International Journal of Mechanical Engineering and Technology (IJMET)
Volume 10, Issue 01, January 2019, pp. 1235-1245, Article ID: IJMET_10_01_125
Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=1
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
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INFLUENCE OF AUSTENITE AND FERRITE
STABILIZERS ON THE MICROSTRUCTURE
AND RELATED MECHANICAL PROPERTIES
OF CARBURIZED STEELS
Sathyashankara Sharma, Pavan Hiremath*, Gowrishankar M C and Manjunath
Shettar
Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology,
Manipal Academy of Higher Education, Manipal – 576104, Karnataka, India.
* Corresponding author
ABSTRACT
Carburization is a thermo-chemical treatment generally employed to enhance the
surface (wear) properties of low carbon steels. The recent carburization studies also
focus considerable positive impact on bulk properties like tensile strength and
toughness. In view of these observations, the present study focuses on mechanical
properties and microstructure of carburized steels. Accordingly, the commercially
available three types of case hardenable steels like plain carbon (EN 3), alloy steels
with only ferrite stabilizer (20MnCr5) and with both ferrite (Cr) and austenite (Ni)
stabilizers (EN 353) were initially normalized to standardize the room temperature
structure before carburizing and machined to ASTM standards to prepare the
specimens. The machined specimens were gas carburized using carburizing furnace
for 2.5 mm case depth and furnace cooled. Tensile and hardness tests were conducted
before and after carburization. The plain carbon steel displayed slight reduction in
tensile strength and the steels with alloying elements increased the tensile strength
considerably. It was also found that Ni and Cr restrict the grain growth and increase
the strength of steel even in furnace cooled condition. Microstructure analysis of
carburized steels revealed markable impact on the type and distribution of room
temperature phases. The carbon content in the case was nearly 0.8 wt. % after
carburization and hardness increase in the surface ranges from 130 to 170% as that
of its original hardness. The combined effect of Ni and Cr also improves
hardenability.
Keywords: Carburization, steels, carbon, case, core.
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Influence of austenite and ferrite stabilizers on the microstructure and related mechanical
properties of carburized steels
Cite this Article:Sathyashankara Sharma, Pavan Hiremath, Gowrishankar M C and
Manjunath Shettar, Influence of Austenite and Ferrite Stabilizers on the
Microstructure and Related Mechanical Properties of Carburized Steels, International
Journal of Mechanical Engineering and Technology, 10(01), 2019, pp. 1235-1245.
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=1
1. INTRODUCTION
In solid state, alloys may be single or mixtures of phases, microscopically depending on the
alloying elements present in it and the solidification rate employed. If a metal is said to be in a
single phase, then it must be microscopically homogeneous [1]. A phase is anything which is
homogeneous and physically distinct. Any structure which is visible as physically distinct
microscopically may be considered as a phase [2]. During heat treatment the alloy may
undergo phase change, and that can be recorded in the form of phase or equilibrium diagrams.
The ferrous alloys also display different phase change during its heating and cooling cycle.
Even the crystal structure/phase changes based on the application temperature, this
phenomenon is called the allotropic behavior [3]. The temperature at which the allotropic
changes takes place in iron is influenced by alloying elements. The knowledge of material
behavior allows the manufacturer to make the best material at reduced cost and improved
quality. In this study some of the heat treatment procedures are followed to review the
properties of ferrous materials in different cases [4]. A low carbon steel containing carbon less
than 0.2 wt. % is sufficiently tough while a high carbon steel containing carbon up to about
1.0 wt. % possesses adequate hardness and wear-resistance. The service conditions of many
machine parts made of steel such as gears, shafts, cams and so on demand very hard and wear
resistant surfaces but with tough cores [5]. Such a combination of properties is not usually
possible from the commercial steels available. By suitable heat treatments, the high carbon
steel can be made very hard; while the low carbon steel will develop sufficient toughness [6].
