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Microstructures and Mechanical Properties of AustemperingCrMo
(SCM 435) Alloy Steel
Cheng-Yi Chen, Fei-Yi Hung+, Truan-Sheng Lui and Li-Hui Chen
Department of Materials Science and Engineering, National Cheng
Kung University, Tainan, Taiwan 701, R. O. China
SCM435 is a CrMo alloy steel and it is usually used to make the
matrix of tempered-martensite by oil bath method, but it has
someproblems about reliability in the thin plate specimens. This
research used the austempering heat treatment on the SCM435 thin
plate specimenwith a type of double loop and obtained the CrMo
bainite structure, and then we could further investigate the
mechanical properties of SCM435bainite materials. Experimental data
showed that the stability of the mechanical properties of the
specimen with austempering temperature 830Cfor 25min was better
than that of 15min. The size of the austenite grains affected the
strength of thin plate bainite specimen. After austemperingat 830C
for 25min and salt bath at 290C, the stability of the ultimate
tensile strength (UTS) was better than that of the salt bath
specimens at310 and 330C. The average hardness of all the specimens
was more than HRA70. The specimen with austempering at 830C for
25min andsalt bath at 290C for 30min had bigger grain size of the
austenite and retained phase of 11.4 vol%. It also had higher
hardness and strength.So, the austempering SCM435 alloy was
improved for tensile strength comparing with traditional oil
tempered-martensite process.[doi:10.2320/matertrans.M2012317]
(Received September 11, 2012; Accepted October 25, 2012;
Published December 25, 2012)
Keywords: SCM435, alloy steel, austempering, mechanical
properties
1. Introduction
The SCM435 is a CrMo steel with high strength and highhardness
properties, and it has been widely used in themachine parts,
shafts, gears and strength screw products.13)
However, this material often has lower reliability due to
theinequality of brittleness in the high-temperature temperingand
quenching. The austempering heat treatment can obtainthe uniform
bainite structure to improve the brittlenessof tempering, and the
austempering heat treatment is acontinuous process that it also has
the efciency anduniformity. For the thin plate specimens, the
mechanicalproperties of bainite structure are better than the
traditionalmartempering structure.In this study, the SCM435 sheet
was made into the double
loop-type thin plate specimens by punch-shear process
tohighlight the stress concentration to study the effects ofthe
brittleness.3,4) The bainite structure has the excellentmechanical
properties3,5,6) and it can reduce the brittlenesseffect of the
thin plate specimen (improve the strength andductility) by
controlling the different phases of matrix(the retained austenite
content). According to the reference,7)
the characteristics of austempering SCM435 still not havebeen
studied and the salt meet the environmental demands.Therefore, this
research controlled the heat treatmentconditions to obtain the
different bainite structures ofSCM435 alloy, and then investigated
the tensile strengthand hardness to obtain the application data of
SCM435alloy.
2. Experimental Procedure
The chemical composition of SCM435 is given in Table 1,the
carbon content is 0.37mass% and contains the otheralloying elements
such as Si, Cr, Mn. This research uses
the double loop-type thin plate specimen (t = 0.78mm)
tohighlight the brittle effects of SCM435 alloy by
punch-shearprocess. Figure 1 shows the geometric dimensions of
thespecimen. The heat treatment conditions of the specimen was830C
(vacuum) holding 15 and 25min for austenitisation,and then made it
in the salt bath furnace immediately fortempering. The salt bath
conditions were placed on eachconstant temperature of 290, 310 and
330C for 30, 60and 120min, then quench in the water. Each
austemperingcondition was called xC-ym by the salt bath
condition,such as 290C-30m.The characteristics of each austempered
specimens are
determined quantitatively by SEM (Hitachi SU8000) andimage
analyzer. The structure phases were identied by XRD(Bruker AXS
Gmbh, Karlsruhe, Germany). The hardnessmeasurement (HRA) and the
tensile properties (tensile rate:1mm1min1) of each specimens were
evaluated in thebrittleness effect. In addition, using ESCA
(Electron Spec-troscopy for Chemical Analysis, PHI 5000 Versa
Probe)analyzed the surface to clarify the characteristics
ofcompounds in the SCM435 bainite matrix.
