Interdependence of Distortion and Residual Stress Relaxation of Cold-Rolled Bearing Rings During Heating
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ODUCTION
THE distortion of bearing rings is still a problem during manufacturing and makes expensive grinding operations necessary to give these rings their final dimensions and shape.[1] The distortion is connected to the whole manufacturing history and depends on the components’ geometry, chemical composition, local variation, microstructure, and phase transformations, as well as the generation and relaxation of stresses.[1–6] The study of these phenomena by experimental investigations and computer simulation is the main objective of the Collaborative Research Centre course SFB 570 Distortion Engineering at the University of Bremen.[1–7,9–12] Residual stresses are well-known influencing factors in the distortion of parts, and their effects have been studied for decades.[2] Prior studies have shown the influence of machining steps and their parameters on the residual stress fields introduced at the surface of rings.[7–11] These results point out that more than the level of residual stresses, the inhomogeneity of their distributions within a ring is the deciding parameter concerning the resulting distortion. Furthermore, the relief of the machining-induced residual stresses during heat treatment is of great importance.[8,11] The JE´RE´MY EPP, HOLGER SURM, JULIEN KOVAC, Research Engineers, THOMAS HIRSCH, Research Director, and FRANZ HOFFMANN, Professor, are with the Foundation Institute of Materials Science, Bremen 28359, Germany. Contact e-mail:epp@ iwt-bremen.de Manuscript submitted July 14, 2010. Article published online November 17, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
production of cold-rolled bearings is used widely, but the interactions between residual stress states and distortion behavior still need to be clarified. In this study, the bearing rings produced by cold rolling have been investigated. Their behavior after heating at different temperatures in terms of distortions, residual stress state changes, and microstructural changes have been studied to identify the active mechanisms responsible for the observed distortions.
II.
EXPERIMENTAL
A. Production of the Rings The rings with an outer diameter of B140 mm, an inner diameter of B126/121 mm, and a height of 33 mm were produced from hot-rolled tubes of steel grade AISI 52100. After rolling, the tubes were submitted to a spheroidizing heat treatment. Afterward, a turning operation was used to obtain blanks with defined mass for the final cold-rolling process. The size and shape deviations after cold rolling were in a small scatter band: outer radius: 70.180 ± 0.014 mm; inner radius: 63.304 ± 0.011 mm/60.912 ± 0.011 mm; outer roundness deviation: 19 ± 5 lm; inner roundness deviation: 16 ± 5 lm. The chemical composition is given in Table I. B. Heat Treatments The bearing rings were heated individually in a multiprocess bell-type furnace, type SOLO, to different VOLUME 42A, MAY 2011—1205
Table I. C 0.95
Chemical Composition in Wt Percent of Steel AISI 52100 (EN 100Cr6)
Si
Mn
P
S
Cr
Mo
Ni
Al
Cu
0.069
0.257
0.0093
0.0076
1.45
0.048
0.132
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