Carbide- matrix interface mechanism of stress corrosion cracking behavior of high-strength CrMo steels
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I.
INTRODUCTION
IT is widely considered tl-9] that the resistance of highstrength steels to stress corrosion cracking (SCC) and hydrogen embrittlement (HE) is closely related to the level of yield strength, Cry,,because experimental results showed that with the rise of Cry,, Kiscc (or Kth) decreases or the rate of crack propagation increases. In established models, tl,2,1~ the dependence of Kiscc (or Kth) on O'ys was mainly attributed to the effect of Ors on hydrostatic stress or flow stress in the near crack-tip region. Some authors suggested that the segregation of impurities, such as P, S, and N, at prior austenitic grain boundaries is the main reason causing low-stress intergranular fracture in hydrogen-containing conditions, 1~1-19] and the resistance to SCC and HE was mainly dependent on both O-y~and the concentration of impurities segregated at grain boundaries.t11,13,16,1S] However, the following experimental facts lead us to question the dependence of KIscc on O'ysalone: (1) At the same level of try,, the K~scc values of widely used lowalloy steels, such as AISI 4340, 300M, and D6AC, are much lower than those of 18Ni maraging steel t2~ and that of 15CrMnMoV steel after thermomechanical treatment. t21'221 (2) The Ors of low-alloy steels depends mainly on both tempering temperature and carbon content. If tempering temperature and carbon content affected Kiscc only by affecting O-y~,the specimens with different carbon contents or tempered at different temperatures, but with the same try,, would have the same Kiscc. However, Sandoz [2~ showed that at the same Crys,carbon content also has a significant effect on the K i s c c of AISI 4340type steels. Therefore, it is necessary to study the effects G.F. LI, Associate Professor, and R.G. WU and T.C. LEI, Professors, are with the Department of Metals and Technology, Harbin Institute of Technology, Harbin 150006, People's Republic of China. Manuscript submitted March 14, 1991. METALLURGICAL TRANSACTIONS A
of both tempering temperature and carbon content, and thus the dependence of Kiscc on O-ys, in more detail. High-strength CrMo steels with different carbon contents were selected for the present study. The variations of KISCC, fracture mode, try~, grain boundary segregation, and microstructure of the steels with both tempering temperature and carbon content were examined in order to determine which factor in the steels has predominant effect of SCC behavior. II.
EXPERIMENTAL PROCEDURE
The chemical compositions of the CrMo steels are listed in Table I. The specimens of 20CrMo were austenitized in a salt bath of 880 ~ for 0.5 hours and quenched in salt water. These of 42CrMo and 60CrMo were austenitized at 860 ~ for 0.5 hours and oil quenched. Then most specimens were tempered at 200 ~ 300 ~ 400 ~ and 500 ~ respectively, for 1 hour and cooled in air. Stress corrosion cracking tests were carried out by using double-cantilever beam (DCB) specimens with a thickness B of 18 mm. Side grooves with a depth of 2 mm were machined to guide crack propagation along a singl
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