A machine part requiring hard surface, but tough core will require a combination of high
carbon at the case (surface) and low carbon for the core, can be achieved by certain heat
treatment called case hardening. Carburizing is the most satisfactory, cheaper and widely used
method of case hardening of low carbon steels [7]. It is the process of diffusing carbon to the
surface layer of steel. The objective of surface hardening is to obtain a hard, wear-resistant
surface with a tough interior. This study deals with the influence of carburization on the
mechanical and microstructural features of three different low carbon steels [8].
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2. MATERIALS AND METHOD
Table 1 Composition of the test specimens
Composition of steels (wt. %)
Elements
EN 3
EN 353
20MnCr5
Carbon (C)
0.178
0.161
0.187
Silicon (Si)
0.188
0.255
0.203
Manganese (Mn)
0.61
0.63
1.17
Phosphorus (P)
0.017
0.012
0.0059
Sulphur (S)
0.0031
0.0080
0.011
Chromium (Cr)
-
0.84
1.11
Nickel (Ni)
-
1.20
-
Iron (Fe)
98.9
96.6
97.2
The compositions of the steels which are considered in this study are shown in table 1. EN
3 is an unalloyed steel with carbon content less than 0.25 wt. %. It is a common grade of steel
without any alloying elements above the permissible limit, generally found in the application
where heavy stress, torque and heat treatment is not involved. 20MnCr5 has high amount of
chromium which is a ferrite stabilizer and helps to retain ferrite even at higher temperatures
there by increasing strength, hardness and wear resistant properties of the steel [3]. EN 353
has both ferrite (Cr) and austenite stabilizers (Ni) which helps in maintaining a good balance
between strength and toughness of the steel [6]. Figure 1 shows the fabricated tensile test
specimen according to ASTM E8 standard [11].
Figure 1 Machined tensile test specimen components to the carburizing furnace
Figure 2 Loading/unloading of steel
Initially the steel samples were normalized by heating them to austenitizing temperature
(930o C) and cooled in air. Gas carburizing was done on the test specimens to get uniform
surface carbon diffusion. Initially degreasing of samples are done by treating in alkaline
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atmosphere [12-13]. Gas carburization was performed on the steels for 18 h in a furnace
maintained at 930oC with continuous supply of propane through spray. Figure 2 shows the
process of loading steels to the furnace.
3. RESULTS AND DISCUSSION
Figures 3, 4 and 5 show the energy dispersive X-ray spectroscopy results showing the
presence of major alloying elements and microstructures of EN 3, 20MnCr5 and EN 353
respectively. EN 3 is a plain carbon steel, air cooling from austenite phase has resulted in
finer pearlite and proeutectoid ferrite grains as shown in figure 3(a). Figure 3(b) shows the
major constituent elements present in region 1 of figure 3(a). Cr is the major alloying element
in 20MnCr5. It has resulted in forming further finer grains than EN 3. Highly distorted
pearlite grains along with proeutectoid ferrite was observed in this steel as shown in figure
4(a). Figures 4(b) and (c) show the major constituent elements at region 1 and region 2 of
figure 4(a) respectively, which indicates the presence of C and Fe in ferrite region and C, Fe
and Cr in the pearlite region. EN 353 which has both Cr and Ni, by altering the shape of
Isothermal transformation diagram and its nose position so that furnace cooling path enters
the ferrite and bainite zone to form bainite structure with ferrite as room temperature
structures. The results show the role played by Cr and Ni in steel to refine the grains and form
new phases. Figure 5(a) shows the microstructure of normalized EN 353 steel. Figures 5(b)
and (c) show the major constituent elements present in region 1 and region 2 of figure 5(a)
respectively, which indicate the presence of Ni, Fe and C in ferrite region and Cr, Ni, Fe and
C in upper bainite region. The elemental analysis clearly indicate Ni dissolved in Ferrite and
Cr forming carbides in the steels.