Table 1 The chemical composition of the SCM435 (mass%).
C Mn Si P S Al Cr Mo Fe
0.37 0.80 0.26 0.010 0.004 0.008 0.83 0.15 Bal.
Fig. 1 The conguration of the thin plate double loop
specimen.
+Corresponding author, E-mail: [email protected]
Materials Transactions, Vol. 54, No. 1 (2013) pp. 56 to 602012
The Japan Institute of Metals
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3. Results and Discussion
Figure 2 shows the microstructures of the austemperedbainite
specimen (830C for 15min) with the different saltbath conditions.
Compared with previous other systems,5,810)
the microstructure of SCM435 alloy was ner (similar
heattreatment condition) and it was a close relationship withthe
alloying elements (carbon content). When austemperingduration
extended from 15 to 25min, the bainite structurehad a coarsening
phenomenon (Fig. 3) and it was becausethe austenite grain growth.
In other words, increasing theaustempering duration of SCM435
specimens, we can obtainthe thick and feathery bainite
structure.XRD spectra of the different salt bath conditions
after
austempering 15 and 25min are showed in Figs. 4 and 5. Thegures
show the peak angles are the same and the phases areconsist of the
bainitic ferrite, Fe2.5C carbides and retained phase. During salt
bath process, the retained phases form inmatrix due to the
diffusion of carbon. In addition to the peaksof the bainite
structure, there is no other obvious peak andwe can conrm the
SCM435 structure to transform into thebainite matrix fully after
austempering heat treatment.Figure 6 shows the comparison of
tensile mechanical
properties of the different salt bath conditions
afteraustempering at 830C for 15min. UTS of the specimen at330C is
signicant decreased with increasing the holdingtime (reduce to
950MPa) and the tensile fracture resistanceof specimen at 330C for
120min was the lowest. In termsof hardness, no matter the
conditions of salt bath 30minor 60min, the hardness was about
HRA72. Furthermore,
increasing salt bath temperature of specimens would decreasethe
hardness of the structure. The main reason is the coarsebainite
structure and higher retained phase content.4,5,911)
Also, this result can conrm the content of precipitationcarbide
is lower with higher salt bath temperature andlonger tempering
duration, so there is no signicant contri-bution to the strength
and hardness of the bainite SCM435alloy.Figure 7 shows the
comparison of mechanical properties
of the different salt bath conditions after austempering at830C
for 25min. The data at 310 and 330C is unstable. Thetensile
strength of the specimens at 290C is reach 1100MPaand has a better
reliability than the other specimens. Forhardness analysis, the
specimen at 330C is more stablethan other two salt bath conditions
(290 and 310C). Thehardness values are about HRA72 and the carbides
in thematrix are distributed uniformity. According to results
ofFigs. 6 and 7, the specimen is austempered at 830C for15min and
precedes the salt bath heat treatment at 290Cthat has more stable
tensile strength and hardness. It can beexplained that the salt
bath conditions is more importancethan austempering time for the
bainite structure of SCM435alloy.The salt bath condition is the
closest relationship for the
mechanical properties, so the failure mechanism of thespecimens
at 290 and 330C are compared to understandbrittle effects of thin
plate specimens.3,8) Figure 8 is thefracture characteristics of the
specimens after each salt bathheat treatments. In Fig. 8(a), we
observe the dimple structureand conrm the characteristics of
ductile failure. In Fig. 8(b),
(a) (b)
(c) (d)Fig. 2 Microstructural characteristics of the austempered
specimen (830C-15min) with the different salt bath conditions: (a)
290C-
60min (b) 290C-120min (c) 330C-60min (d) 330C-120min.
Microstructures and Mechanical Properties of Austempering CrMo
(SCM 435) Alloy Steel 57
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(a) (b)
(c) (d)Fig. 3 Microstructural characteristics of the austempered
specimen (830C-25min) with the different salt bath conditions: (a)
290C-
60min (b) 290C-120min (c) 330C-60min (d) 330C-120min.
Diffraction angle, 2 / degree
Inte
nsity
(a.u.
)
Fig. 4 XRD of the austempered specimen (830C-15min) with
thedifferent salt bath conditions.
Diffraction angle, 2 / degree
Inte
nsity
(a.u.
