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Powder Metallurgy Progress, Vol.14 (2014), No 3
129
ANALYSIS OF PM-STEELS CONTAINING BORON WITH LASER
ABLATION - INDUCTIVELY COUPLED PLASMA MASS
SPECTROMETRY
C. Gierl-Mayer, V. Bauer, A. Limbeck, H. Danninger
Abstract
The activated sintering of steels by addition of boron is an attractive
method to attain high densities. But still there is the danger of an
enrichment of brittle phases on grain boundaries, as the solubility of
boron is extremely low, which, on the other hand, is a key factor for the
sintering with activating liquid phase. Chromium as an alloying element
changes the solubility a little, and therefore the sensitivity against the
formation of these brittle phases. Also the sintering atmosphere plays a
major role, as boron tends to react with hydrogen and nitrogen. The
quantitative analysis of boron is known to be very tricky, as the usual
methods as XRF and SEM-EDS, are not very sensitive because of the low
atomic number of boron. In this investigation the new method of Laser
Ablation -Inductively Coupled Plasma Mass Spectrometry, LA-ICP-MS,
is demonstrated on boron containing steels. This new method allows the
quantitative analytics of boron in a lateral manner. The paper describes
the adaption of the method and the correlation of boron contents with
microstructural constituents, especially the influence of the alloying
element chromium. A correlation between boron concentration and
mechanical properties is drawn.
Keywords: advanced analytics, microstructure, mechanical properties,
activated sintering
INDRODUCTION
The activating effect of boron on the sintering of iron was already published in
1955 [1], and described by many authors over the years [2-10]. But still there is no real
industrial application for different reasons. One of the major problems is the reactivity of
the added boron with usual sintering atmospheres. With nitrogen, the main constituent of
the most used atmosphere, it reacts and forms h-BN which deactivated the boron source
[11, 12]. With hydrogen, the formation of B2H3 is reported, and boron loss to the
atmosphere is the reason for gradual deactivation of the sample, from outside to inside. So
the only applicable atmosphere is vacuum, which is for PM structural parts usually not cost
effective.
The sensitivity for formation of brittle phases with boron on grain boundaries is
system immanent, because one of the most important precautions for activated sintering is
the low solubility of the liquid phase in the matrix to be effective during the whole sintering
process. If this phase is a brittle, as it is with boron addition, mechanical properties,
especially elongation of the material, are strongly dependent on the presence, or better its
absence, of a continuous network. Some alloying elements like chromium influence the
B
Christian Gierl-Mayer, Viktor Bauer, Andreas Limbeck, Herbert Danninger, Institute of Chemical Technologies
and Analytics, Vienna University of Technology, Getreidemarkt 9/164-CT, A-1060 Wien/Vienna, Austria
Powder Metallurgy Progress, Vol.14 (2014), No 3
130
solubility of boron, but still there exists a limit of boron addition to avoid embrittlement of
the material. It is interesting to know how much boron can be dissolved within the matrix,
but boron, as a light element, is very tricky to analyse quantitatively. All physical methods
based on X-rays are not very sensitive, due to the low atomic number of boron.
Fig.1. LA-ICP-MS schematic [13].
The new method of laser-ablation Inductively Coupled Plasma Mass Spectrometry
(LA-ICP-MS) opens the possibility to analyse boron quantitatively in a lateral manner. The
method is based on the evaporation of the sample with a focused laser beam, transfer of the
evaporated material into Ar-plasma and subsequent element specific analysis of He focused
ions with a mass spectrometer (see Fig.1). LA-ICP-MS is a highly accepted, widely used
method for the determination of major, minor and trace elements in solids, as well as
isotope-ratio measurements [13]. The laser produces evaporation of the material spotwise
and therefore lateral analysis becomes possible. Major limitations associated with LA-ICPMS are the non-sample related variation of the analytic response during the ablation
process, defined as elemental fractionation [14].
In the following part the first experiments with LA-ICP-MS on PM-steels are
reported.