3.1. Tensile Test
After carburization the core remained as before with same carbon wt.% but, in the case,
pearlite phase was covered almost full surface due to the attainment of eutectoid composition
(0.8 wt.%) due to carburization, which can be seen in figures 6, 7 and 8. Figure 9 shows the
average stress strain curves of three selected materials. Figures 10 (a), (b) and (c) show the
results of tensile test. As shown in figure 9(a) plain carbon steel displayed yielding at 574
MPa. As it is in normalized condition, it displayed proper yield point than the carburized
samples. After carburizing, partial yielding was found at around 295 MPa (figure 9(b)). It is
also observed that the % elongation is comparatively high in carburized condition. As EN 3 is
a plain carbon steel without any alloying element displays grain coarsening phenomena while
soaking at high temperatures. Hence, the carburized EN 3 steel has coarser grains than the
normalized one as it is furnace cooled hence there is a drop in the strength of steel [9].
Microstructures of case and core are shown in figures 6 (a) and (b). The case region was
completely transformed to coarse pearlite and in the core, pearlite, ferrite and needle like
arrangement of fine pearlite next to pearlite bay was observed as a result of different cooling
rates of the core and the case.
Figures 9(c) and (d) show the average stress strain curves for 20MnCr5 which has Cr.
20MnCr5 did not display a definite yield point in both normalized and carburized condition.
Yielding took place just before the fracture of the specimen. Cr in steel forms chromium
carbide restricts grain growth and increases the tensile properties of the steel [10]. As a result
of Cr and C reaction chromium carbide precipitates are formed during the slow cooling from
austenitic state to room temperature. The chromium carbide precipitates thus formed bind the
freshly generated ferrite grain boundaries and leads in forming fine ferrite grains. Figures 7
(a) and (b) show the microstructures of case and core of 20MnCr5. Feathery bainite along
with ferrite was observed in the core and in case coarse pearlite was found in lamellar and
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Sathyashankara Sharma, Pavan Hiremath, Gowrishankar M C and Manjunath Shettar
distorted form [16-17]. There is an enlargement of carbide bay in this steel than EN 3 and also
20MnCr5 displayed finer grain size than EN 3 which clearly indicates Cr in the form of
chromium carbide has greater influence in refining the grain size and increasing the strength
of steel. Ultimate tensile strength of 20MnCr5 increased after carburization and % elongation
was reduced because of carburization.
Figure 3 (a) SEM image of normalized EN 3 (b) Energy dispersive X-ray spectroscopy results
showing the presence of major alloying elements in EN 3 steel at region 1
Figure 4 (a) SEM image of normalized 20MnCr5 (b) Energy dispersive X-ray spectroscopy results
showing the presence of major alloying elements in 20MnCr5 steel at region 1 (c) region 2
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Influence of austenite and ferrite stabilizers on the microstructure and related mechanical
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Figure 5 (a) SEM image of normalized EN 353 (b) Energy dispersive X-ray spectroscopy results
showing the presence of major alloying elements in EN 353 steel at region 1 (c) region 2
EN 353 has both Cr and Ni in it, displayed larger increase in tensile strength even in
furnace cooled condition. In steel, Ni as refractory element as well as hardenability agent, it
promotes toughness property, whereas Cr a strong carbide former and ferrite stabilized
enhances bulk hardness [4]. Hence, Cr as well as Ni in steel lead to combined positive effect
on tensile properties in carburized condition. Figures 9 (e) and (f) show the stress strain
curves of EN 353. Figures 8 (a) and (b) show the microstructures of EN 353 case and core
after carburization. In the core very finer pearlite islands were found along with ferrite grains
and acicular bainite, case displayed complete pearlite with small regions of distorted acicular
bainite [18-19]. Even this steel did not display a definite yield point and the yielding took
almost near the fracture. But EN 353 displayed the highest strength after carburization as a
result of obstruction to grain growth due to austenite and ferrite stabilizers. It is also observed
that Ni and Cr in presence of high carbon (nearly 0.8 wt. % C in case) has great influence on
the tensile strength of the steels. Here the grains are finer than 20MnCr5. Ultimate tensile
strength of EN 353 increased after carburization and % elongation was reduced because of
carburization [15].
