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 830°Cfor 25min was better than that of 15min. The size of the austenite grains affected the strength of thin plate bainite specimen. After austemperingat 830°C for 25min and salt bath at 290°C, the stability of the ultimate tensile strength (UTS) was better than that of the salt bath specimens at310 and 330°C. The average hardness of all the specimens was more than HRA70. The specimen with austempering at 830°C for 25min andsalt bath at 290°C 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 efficiency 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 doubleloop-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 was830°C (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 330°C for 30, 60and 120min, then quench in the water. Each austemperingcondition was called x°C-ym by the salt bath condition,such as 290°C-30m.

The characteristics of each austempered specimens aredetermined quantitatively by SEM (Hitachi SU8000) andimage analyzer. The structure phases were identified by XRD(Bruker AXS Gmbh, Karlsruhe, Germany). The hardnessmeasurement (HRA) and the tensile properties (tensile rate:1mm·1min¹1) 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 configuration of the thin plate double loop specimen.

+Corresponding author, E-mail: fyhung@mail.ncku.edu.tw

Materials Transactions, Vol. 54, No. 1 (2013) pp. 56 to 60©2012 The Japan Institute of Metals

3. Results and Discussion

Figure 2 shows the microstructures of the austemperedbainite specimen (830°C for 15min) with the different saltbath conditions. Compared with previous other systems,5,8­10)

the microstructure of SCM435 alloy was finer (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 afteraustempering 15 and 25min are showed in Figs. 4 and 5. Thefigures 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 confirm the SCM435 structure to transform into thebainite matrix fully after austempering heat treatment.

Figure 6 shows the comparison of tensile mechanicalproperties of the different salt bath conditions afteraustempering at 830°C for 15min. UTS of the specimen at330°C is significant decreased with increasing the holdingtime (reduce to ³950MPa) and the tensile fracture resistanceof specimen at 330°C 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 confirm the content of precipitationcarbide is lower with higher salt bath temperature andlonger tempering duration, so there is no significant contri-bution to the strength and hardness of the bainite SCM435alloy.

Figure 7 shows the comparison of mechanical propertiesof the different salt bath conditions after austempering at830°C for 25min. The data at 310 and 330°C is unstable. Thetensile strength of the specimens at 290°C is reach 1100MPaand has a better reliability than the other specimens. Forhardness analysis, the specimen at 330°C is more stablethan other two salt bath conditions (290 and 310°C). 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 830°C for15min and precedes the salt bath heat treatment at 290°Cthat 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 themechanical properties, so the failure mechanism of thespecimens at 290 and 330°C 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 confirm the characteristics of ductile failure. In Fig. 8(b),

(a) (b)

(c) (d)

Fig. 2 Microstructural characteristics of the austempered specimen (830°C-15min) with the different salt bath conditions: (a) 290°C-60min (b) 290°C-120min (c) 330°C-60min (d) 330°C-120min.

Microstructures and Mechanical Properties of Austempering Cr­Mo (SCM 435) Alloy Steel 57

(a) (b)

(c) (d)

Fig. 3 Microstructural characteristics of the austempered specimen (830°C-25min) with the different salt bath conditions: (a) 290°C-60min (b) 290°C-120min (c) 330°C-60min (d) 330°C-120min.

Diffraction angle, 2 θθ / degree

Inte

nsi

ty (

a.u

.)

Fig. 4 XRD of the austempered specimen (830°C-15min) with thedifferent salt bath conditions.

Diffraction angle, 2 θθ / degree

Inte

nsi

ty (

a.u

.)

Fig. 5 XRD of the austempered specimen (830°C-25min) with thedifferent salt bath conditions.

Ten

sile

str

eng

th, U

TS

/ M

Pa

Har

dn

ess,

H /

HR

A

Fig. 6 The mechanical properties of the austempered specimen (830°C-15min) with the different salt bath conditions.

Ten

sile

str

eng

th, U

TS

/ M

Pa

Har

dn

ess,

H /

HR

A

Fig. 7 The mechanical properties of the austempered specimen (830°C-25min) with the different salt bath conditions.

C.-Y. Chen, F.-Y. Hung, T.-S. Lui and L.-H. Chen58

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-baseaustempering 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 find all thespecimens have a small amount of carbide generated (thiscarbide is Fe2.5C). Furthermore, increasing austempered time,the tensile strength of SCM435 is decreased significantly.It is clear that the retained £ phase (the specimen at 290°C,£ = 11.4 vol%) of the matrix is the main reason to effect thefailure mechanism.

Figure 11 shows the microstructure of each specimenwhich 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 (830°C-25min) with the different salt bath conditions: (a) 290°C-60min(b) 290°C-120min (c) 330°C-60min (d) 330°C-120min.

(a)

(b) Binding Energy, BE / eV

Binding Energy, BE / eV

Inte

nsi

ty, C

/S /

104

Inte

nsi

ty, C

/S /

10 4

Fig. 9 ESCA of the austempered specimen (830°C-15min) with thedifferent salt bath conditions: (a) 290°C-30min (b) 290°C-120min.

Microstructures and Mechanical Properties of Austempering Cr­Mo (SCM 435) Alloy Steel 59

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 finer bainite structure.Extending the austempered heat treatment time woulddecrease the hardness due to the structure had lowercarbon content. For 290°C specimen, it had a finerbainite 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 thefinancial 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

nsi

ty, C

/S /

104

Inte

nsi

ty, C

/S /

104

Fig. 10 ESCA of the austempered specimen (830°C-15min) with thedifferent salt bath conditions: (a) 290°C-30min (b) 290°C-120min.

C.-Y. Chen, F.-Y. Hung, T.-S. Lui and L.-H. Chen60