(SCM 435) Alloy Steel

Microstructures and Mechanical Properties of Austempering CrMo (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 some problems about reliability in the thin plate specimens. This research used the austempering heat treatment on the SCM435 thin plate specimen with a type of double loop and obtained the CrMo bainite structure, and then we could further investigate the mechanical properties of SCM435 bainite materials. Experimental data showed that the stability of the mechanical properties of the specimen with austempering temperature 830°C for 25 min was better than that of 15 min. The size of the austenite grains affected the strength of thin plate bainite specimen. After austempering at 830°C for 25 min and salt bath at 290°C, the stability of the ultimate tensile strength (UTS) was better than that of the salt bath specimens at 310 and 330°C. The average hardness of all the specimens was more than HRA70. The specimen with austempering at 830°C for 25 min and salt bath at 290°C for 30 min 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 Table 1 The chemical composition of the SCM435 (mass%).

CMnSiP S AlCrMoFe The SCM435 is a CrMo steel with high strength and high hardness properties, and it has been widely used in the 0.37 0.80 0.26 0.010 0.004 0.008 0.83 0.15 Bal. machine parts, shafts, gears and strength screw products.13) However, this material often has lower reliability due to the inequality of brittleness in the high-temperature tempering and quenching. The austempering heat treatment can obtain the uniform bainite structure to improve the brittleness of tempering, and the austempering heat treatment is a continuous process that it also has the efficiency and uniformity. For the thin plate specimens, the mechanical properties of bainite structure are better than the traditional fi martempering structure. Fig. 1 The con guration of the thin plate double loop specimen. In this study, the SCM435 sheet was made into the double loop-type thin plate specimens by punch-shear process to highlight the stress concentration to study the effects of the double loop-type thin plate specimen (t = 0.78 mm) to the brittleness.3,4) The bainite structure has the excellent highlight the brittle effects of SCM435 alloy by punch-shear mechanical properties3,5,6) and it can reduce the brittleness process. Figure 1 shows the geometric dimensions of the effect of the thin plate specimen (improve the strength and specimen. The heat treatment conditions of the specimen was ductility) by controlling the different phases of matrix 830°C (vacuum) holding 15 and 25 min for austenitisation, (the retained austenite content). According to the reference,7) and then made it in the salt bath furnace immediately for the characteristics of austempering SCM435 still not have tempering. The salt bath conditions were placed on each been studied and the salt meet the environmental demands. constant temperature of 290, 310 and 330°C for 30, 60 Therefore, this research controlled the heat treatment and 120 min, then quench in the water. Each austempering conditions to obtain the different bainite structures of condition was called x°C-y m by the salt bath condition, SCM435 alloy, and then investigated the tensile strength such as 290°C-30 m. and hardness to obtain the application data of SCM435 The characteristics of each austempered specimens are alloy. determined quantitatively by SEM (Hitachi SU8000) and image analyzer. The structure phases were identified by XRD 2. Experimental Procedure (Bruker AXS Gmbh, Karlsruhe, Germany). The hardness measurement (HRA) and the tensile properties (tensile rate: The chemical composition of SCM435 is given in Table 1, 1 mm·1 min¹1) of each specimens were evaluated in the the carbon content is 0.37 mass% and contains the other brittleness effect. In addition, using ESCA (Electron Spec- alloying elements such as Si, Cr, Mn. This research uses troscopy for Chemical Analysis, PHI 5000 Versa Probe) analyzed the surface to clarify the characteristics of +Corresponding author, E-mail: [email protected] compounds in the SCM435 bainite matrix. Microstructures and Mechanical Properties of Austempering CrMo (SCM 435) Alloy Steel 57

(a) (b)

