Formation of white layers in steels by machining and their characteristics
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I. INTRODUCTION
IT is well known that a thin, hard layer, which appears to resist etching, is sometimes formed on steel surfaces subjected to machining,[1–5] abrasion,[6,7,8] or sliding.[9,10,11] Such a surface layer has also been produced in steel by impact loading of specimens in a Charpy machine, highspeed punching and blanking,[12,13] explosive forming,[14] and even by pressing an oversized ball bearing through a hole at high velocity.[15] The layer, typically less than 50 m in thickness, is often described as being hard and brittle. Because of its “featureless” appearance under the optical microscope, even after exposure to chemical attack by etchants that are commonly used for developing steel microstructures, the layer has been labeled as a “white etching layer” or simply a “white layer” (WL). The formation of a WL during machining or high-speed sliding has sometimes been attributed to the steel’s surface being heated to above the austenitizing temperature during the process, followed by rapid self-quenching of the surface by the bulk of the material.[7,8] It is possible that such a phase transformation may also cause localization of plastic flow.[16] Litmann and Wulff[7] used thermocouples to measure the surface temperature of 52100 steel during grinding and showed that a WL could be observed whenever the surface temperature exceeded the austenitizing temperature. However, they noted S. AKCAN, formerly with the Center for Materials Processing and Tribology, Schools of Engineering, Purdue University, is with GE Medical Systems, Milwaukee, WI. S. SHAH, formerly with the Center for Materials Processing and Tribology, Schools of Engineering, Purdue University, is with GE Superabrasives, Worthington, OH. S.P. MOYLAN and S. CHANDRASEKAR are with the Center for Materials Processing and Tribology, Schools of Engineering, Purdue University, West Lafayette, IN 47907-1287. Contact e-mail: [email protected] P.N. CHHABRA, formerly with the Center for Materials Processing and Tribology, Schools of Engineering, Purdue University, is with Applied Materials, Santa Clara, CA. H.T.Y. YANG is with the Department of Mechanical and Environmental Engineering, University of California, Santa Barbara, CA 93106. Manuscript submitted April 17, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
that their experiments by no means established that austenitization and subsequent quenching was a prerequisite for WL formation. The WLs have also been observed on the crests of corrugated steel-rail tracks and wheels, where the contact temperature has been estimated as being well below the austenitization temperature, and there is little evidence of flow localization.[10] Knoop and Vickers hardness measurements made on WLs suggest that they are harder than the bulk material, even when the bulk microstructure is martensite.[2,3,4] However, these hardness measurements are misleading because the indentation size, even at low loads (20 to 50 g), used to make the indents was much greater than the WL thickness. Furthermore, at low loads, there i
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