)
Fig. 5 XRD of the austempered specimen (830C-25min) with
thedifferent salt bath conditions.
Ten
sile
stre
ngth
, UTS
/ M
Pa
Har
dnes
s, H
/ H
RA
Fig. 6 The mechanical properties of the austempered specimen
(830C-15min) with the different salt bath conditions.
Ten
sile
stre
ngth
, UTS
/ M
Pa
Har
dnes
s, H
/ HR
A
Fig. 7 The mechanical properties of the austempered specimen
(830C-25min) with the different salt bath conditions.
C.-Y. Chen, F.-Y. Hung, T.-S. Lui and L.-H. Chen58
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the tensile fracture structure is level and
ladder-splitting.There are only a few dimple microstructures, so
the brittlebehavior is the major mechanism for the failure mode.In
Figs. 8(c) and 8(d), although the dimple microstructuresare
observed, but the main mechanism are still splitting,and it is
belong to brittle materials fracture mode, so ithas a lower tensile
strength. From failure mechanism,the brittle of thin plate specimen
has high sensitivity (t =0.78mm). If the SCM435 has no appropriate
austemperingconditions, even the upper bainite structures still
occur thebrittle failure.According to literature,12) the carbides
of the iron-base
austempering specimen are relationship with material
failuremode. A short time austempering heat treatment can obtainthe
structural toughness, but longer austempering heattreatment can
improve the hardness and brittleness. Basedon this, we use ESCA to
analyze the specimen after salt bathto understand the carbide
system. Figures 9 and 10 are thegraphs of surface analysis, compare
with them can nd all thespecimens have a small amount of carbide
generated (thiscarbide is Fe2.5C). Furthermore, increasing
austempered time,the tensile strength of SCM435 is decreased
signicantly.It is clear that the retained phase (the specimen at
290C, = 11.4 vol%) of the matrix is the main reason to effect
thefailure mechanism.Figure 11 shows the microstructure of each
specimen
which corresponds to the mechanical properties at differentsalt
bath temperatures. The content of the bainite structureincreased
with reducing the salt bath temperature. Theretained phase has the
contribution to improve thebrittleness effect and enhance the
tensile reliability.
(a) (b)
(c) (d)Fig. 8 The fractured characteristics of the austempered
specimen (830C-25min) with the different salt bath conditions: (a)
290C-60min
(b) 290C-120min (c) 330C-60min (d) 330C-120min.
(a)
(b) Binding Energy, BE / eV
Binding Energy, BE / eV
Inte
nsity
, C/
S / 1
04In
tens
ity, C/
S / 1
0 4
Fig. 9 ESCA of the austempered specimen (830C-15min) with
thedifferent salt bath conditions: (a) 290C-30min (b)
290C-120min.
Microstructures and Mechanical Properties of Austempering CrMo
(SCM 435) Alloy Steel 59
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4. Conclusion
(1) The SCM435 alloy steel had some problems aboutreliability
when it used the martempering heat treat-ment. The SCM435 alloy
steel transformed into thebainite structure by austempering heat
treatment thathad some retained phase and a few Fe2.5C carbides
toraise the reliability.
(2) The tensile strength and hardness of the SCM435alloy were
improved when the grain size of theaustenite had growth and formed
ner bainite structure.Extending the austempered heat treatment time
woulddecrease the hardness due to the structure had lowercarbon
content. For 290C specimen, it had a nerbainite structure and
higher retained phase that could
improve the temper brittleness of the SCM435 thinplate
specimen.
Acknowledgements
The authors are grateful to National Cheng KungUniversity, the
Center for Micro/Nano Science and Tech-nology (D101-2700) and NSC
101-2221-E-006-114 for thenancial support.
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Fig. 11 The diagram of the strength and reliability with the
differentaustempered temperature.
(a)
(b) Binding Energy, BE / eV
Binding Energy, BE / eV
Inte
nsity
, C/
S / 1
04In
tens
ity, C/
S / 1
04
Fig. 10 ESCA of the austempered specimen (830C-15min) with
thedifferent salt bath conditions: (a) 290C-30min (b)
290C-120min.
C.-Y. Chen, F.-Y. Hung, T.-S. Lui and L.-H. Chen60