High cycle fatigue behavior of gas-carburized medium carbon Cr-Mo steel
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I.
INTRODUCTION
THE reason for high fatigue strength of carburized and quenched steel is high hardness and compressive residual stress near the surface, tl,21 The expected microstructure near the surface of carburized steel is martensite with some amount of retained austenite. The general trend for carburizing steel has been to hold the carbon content to around 0.2 pct to produce tough cores and good residual stress distribution. However, the need to use steel with carbon contents of less than 0.2 pct might be questioned for certain types of components that require a high core strength. Alternatives would be to use a low-carbon core with either a deeper case depth or alloyed to develop the required core strength; both increase the costs. Steels with higher carbon cores could also be used. However, a problem is that fatigue limits decrease with increasing core carbon content.t3] Many factors affect the fatigue resistance of carburized steel. These factors include hardness, residual stress, surface finish, microstructure, grain size, globular and network carbides, intergranular oxidation, microcracking, and the presence of retained austeniteY ,21 Many of these factors may interact to influence wear and fatigue performance. However, it is generally accepted that the presence of internal oxidation decreases the fatigue strength. [4,5,61Reheating treatments result in the increase of fatigue limits because of refinement of microstructure, reduced retained austenite level, and decreased microcracking.t2.7.8] The endurance limit is increased with increasing the case depth.[3,9.1o] Though the effect of retained austenite on mechanical properties is still controversial,tX~.~2,~3]low amounts of retained austenite are beneficial to high cycle bending fatigue properties, t7,14,15] This article reports on the high cycle fatigue properties of medium carbon steel that contains around 0.4 pct carbon HYUNG-JUN KIM, Research Engineer, and YOUNG-GAK KWEON, Head of Maintenance Technology Research Team, are with the Steel Products Division, Research Institute of Industrial Science and Technology, Pohang, Korea 790-600. Manuscript submitted August 16, 1995. METALLURGICALAND MATERIALSTRANSACTIONS A
and compares the results with those of a widely used carburizing steel that contains around 0.2 pct carbon in terms of grain size, case depth, and fatigue limit. II.
EXPERIMENTAL
Two kinds of low-alloy steel were used in this investigation. Chemical compositions analyzed on optical emission spectrometer (DV-4, Baird) are listed in Table I. SNCM21 is one of the most used carburizing steels that conforms to AISI 8620, and SCM4 is equivalent to AISI 4140. 4140 steel is less commonly carburized but used to produce high core strength and adequate case depth. After normalizing at 870 ~ for 4140 steel and at 920 ~ for 8620 steel, the 20-ram-diameter steel bar was machined to the shape of the specimen described in Figure 1 and finished by grinding using #1500 emery paper. Therefore, the specimen does not have the structure anomalies near the surfa
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