EXPERIMENTAL
To develop and describe the power of the new analytic method LA-ICP, Fe3w%Cr-0.5w%Mo-0.5w%C (prealloyed steel powder Astaloy CrM, produced by Höganäs
AB, Sweden) was chosen as a model system. Boron was added as crystalline boron with the
following contents: 0 (reference), 0.075 w%, 0.15 w%, 0.3 w% and 0.6 w%.
Sample preparation
Samples with the size of 55x10x8 mm³ (Charpy bars) were pressed at 600 MPa,
after mixing the powder in a tubular mixer, in a floating die with die wall lubrication.
Sintering was performed in a Dilatometer NETZSCH DIL 402C in argon and in hydrogen
(both 99.999 quality) at 1250°C, heating and cooling ramp of 10 K/min, 60 min,
isothermal. After sintering, Archimedes density was measured and samples were impact
tested. The ends of the samples were cut off and mounted in Bakelite for metallographic
preparation and further analysis by LA-ICP-MS.
Powder Metallurgy Progress, Vol.14 (2014), No 3
131
Analysis parameters
For analysis, a quadrupole ICP-MS (iCAP Q, ThermoFisher Scientific) instrument
was used. The ablation system (New Wave 213, ESI), containing a frequency quintupled
213 nm Nd:YAG laser, was connected to the iCAP Q via tubing. NIST 612 was utilized as
a tuning material.
To use LA-ICP-MS for quantitative analysis calibration had to be developed, as no
reference material was available. For calibration purposes powder mixtures of Fe21B with
AstaloyCrM were prepared with different boron contents. These mixes were pressed in a
laboratory press to samples with diameter 13 mm and thickness 1.25 mm. At first it was
tried to measure these samples directly, but results, especially standard deviation, were too
high. The second attempt was to sinter the samples in vacuum at 1100°C before analysis,
which improved standard deviation considerably. Exact composition of prepared standards
was determined by ICP-MS, after dissolving (1ml concentrated HCl/HNO3 3:1) and
diluting them. The calibration curve used is shown in Fig.2.
y = 2E‐05x + 0,0041
11B/58Fe
2
R = 0,9957
1,20E‐01
11B/58Fe signal ratio []
1,00E‐01
8,00E‐02
6,00E‐02
4,00E‐02
2,00E‐02
0,00E+00
‐500
0
500
1000
1500
2000
2500
3000
3500
4000
4500
calculated sample concentration [ppm]
Fig.2. Correlation between LA-ICP-MS normalised signals of B-concentration of standards
using Fe; averaged values of 5 time scans per sample.
Spot size for the LA analysis was 200 µm, Laser frequency 10 Hz, laser power
5.71 J/cm2, scan speed 300 µm/s and carrier gas flow of 0.5 l/min He. Line scan duration
was set to 4 min per time scan which is about 1.5 mm2 scan area to ensure homogeneity.
For MS-Analysis of boron, B11 was used, calculated in relation to Fe58.
RESULTS AND DISCUSSION
The activating effect of the boron addition can be seen on the dilatometric graph in
both atmospheres (see Fig.1). The blue graph (0.6 w% B) for hydrogen atmosphere and the
violet graph (0.3 w% B) run into overflow, as the maximum shrinkage of the system (2500
µm) is reached) and no further densification can be detected. The graphs show that even a
very small addition of boron already gives a noticeable effect on sintering. Certainly 0.6
w% boron is too much, as both samples (argon and hydrogen) almost lost their shape due to
too much liquid phase formation.
Powder Metallurgy Progress, Vol.14 (2014), No 3
a) Argon
132
b) Hydrogen
Fig.3. Dilatometric graphs of Fe-3Cr-0.5Mo-0.5C with different amounts of boron; 1250°C
60 min, 10 K/min.
The physical and mechanical properties are reported in Table 1. Impact energy
should only be taken as an indicator of embrittlement, as only 1 sample of each
composition was measured. The microhardness was measured within the core of the
metallic matrix only, not in the solidified eutectic, where microhardnesses > 800 HVm0.1
were measured. It is interesting that even the lowest content of boron resulted in significant
densification and higher hardness. Microhardness especially did not reach higher values at
higher boron contents. For the samples sintered in argon, densification obviously stopped
because of the formation of large gas-filled porosity, which cannot be found in the samples
sintered in hydrogen.
a) Argon
b) Hydrogen
Fig.4. Unetched microstructure of Fe-3Cr-0.5Mo-0.5C-0.3B sintered in different
atmospheres.
