In-Situ X-Ray Investigations and Computer Simulation during Continuous Heating of a Ball Bearing Steel
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TION
AT the end of manufacturing chains, engineering components are often subjected to heat treatments, e.g., quench hardening to obtain the desired properties. However, in almost all cases, these treatments lead to undesirable dimensional and shape changes. The origins of these distortions are associated with the entire manufacturing history and depend on the componentsÕ geometry, local chemical composition, and microstructure as well as the generation and relaxation of residual stresses.[1] The study of these phenomena by experimental investigations as well as by computer simulation is the purpose of the Collaborative Research Center SFB 570 ‘‘Distortion Engineering’’ of the University of Bremen.[1–3] The austenitizing state before quenching is important for the prediction of steel component distortions during heat treatment cycles. Therefore, a good knowledge of microstructure evolutions throughout the austenitizing process is required.[3] X-ray diffraction is a nondestructive method for in-situ investigation of phase transformations, stress, and texture changes in near surface regions of samples and components. In this study of phase transformation kinetics during continuous heating to austenitizing temperature, a comparison between in-situ X-ray diffraction and computer simulation based on dilatometer tests was made. Lattice parameter changes during heating in comparison with simulation were also investigated. For the study of transformation kinetics, one essential requirement is an JE´RE´MY EPP and HOLGER SURM, Research Engineers, and THOMAS HIRSCH, Research Director, are with the Foundation Institute of Materials Science, IWT Bremen, 28359, Bremen, Germany. Contact e-mail: [email protected] OLAF KESSLER, formerly with the Foundation Institute of Materials Science, IWT Bremen, is now Professor of Materials Science at the University of Rostock, Rostock, Germany. Manuscript submitted March 13, 2007. Article published online September 13, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
acquisition rate high enough to receive reliable information in comparison with the kinetic rates. A high time resolution during X-ray diffraction is also necessary, and therefore, special instrumentations for use at elevated temperatures are required.[4] Thus, a rotating anode and a position-sensitive area detector as reported in a previous study were used for these investigations.[5] II.
EXPERIMENTAL
A. Material For the X-ray experiments, rods of AISI 52100 (EN 100Cr6) ball bearing steel were machined to discs with diameter of 22 mm and thickness of 2 mm. Samples of 4 mm in diameter and 10 mm in length were manufactured for the dilatometer tests. Their chemical composition is given in Table I. The steel rods were submitted to a spheroidizing heat treatment before machining to obtain globular carbides and, therefore, to improve their machinability. This treatment involves the following steps: heating up to 800 C; soaking at 800 C for 2 hours; cooling to 710 C in 5 hours; soaking at 710 C for 20 minutes; and slow cooling to ro
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