Transformation of retained austenite in carburized 4320 steel
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I.
INTRODUCTION
C A R B U R I Z E D steels containing retained austenite can be dimensionally unstable. A phase transformation of austenite to either martensite or bainite is accompanied with a volume expansion in steels. If a volume expansion occurs, the performance of components manufactured from these materials may suffer. For example, a volume expansion in the case-carburized cone of a railroad bearing can cause the cone to overcome the press fit on the axle journal and become loose, m It is important to understand why these dimensional and volumetric changes occur and under what stress and temperature conditions these changes will take place. A dimensional change can occur through either a stressinduced phase transformation of retained austenite [facecentered cubic (fcc) structure] to martensite [body-centered tetragonal (bct) structure] or a thermal-induced transformarion of retained austenite to bainite [body-centered cubic (bcc) structure]. In either case, there is a localized volume expansion of about 4 pct. tel A number of other mechanisms can cause dimensional instability; t31 however, in steels with retained austenite, the changes in dimensions due to the phase transformation can be significant. A number of experiments were conducted on a carburized steel which contained 35 pct retained austenite by volume. These experiments were designed to study the effects of stress-induced and thermal-induced transR.W. NEU, Visiting Assistant Professor, and HUSEYIN SEHITOGLU, Associate Professor, are with the Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL 61801. Manuscript submitted July 9, 1990. METALLURGICAL TRANSACTIONS A
formation and how they are related. The extent of transformation is determined by precise measurement of the volume change of the specimen. The characteristics of stress-induced and thermal-induced transformation are summarized in light of the experimental work conducted during this investigation.
II. M A T E R I A L A N D EXPERIMENTAL TECHNIQUES The material used in this study was carburized 4320 steel. Smooth fatigue specimens with a 12.7-ram (0.5-in.) gage length and 5.08-mm (0.200-in.) diameter, as shown in Figure 1, were used. To obtain a through-carburized gage section with uniform structure, a special specimen preparation was required. The gage section of each specimen was first rough machined to 5.84-mm (0.230-in.) diameter. Carburizing was then performed on these specimens in an endothermic atmosphere at a temperature of 982 ~ (1800 ~ for 18 hours. After carburizing, the specimens were austenitized at 857 ~ (1575 ~ for 1 hour, hardened by quenching in oil, and then tempered for 1.5 hours at 177 ~ (350 ~ The specimens were machined to final dimensions utilizing a slow grind process. The carbon content in the gage section was 1.1 pct. Using X-ray diffraction techniques, t41 the average amount of retained austenite (Aret) in the specimens before testing was 35 pct. After testing, the austenite content was measured again by the same technique. Th
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