Defects, Microstructure and Properties of the Industrial 35 CrMoV Steel
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DEFECTS, MICROSTRUCTURE AND PROPERTIES OF THE INDUSTRIAL 35 CrMoV STEEL WU WANGZI *AND QIAO GUIWEN * • Shenyang Institute of Technology, 2-1 Wenhua Road, Shenyang 110015, P.R. China • * Institute of Metal Research, Academia Sinica, 72 Wenhua Road, Shenyang, 110015, P.R. China
ABSTRACT 35 CrMoV steel is widely used in industries. To select the best technology for improving the mechanical properties, a series of heat treatments were tested. Metallographic examination indicates that the microstructure is a kind of non-typical bainite and, to a certain extent, similar to tempered troostites. TEM observation shows that rod-like alloying cementite distributed in ferrite grains laid parallel to each other and ultra-fine alloying carbides also dispersed in the lath-like ferrite grains. Moreover, there exists high density of dislocation tangles and microtwins. Detailed discussions are made on the solid solution strengthening, the work hardening process and the crack propagation by view of microstructure, defects and stress concentration.
Introduction The addition of alloying elements even in small concentrations can alter the properties and structure of ferrite and bainite [1]. Bainitic structures are well known for their complexity and the corresponding structures after different treatments were investigated using replicas and thin foils in the electron microscope [2]. 35 CrMoV steel is widely used in industries. Good synthetical properties have been obtained by isothermally transforming austenite to the various bainitic structures. The research described in this paper has sought to find a suitable heat treatment. Proof and ultimate tensile strengths and elogation as well as area reduction rates were determined following the different treatments, and the corresponding structures were investigated using TEM, SEM and optical metallography.
Materials and Experiments The chemical compositions of the tested steels are listed in Table 1. Table 1. Chemical composition (wt.%)
I 35CrMoV
C
Si
0.35
0.23
Mal 0.58
P 0.031
0.035
Cr
Mol
1.18
025
VI 0.15
The tensile and Charpy specimens were prepared by following the chinese national standard GB223-63. The critical temperatures Act and Ac 3 were measured using dilatometer as 784C and 820C . To obtain fully solved alloying elements, the austenisation temperature was selected at 910C for 20min. The heat treatments were Mat. Res. Soc. Symp. Proc. Vol. 209. ©1991 Materials Research Society
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chosen as follows: Group 14: 91012 / 20min, Quenched in oil, 40012 tempered/ lhr. Group 24: 910t2 / 20min, Quenched in oil, 600tI tempered/ lhr. Group 34: 9101C / 20min, isothermal quenched at 4001C, tempered/ lhr in (50%KNO 3+50%NaNO 2)bath. The specimens were fine ground to the designed sizes after heat treatment and tested at 2.5 mm / min rate for tensile experiments and notched for Charpy tests. The fracture surfaces were examined with S-360 and S4-10 scanning electron microscopes and replicas were also made for TEM observations. Moreover, the thin foils were cut from the specimen
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