Microstructure formation and properties of abrasion resistant cast steel

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of
ISSN (1897-3310)
Volume 10
Issue Special1/2010
295-300
FOUNDRY ENGINEERING
57/1
Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences
Microstructure formation and properties of
abrasion resistant cast steel
J. Krawczyk*, S. Parzych
Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology,
Mickiewicza Al. 30, 30-059 Kraków
*Correspondence address: e-mail: [email protected], [email protected]
Received: 26.02.2010; accepted in revised form: 30.03.2010
Abstract
The so-called adamitic cast steels are characterised by a high abrasion resistance. These cast steels are of a pearlitic matrix with
uniformly distributed hypereutectoid cementite precipitates. Apart from hypereutectoid cementite very often transformed ledeburite also
occurs in the microstructure of these cast steels. Such cast steels contain chromium (app. 1 %) and nickel (app. 0.5 %) as alloy additions
and sometimes their silicon content is increased. The presence of molybdenum is also permissible (app. 0.4 %). The basic problem in
application of these steels for structural elements constitutes their insufficient crack resistance. An improvement of mechanical properties
by changes of morphology of hypereutectoid cementite and transformed ledeburite precipitates by means of the heat treatment application
was the aim of this study. G200CrNiMo4-3-3 cast steel was the investigated material. Changes in the morphology of hypereutectoid
cementite and transformed ledeburite obtained due to the heat treatment are described in detail in the present paper. An influence of the
microstructure changes on impact toughness of the investigated cast steel is presented. Investigations performed within this study will
serve for the microstructure optimisation on account of functional qualities of this cast steel.
Key words: wear resistant alloys, cast steel, hypereutectoid cementite, transformed ledeburite, mechanical properties.
1. Introduction
Casting alloys on iron matrix are applied for structural
elements in railway systems [1-3]. Important parameters of these
alloys are their abrasion and crack resistance as well as welding
ability. These properties depend on the chemical composition and
microstructure. An influence of the microstructure of iron matrix
casting alloys on their functional qualities is still widely discussed
in references [4-27]. One of the methods of forming optimal
casting properties of such alloys is their heat treatment [23-27].
The determination of the heat treatment influence on the
morphology of hypereutectoid cementite and transformed
ledeburite in G200CrNiMo4-3-3 cast steel and on its mechanical
properties was the aim of the present paper.
2. Investigated material and its heat
treatment
G200CrNiMo4-3-3 cast steel, of the chemical composition
given in Table 1, was the tested material. The microstructure of
this cast steel in as-cast condition is shown in Figure 1. This
microstructure is characterised by a network of hypereutectoid
cementite and transformed ledeburite occurring along grain
boundaries of a primary austenite, while in areas near boundaries
of prior austenite grains precipitates of hypereutectoid cementite
in the Widmannstätten structure occur. Spheroidal cementite
precipitated inside the primary austenite grain can be also seen.
Table 1. Chemical composition (weight %.) of G200CrNiMo4-33 cast steel
C
Mn
Si
P
S
Cr
Ni
Mo
2.03
0.64
0.56
0.024
0.008
1.30
0.47
0.33
3. Heat treatment
Microstructures of the investigated cast steel after the heat
treatment are presented in Figure 2.
The heat treatment of cast steel consisted of heating it up to a
temperature of 1000 oC, holding at this temperature and a slow
cooling to a temperature being below the diffusive transformation
range. Then the heating was repeated to dissolve only a part of
secondary carbides in austenite and to normalise prior austenite
grains. The final cooling was done in two stages. The first stage
consisted of fast enough cooling to temperatures below diffusive
transformations in order to obtain fine-lamellar pearlite of a better
hardness and a higher crack resistance. A short isothermal holding
ARCHIVES OF FOUNDRY ENGINEERING Volume 10, Special Issue 1/2010, 295-300
295
in a temperature being below diffusive transformations, combined
with a slow cooling to 450 oC, should cause the temperature
equalization in the element undergoing the heat treatment. Then
cast steel was cooled to an ambient temperature.