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Figure 6(a) Core of EN 3 after carburization (b) Case of EN 3 after carburization. Magnification
1000x (dark region: pearlite light region: ferrite)
Figure 7(a) Core of 20MnCr5 after carburization (b) Case of 20MnCr5 after carburization.
Magnification 1000x (dark region: pearlite light region: ferrite)
Figure 8(a) Core of EN 353 after carburization (b) Case of EN 353 after carburization. Magnification
1000x (dark region: pearlite light region: ferrite)
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Influence of austenite and ferrite stabilizers on the microstructure and related mechanical
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Figure 9 Stress strain curves (a) EN 3 Normalized (b) EN 3 Carburized (c) 20MnCr5 normalized (d)
20MnCr5 Carburized (e) EN 353 Normalized (f) EN 353 Carburized
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Sathyashankara Sharma, Pavan Hiremath, Gowrishankar M C and Manjunath Shettar
Figure 10 Results of tensile strength (a) Load at peak (b) Ultimate tensile strength (c) Percentage
elongation
3.2. Hardness test
The steel samples were checked for hardness before and after carburization. The hardness
values of as received steels are shown in table 2. It is observed that EN 3 plain carbon steel
displayed least hardness among the three. 20MnCr5 and EN 353 displayed higher hardness
than EN 3 in as received condition, as a result of formation of bainitic regions along with
ferrite and fine pearlite. Due to the presence of Ni and Cr as Isothermal diagram shape
modifier and supports too enhance the relative stability of austenite below the critical
temperature during continuous cooling shows bainite and pearlite regions at room
temperatures.
After carburization the steel samples are cut perpendicular to the length of the rod to find
the hardness distribution from core to the case. The results of hardness test are shown in
figure 11. As carburized steels are furnace cooled, there is a drop in the hardness of steel at
the core in all the cases, but due to carburization, a large increase in the hardness in the
case/surface of all steels are found. The presence of Cr in 20MnCr5 and Cr along with Ni in
EN 353 resulted in higher increase of hardness as well as depth of hardened zone in these
steels. Hence, the hardness values obtained in the subsurface layers of the steels
(hardenability) increase with number and percentage of alloying elements (figure 11 (a) and
(b)) which is in line with the results published by Razzak [9]. Nearly 130, 140 and 170%
increase in surface hardness is found on EN 3, 20MnCr5 and EN 353 steels respectively after
carburization.
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Table 2: Hardness of the steels in as received condition
Trials
Hardness of steels (HRC)
EN 3 20MnCr5 EN 353
1
11
21.5
26.5
2
12
22
27
3
10.5
21.5
26
4
13
20.5
27
5
12.5
22
26.5
Average
11.8
21.5
26.6
Figure 11(a): Hardness distribution in EN 3 after carburization (b): Hardness distribution in 20MnCr5
after carburization (c): Hardness distribution in EN 353 after carburization
4. CONCLUSION
Carburization technique is wisely an acceptable technique for case hardening purposes. The
strength of plain carbon steel EN 3 is reduced after carburization, but in presence of alloying
elements the strength of carburized specimens increased, which indicates the influence of
alloying elements on the tensile strength of low carbon steels. Cr precipitates as chromium
carbide and Ni dissolves in solid solution during carburization. Cr is found to be more
influential in formation of carbide and larger feathery bainite region in the core of 20MnCr5.
It was found that the yielding in presence of alloying elements reached closer to the ultimate
strength. Ni and Cr along with eutectoid carbon increases the tensile strength drastically
whereas Cr and Ni with 0.2% carbon has got less strength than the steel which has only Cr as
an alloying element. It implies Cr is more effective in low carbon steels for tensile properties.
Carburization followed by furnace cooling resulted in forming larger pearlite in the case
region of all steels and the presence of both alloying elements in the steels influenced in
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forming smaller regions of bainite in the core as well as case. The combined effect of Cr and
Ni not only increases hardness but also improves hardenability.
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