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

3. Results and Discussion increasing salt bath temperature of specimens would decrease the hardness of the structure. The main reason is the coarse Figure 2 shows the microstructures of the austempered bainite structure and higher retained £ phase content.4,5,911) bainite specimen (830°C for 15 min) with the different salt Also, this result can confirm the content of precipitation bath conditions. Compared with previous other systems,5,810) carbide is lower with higher salt bath temperature and the microstructure of SCM435 alloy was finer (similar heat longer tempering duration, so there is no significant contri- treatment condition) and it was a close relationship with bution to the strength and hardness of the bainite SCM435 the alloying elements (carbon content). When austempering alloy. duration extended from 15 to 25 min, the bainite structure Figure 7 shows the comparison of mechanical properties had a coarsening phenomenon (Fig. 3) and it was because of the different salt bath conditions after austempering at the austenite grain growth. In other words, increasing the 830°C for 25 min. The data at 310 and 330°C is unstable. The austempering duration of SCM435 specimens, we can obtain tensile strength of the specimens at 290°C is reach 1100 MPa the thick and feathery bainite structure. and has a better reliability than the other specimens. For XRD spectra of the different salt bath conditions after hardness analysis, the specimen at 330°C is more stable austempering 15 and 25 min are showed in Figs. 4 and 5. The than other two salt bath conditions (290 and 310°C). The figures show the peak angles are the same and the phases are hardness values are about HRA72 and the carbides in the consist of the bainitic ferrite, Fe2.5C carbides and retained £ matrix are distributed uniformity. According to results of phase. During salt bath process, the retained £ phases form in Figs. 6 and 7, the specimen is austempered at 830°C for matrix due to the diffusion of carbon. In addition to the peaks 15 min and precedes the salt bath heat treatment at 290°C of the bainite structure, there is no other obvious peak and that has more stable tensile strength and hardness. It can be we can confirm the SCM435 structure to transform into the explained that the salt bath conditions is more importance bainite matrix fully after austempering heat treatment. than austempering time for the bainite structure of SCM435 Figure 6 shows the comparison of tensile mechanical alloy. properties of the different salt bath conditions after The salt bath condition is the closest relationship for the austempering at 830°C for 15 min. UTS of the specimen at mechanical properties, so the failure mechanism of the 330°C is significant decreased with increasing the holding specimens at 290 and 330°C are compared to understand time (reduce to ³950 MPa) and the tensile fracture resistance brittle effects of thin plate specimens.3,8) Figure 8 is the of specimen at 330°C for 120 min was the lowest. In terms fracture characteristics of the specimens after each salt bath of hardness, no matter the conditions of salt bath 30 min heat treatments. In Fig. 8(a), we observe the dimple structure or 60 min, the hardness was about HRA72. Furthermore, and confirm the characteristics of ductile failure. In Fig. 8(b), 58 C.-Y. Chen, F.-Y. Hung, T.-S. Lui and L.-H. Chen

(a) (b)

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

Diffraction angle, 2 θ / degree Diffraction angle, 2 θ / degree

Fig. 4 XRD of the austempered specimen (830°C-15 min) with the Fig. 5 XRD of the austempered specimen (830°C-25 min) with the different salt bath conditions. different salt bath conditions. / MPa / MPa / HRA UTS UTS / HRA / HRA H H Hardness, Hardness, Tensile strength, Tensile Tensile strength, Tensile

Fig. 6 The mechanical properties of the austempered specimen (830°C- Fig. 7 The mechanical properties of the austempered specimen (830°C- 15 min) with the different salt bath conditions. 25 min) with the different salt bath conditions. Microstructures and Mechanical Properties of Austempering CrMo (SCM 435) Alloy Steel 59

(a) (b)

Fig. 8 The fractured characteristics of the austempered specimen (830°C-25 min) with the different salt bath conditions: (a) 290°C-60 min (b) 290°C-120 min (c) 330°C-60 min (d) 330°C-120 min. the tensile fracture structure is level and ladder-splitting. There are only a few dimple microstructures, so the brittle (a) behavior is the major mechanism for the failure mode. In Figs. 8(c) and 8(d), although the dimple microstructures are observed, but the main mechanism are still splitting, 4 and it is belong to brittle materials fracture mode, so it has a lower tensile strength. From failure mechanism, / 10 = the brittle of thin plate specimen has high sensitivity (t C/S 0.78 mm). If the SCM435 has no appropriate austempering conditions, even the upper bainite structures still occur the brittle failure.

According to literature,12) the carbides of the iron-base Intensity, austempering specimen are relationship with material failure mode. A short time austempering heat treatment can obtain the structural toughness, but longer austempering heat Binding Energy, BE / eV treatment can improve the hardness and brittleness. Based (b) on this, we use ESCA to analyze the specimen after salt bath to understand the carbide system. Figures 9 and 10 are the graphs of surface analysis, compare with them can ﬁnd all the 4

specimens have a small amount of carbide generated (this / 10 carbide is Fe2.5C). Furthermore, increasing austempered time, the tensile strength of SCM435 is decreased signiﬁcantly. C/S It is clear that the retained £ phase (the specimen at 290°C, £ = 11.4 vol%) of the matrix is the main reason to effect the

failure mechanism. Intensity, Figure 11 shows the microstructure of each specimen which corresponds to the mechanical properties at different salt bath temperatures. The content of the bainite structure increased with reducing the salt bath temperature. The Binding Energy, BE / eV retained £ phase has the contribution to improve the Fig. 9 ESCA of the austempered specimen (830°C-15 min) with the brittleness effect and enhance the tensile reliability. different salt bath conditions: (a) 290°C-30 min (b) 290°C-120 min. 60 C.-Y. Chen, F.-Y. Hung, T.-S. Lui and L.-H. Chen

(a) 4 / 10 C/S Intensity, Intensity,

Binding Energy, BE / eV (b) Fig. 11 The diagram of the strength and reliability with the different austempered temperature. 4

/ 10 improve the temper brittleness of the SCM435 thin

C/S plate specimen.

Acknowledgements

Intensity, Intensity, The authors are grateful to National Cheng Kung University, the Center for Micro/Nano Science and Tech- nology (D101-2700) and NSC 101-2221-E-006-114 for the Binding Energy, BE / eV ﬁnancial support.

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

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