a)
a)
b)
b)
c)
c)
Fig. 1. Microstructure of G200CrNiMo4-3-3 cast steel in as-cast
condition: a) network of transformed ledeburite and
hypereutectoid cementite, b) morphology of hypereutectoid
cementite precipitated in the Widmannstätten structure,
c) morphology of hypereutectoid cementite precipitated inside the
primary austenite grain and cementite precipitated in the
Widmannstätten structure. Etched with 2% nital
296
Fig. 2. Microstructure of G200CrNiMo4-3-3 cast steel after the
heat treatment: a) network of transformed ledeburite and
hypereutectoid cementite, b) morphology of hypereutectoid
cementite precipitated in the Widmannstätten structure,
c) morphology of hypereutectoid cementite precipitated inside the
primary austenite grain and cementite precipitated in the
Widmannstätten structure. Etched with 2% nital
4. The obtained results and their
discussion
The applied heat treatment significantly influences the
microstructure change. It was estimated, by means of the
ARCHIVES OF FOUNDRY ENGINEERING Volume 10, Special Issue 1/2010, 295-300
maximum indicates that a part of the ledeburitic cementite
precipitates was spheroidized after the heat treatment.
45
40
as-cast condition
35
absolute frequency
stereological lineal analysis, that – due to the heat treatment - the
fraction of transformed ledeburite (together with the built on
hypereutectoid cementite) was decreased from 10±1 % to 8±1 %.
The size distribution of the transformed ledeburite precipitates
described by the length distribution of the chords li cut across it is presented in Figure 3. Due to the applied heat treatment the
dimensions of the transformed ledeburite precipitates are smaller.
There can be two reasons of such changes. The first one: the
eutectic (ledeburite) formed during crystallization is unbalanced
and it was only when the heat treatment was performed that its
fraction decreased to the one close to the equilibrium system. The
second one: in as-cast condition the transformed ledeburite
precipitates are increased by large amounts of built on
hypereutectoid cementite (during cooling – especially within the
austenitic range).
after heat treatment
30
25
20
15
10
5
0,
02
0,
08
0,
14
0,
19
0,
25
0,
31
0,
36
0,
42
0,
47
0,
53
0,
59
0,
64
0,
70
0,
75
0,
81
0,
87
0,
92
0,
98
m
or
e
0
35
shape factor
as-cast condition
30
absolute frequency
after heat treatment
Fig. 4. Shape factor distribution of the transformed ledeburite
precipitates
25
20
15
10
5
0
2
12
22
32
42
52
62
72
82
92
chord length li, µm
102
112
123
133 more
Fig. 3. The length distribution of chords li of the transformed
ledeburite precipitates cut across by random secants
Due to the heat treatment the total grain surface Sv increased
from 7.6±0.1 mm-1 to 17.0±0.8 mm-1. This is the result of
diminishing the average flat grain diameter ī from 260 µm (in ascast condition) to 120 µm (after the heat treatment). However,
smaller amount of transformed ledeburite (with built on
hypereutectoid cementite) causes also decreasing of the relative
grain boundaries hold by it - as compared to as-cast condition (Svled). Svled was decreased from 4.2±2 mm-1 to 3.0±2 mm-1, due
to the heat treatment. This caused a smaller ‘filling’ of grain
boundaries by transformed ledeburite (with built on
hypereutectoid cementite) from approximately 55 % to 17 %. In
order to investigate a spheroidizing process of transformed
ledeburite (with built on hypereutectoid cementite) the average
thickness of its precipitate network đ, was determined. It was
found that due to the heat treatment this thickness slightly
increased (regardless of a decreased fraction of transformed
ledeburite) from approximately 24 µm (in as-cast condition) to
approximately 27 µm (after the heat treatment). A spheroidization
of transformed ledeburite (with built on hypereutectoid cementite)
is also confirmed by calculations of its shape factor F, determined
