the results could be reached. By these, the
importance of holding time in order to adjust the final chemistry of the sintered parts was
demonstrated. However, increasing holding time is not a goal for any production plant of
PIM parts, which makes control of carbon by selecting a correct gas very decisive.
47
Suitable results were acquired if samples were debound and sintered under
100%Ar or 75%H2. Carbon was monitored in the cycle containing H2 by adjusting the
range and intensity of CH4 production, whereas under 100%Ar the production of CO and
CO2 was enough to hold the unchanged initial carbon level. Nevertheless, it is important
to keep in mind that for Fe OM, under Ar, the proper carbon content was retained, while
for Fe CC, the accurate carbon control was not achieved. This way, it might be expected
that for different powder mixtures, new gas conditions will be required.
4.3.3 Conclusions

Debinding was delayed according to the reduction of the hydrogen ratio contained
in the stream gas. This behavior was linked to the high heat capacity and
conductivity of H2 [1]. It might be assumed that hydrogen accelerates heating and
this way, providing a premature occurrence of any reaction.

Carbon control can be accomplished by monitoring of methane levels. Higher H2
fraction flowing into the furnace led to increasing of methane production and
consequently, less carbon in the sintered component.

The extremely low carbon level measured in the samples sintered under pure
hydrogen was related to the high reductive potential of pure H2 atmospheres. This
was clear via MS due to the early end of the CH4 peak compared to the conditions
containing mixed gases. A proportion containing 75%H2 + 25%Ar was adequate
to control the carbon content for this Fe2Ni alloy, processed according to this
standard debinding and sintering cycle. If customized new settings (heating rate,
holdings, etc.) for this production, the carbon levels will be also modified.

It has been shown that carbon can be controlled either by atmosphere and/or
adequate dwell time for debinding (under atmosphere containing hydrogen).

Instead of methane, carbon monoxide and carbon dioxide were the main products
obtained in the cycles conducted under flowing argon. The oxygen ratio in the
powder was sufficient to allow complete removal of any remaining binder. Argon
demonstrated to be carburizing or neutral according to the powder features. For
this alloy, carbon was well controlled in Ar at the desired composition (≤ 0.35%).
48
5. SUMMARY AND OUTLOOK
Mass spectrometry allows looking into reactions occurring between powder,
binder and process gas. During processing of the two carbonyl iron powders examined
here, several contrasts were revealed and most of them linked to the initial powder
features. This behavior may spread onto several new investigations in future for different
alloys like chromium steels, hard metals, etc. Ongoing experiments are already performed
with 316L stainless steel processed via master alloyed and pre-alloyed powders.
Unfortunately, these experiments presented here did not allow determining
pronounced differences of polyamide and polyethylene degradation products. It is
expected to identify those, if new experiments using a reduced nitrogen/oxygen powder,
which has a small amount of outgassing coming from powder reactions, are carried out.
It has been shown that the process gas affects directly the carbon content in the
sintered part. Decarburization was enhanced with methane reaction, during sintering
under hydrogen flowing atmospheres. Monitoring of CH4 levels will allow a systematic
control of carbon content.
Upcoming work will be related to accurately customize all process variables for
sintering in batch and continuous furnaces. Understanding progressively PIM furnaces
will foresee fast troubleshooting and proper parameter selection for new developments.
Furthermore, reduction of processing time will be on hand for PIM producers, if known
what in fact occurs inside of a furnace. Some initial tests already allowed successful
batch processing of Fe2Ni in less than 6 hours of heating.
49
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53
APPENDIX A – TYPICAL MASS SPECTRA
54
APPENDIX A – TYPICAL MASS SPECTRA
55
APPENDIX A – TYPICAL MASS SPECTRA
56
APPENDIX A – TYPICAL MASS SPECTRA
57
APPENDIX A – TYPICAL MASS SPECTRA
58
APPENDIX A – TYPICAL MASS SPECTRA
59
APPENDIX A – TYPICAL MASS SPECTRA
Typical mass spectrum of polyethylene pyrolysis products [30].
60