The effects of heat treatment on fracture toughness and fatigue crack growth Rates in 440C and BG42 steels
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INTRODUCTION
HIGH-carbonhigh-chromium martensitic stainless steels, such as 440C and BG42, are used in rolling element bearings where corrosion resistance is required, but problems of fatigue, fracture toughness, and dimensional stability also arise, and these factors influence the choice of heat treatments. These steels develop high strengths and exhibit secondary hardening on tempering, with a peak at about 480 ~ (900 ~ for 440C and 525 ~ (975 ~ for BG42. In 440C, hardness levels of about 60 Rc can be achieved on tempering at 150 ~ (300 ~ even though the retained austenite content may be at about 20 vol pct. On tempering at the secondary hardening peak, the retained austenite is reduced to about 6 pct and the hardness is about 58 R~. These properties are shown in Figures 1 through 3. Refrigeration treatments are used in order to reduce the retained austenite contents and thus to increase the hardness (Figure 1). Multiple tempering cycles are also used, particularly near the secondary hardening peak, in an effort to reduce the austenite contents and thus to improve dimensional stability. By varying the heat treating variables, such as austenitizing temperature, tempering temperature, and refrigeration treatments, different combinations of hardness and retained austenite can be produced. The influence of this interplay of retained austenite and tempered martensite on fatigue crack BINGZHE LOU is with Shaanxi Mechanical Engineering Institute, Xian, People's Republic of China, and was Visiting Scientist at Massachusetts Institute of Technology. B. L. AVERBACH is Professor of Materials Science, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted November 9, 1982. METALLURGICAL TRANSACTIONS A
propagation and fracture toughness in these steels had not been established, and this was the objective of the work reported here. It has been shown that the presence of retained austenite can improve the fracture toughness, K,c, of high strength steels, '-6 and two mechanisms have been proposed to explain this improvement. Webster2 pointed out that the advancing crack may stop, branch, and grow around an area of stable retained austenite, thereby resulting in increased energy absorption. When retained austenite is mechanically
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Fig. 1 - - Effect of austenitizing temperature and tempering temperature on hardness of BG42 and 440C steels. VOLUME 14A, SEPTEMBER 1983~ 1899
unstable, however, strain-induced martensitic transformation may occur in the plastic zone ahead of the advancing crack, :-6 and this also requires additional energy absorption. The beneficial effects of retained austenite and its martensitic transformation on fatigue crack growth at nearthreshold levels 7 and within the power low regime 8 have also been reported. The thermal decomposition of reta
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