as the ratio of a circumference of a circle - of the same surface
area as a particle section - to the circumference of this particle
section. The average shape factor of transformed ledeburite (with
built on hypereutectoid cementite) was increased from 0.23 (in ascast condition) to 0.25 (after the heat treatment). The shape factor
distribution of the transformed ledeburite precipitates depending
on the cast steel condition is presented in Figure 4. The second
Decreased, due to the heat treatment, amount of transformed
ledeburite evidently caused an increase of the hypereutectoid
cementite fraction. Applying the point-count method (10 times
pressing of the network of 361 nodes, at a magnification of 568) it
was estimated that the cementite fraction, determined – on the
bases of its morphology – as being hypereutectoid, increased from
approximately 21 % (in as-cast condition) to app. 35 % (after the
heat treatment). Thus, this increase is significantly more
pronounced than the previously described loss of transformed
ledeburite. This difference can be the result of several reasons: a
change of the transformed ledeburite morphology (building of its
eutectic structure, it means: pearlitic areas limited by ledeburitic
cementite), a decrease of a carbon concentration in the vicinity of
ledeburitic and hypereutectoid cementite (formation of thicker
ferritic coatings) as well as a classification of coagulated and
spheroidized particles of pearlitic cementite as hypereutectoid
cementite. Another reason can be quenching of the pearlitic
matrix by carbon versus its equilibrium state in as-cast condition.
Thus, the fraction of cementite precipitates, not classified as
ledeburitic cementite or lamellar pearlitic cementite, increased in
the cast steel microstructure after the heat treatment. However,
this type of precipitates can have a substantial influence on the
properties of the investigated cast steel. As the result of the heat
treatment not only the fraction of hypereutectoid cementite was
changed but also its morphology. Dimension changes of its
precipitates are described by the distribution of chord length li
(Fig. 5), while precipitate shapes are described by the shape factor
(Fig. 6). Analyses of such changes indicate e.g. fragmentation and
spheroidization of hypereutectoid cementite precipitated in the
Widmannstätten structure as a result of the applied heat treatment.
Thus, the average shape factor of the hypereutectoid cementite
precipitates was changed due to the heat treatment and increased
from 0.56 (in as-cast condition) to 0.65 (after the heat treatment).
Microstructure changes of the investigated cast steel, occurred
due to the heat treatment, influenced its properties. The cast steel
hardness increased from 297±5 HBW (in as-cast condition) to
322±5 HBW (after the heat treatment).
Microstructure changes influenced significantly the cast steel
behaviour during the static tensile test. During this test, the
sample ruptures occurred already within the range of elastic
ARCHIVES OF FOUNDRY ENGINEERING Volume 10, Special Issue 1/2010, 295-300
297
strains both in as-cast condition and after the heat treatment. To
this end neither elongations nor reductions of area were observed.
Only the maximum stresses, at which tensile strength samples
ruptured, were determined. Since the determination of the tensile
strength requires over passing of the yield strength the above
mentioned stresses can not be called the tensile strength.
However, samples in as-cast condition were – during the static
tensile test - ruptured in a wide range of stresses 379±113 MPa,
while samples after the heat treatment were not ruptured at
stresses lower than 400 MPa, which means 427±15 MPa. Thereby
the obtained result can essentially help in undertaking the decision
concerning the application of this cast steel in the selected
structural elements.