Powder Metallurgy Progress, Vol.14 (2014), No 3
133
a) 0% B
b) 0.075% B
c) 0.3% B
Fig.5. Etched Microstructure (Nital) and fracture surfaces (SEM) of Fe-3Cr-0.5Mo-0.5C
with different amounts of boron, sintered in hydrogen.
The reason for the embrittlement can be seen on the micrographs of Fig.5, which
show a dramatic change in the fracture mechanism. The boron-free material shows only
ductile fracture and an addition of only 0.075 w% boron leads to brittle fracture. The higher
the amount of boron, the fewer ductile dimples can be found on the broken surfaces, and
the more the fracture mechanism changes from transgranular to intergranular fracture.
Powder Metallurgy Progress, Vol.14 (2014), No 3
134
Tab.1. Physical and mechanical properties Fe-3Cr-0.5Mo-0.5C with different contents of
boron.
B-content
[w%]
0
0
0.075
0.075
0.15
0.15
0.30
0.30
0.60
0.60
Green
density
[g/cm³]
6.83
6,76
6.72
6.72
6.66
Atmosphere
Ar
H2
Ar
H2
Ar
H2
Ar
H2
Ar
H2
Sintered
density
[g/cm³]
7.07
7.11
7.26
7.27
7.38
7.54
7.35
7.77
7.40
7.74
Impact
energy
[J/cm²]
23
19
14
8.5
6.5
4.0
2.3
2.5
1.5
1.5
Hardness
HV30
111
108
174
152
203
248
175
299
198
315
Microhardness
HVm0.1
340
333
383
416
411
408
412
404
383
374
Analytics
Three different methods to discover the boron content were tested:
• Analysis of surface and core content: Results show that samples sintered in hydrogen
have lower boron contents on the surface than in the core (2 mm sub-surface), compared
to the samples sintered in argon (see Fig.6a).
• Determination of depth profile via line scan with spot diameter of 40 µm on the sample
cross section with 0.3 w% B starting from the surface. The results also show lower
concentration of boron in samples sintered in hydrogen compared to those sintered in
argon (see Fig.6b).
a) Surface and Core Analysis
b) Depth profile Analysis
Fig.6. 11B concentration in surface and core (a) and as function of sample depth, starting
from the surface (b) of 0.3 w% B samples sintered in hydrogen and argon (200 µm spotsize,
300 µm laser feed, 6 J/cm² (a) 6.49 J/cm2 (b) laser power, 10 Hz, 0.5 l/min He).
•
Lateral B-distribution (Imaging): The results of imaging are shown in Fig.4. The bottom
right of the picture represents the edge of the sample, the length of the pictures is 1 mm,
with a lateral resolution of 20 µm. It is interesting to see that, although there are also
some residues of former liquid phase on the edge of the sample sintered in hydrogen, the
boron is much more evenly distributed in the sample sintered in argon.
Powder Metallurgy Progress, Vol.14 (2014), No 3
135
a) Argon
b) Hydrogen
Fig.7. Lateral distribution of Boron on Fe-3Cr-0.5Mo-0.5C-0.3B sintered in Hydrogen and
Argon. (20 µm spotsize, 20 µm laser feed, 10.28 J/cm2 laser power, 10 Hz, 0.5 l/min He)
CONCLUSIONS
It was demonstrated that Laser-Ablation ICP-Analytics is an interesting method to
determine boron quantitatively in a lateral manner. There is definitively still a lot of work to
do to optimise the method. The biggest obstacle is missing of standards to calibrate the
system. Therefore fabrication of standards with known boron contents is essential for the
success of the method. All results show that there is a distinct loss of boron on the surface
in both atmospheres, but much more in hydrogen.
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