a)
absolute frequency
90
80
as-cast condition
70
after heat treatment
b)
60
50
40
30
20
10
9,
91
10
,8
9
11
,8
7
12
,8
6
13
,8
4
14
,8
2
m
or
e
8,
93
7,
94
6,
96
5,
98
5,
00
4,
01
3,
03
2,
05
1,
07
0
chord length li, mm
Fig. 5. Distribution of chord lengths li of the hypereutectoid
cementite particles cut by random secants
Fig. 7. Fracture toughness with V notch: a) as-cast condition,
b) after the heat treatment. SEM
300
250
as-cast condition
absolute frequency
after heat treatment
200
150
100
50
0,
07
0,
13
0,
19
0,
25
0,
31
0,
37
0,
43
0,
49
0,
55
0,
61
0,
67
0,
73
0,
79
0,
85
0,
91
0,
97
m
or
e
0
shape factor
Fig. 6. Distribution of shape factor dimensions of the
hypereutectoid cementite particles
The investigated cast steel is characterised by a very low
impact strength. KCU2 and KCV impact strengths measured for
as-cast condition were the same, equal 1.4±0.2 J/cm2. This can be
due to an occurrence of a large number of structural notches in
this cast steel. Application of the heat treatment lowered a little
KCU2, to 1.2±0.2 J/cm2, and slightly increased KCV to
1.7±0.2 J/cm2. In both cases the brittle fractures prevail (Fig. 7).
However, in samples after the heat treatment, some places
demonstrating features of a ductile fracture can be found.
298
Very low impact strength and its small changes due to the
heat treatment allow only for their cautious interpretation.
However, certain assumptions can be put forward, e.g. that the
fragmentation of the transformed ledeburite network favours
impact strength of V notched samples; while precipitation of
larger amounts of hypereutectoid cementite inside the prior
austenite grain lowers impact strength of notched U samples.
Differences can be caused by various stress concentrations
depending on the notch shape. The concentration of dynamic
stresses (V notch) causes - to a higher degree - a shorter crack
path than in the case of U notch. Whereas smaller stresses but
distributed within a larger sample volume (U notch) facilitate
effects of structural notches.
An occurrence of a continuous network of ledeburitic and
hypereutectoid cementite as well as smaller amounts of
hypereutectoid cementite inside the primary austenite grain causes
that (in the case of applying U notch) a crack development path
becomes longer, which results in a slightly higher KCU2 impact
toughness of samples in as-cast condition than after the heat
treatment. Profiles of sample fractures with U notch in as-cast and
after the heat treatment seem to confirm the above statements
(Fig. 8).
ARCHIVES OF FOUNDRY ENGINEERING Volume 10, Special Issue 1/2010, 295-300
a)
2. Decreasing the transformed ledeburite fraction – due to the heat
treatment – causes an increase of the hypereutectoid cementite fraction,
however it is not a proportional dependence.
3. The heat treatment allows to shape the morphology of the
hypereutectoid cementite precipitates in G200CrNiMo4-3-3 cast
steel:
- partial fragmentation and spheroidization of hypereutectoid
cementite - precipitated in the Widmannstätten structure can be performed,
- more spheroidal precipitates of hypereutectoid cementite can
be ‘introduced’ inside grains.
4. The heat treatment and corresponding microstructure changes
have significant influences on mechanical properties of
G200CrNiMo4-3-3 cast steel:
- allows to increase hardness,
- allows to stabilise strength to prevent samples ruptures at
stresses lower than 400 MPa,
- favours an increase of impact toughness of samples with V
notch,
- can decrease impact toughness of samples with U notch.
b)
Acknowledgments
The Authors wish to express their gratitude to Prof. Jerzy
Pacyna, M.Sc. Jarosław Radwan, Ph.D. Kazimierz Satora, Ph.D.
Piotr Bała and Ph.D. Bogdan Pawłowski for their help in the
preparation of this paper.
The research program was financed by the Ministry of Science
and Higher Education and performed within the Supervisor’s
Grant No N N507 473337.
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Fig. 8. Profile of toughness sample fracture KCU: a) as-cast
condition, b) after the heat treatment. The fracture was
electrolytically covered by a nickel layer. Etched with 2% natal
[2]
5. Conclusions
Investigations performed within this study enable to formulate the
following conclusions:
1. The heat treatment allows to shape the morphology of the
transformed ledeburitic precipitates in G200CrNiMo4-3-3 cast steel:
- fraction of precipitates classified as transformed ledeburite can be
decreased,
- fragmentation of the transformed ledeburite precipitates can be
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- the heat treatment does not cause thinning of the network of the
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