Effect of Prior Cold Work on the Martensite Transformation in SAE 52100
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Numerous publications refer to the phase transformations and properties of SAE 52100 steel, and this paper concerns itself with the effect of prior cold deformation on the martensitic hardening response. The Acl and Ac3 temperatures are lowered due to cold work as is the Ms with a resultant increase in the retained austenite content for a given hardening cycle. Significantly, the prior cold deformation results in a refinement of the austenite grain size. The low angle dislocation cells produced by the cold deformation recover during the heating to the austenitizing temperature to form fine ferrite subgrains. The intersections of the fine ferrite subgrains with the spheroidal carbides in the soft annealed microstructures are preferential sites for nucleation of austenite. This results in finer austenite grains, which produces accelerated carbide dissolution and austenite alloy enrichment compared to unworked, soft annealed structures. The mechanism for the accelerated austenitization is significant in predicting heat treatment response from published phase transformation data for SAE 52100 steel.
I.
INTRODUCTION
S A E 52100 steel is used for practically all through hardened bearings produced today, and is a very versatile tool steel. As with most hypereutectoid tool steels, high hardness and yield strength is achieved by austenitization such that only the required amount of alloy is taken into solution by partial carbide dissolution before martensitic quenching or isothermal transformation to bainite. The undissolved carbides in the austenite prevent excessive austenite grain growth. The phase transformations occurring during the hardening of this steel, and the resultant microstructure, have been the subject of numerous investigations. Stickels 1'2 has published a comprehensive paper on the influence of the spheroidal carbides on the heat treatment response, and Orlich 3 has documented the phase transformation temperatures. Additional information on the phase transformations in SAE 52100 is available also in References 4 to 8. The influence of heat treatment on the yield strength has been published by Schlicht and Zwirleing, 9 and Stickels 1~ has measured the flow stress and strain hardening coefficients. Data on the fracture toughness properties after various heat treatments are published by Kar et al. 11 and Averbach. 12 Because of the technological importance of this steel in engineering applications, many more publications are available on the heat treatment and phase transformations in this steel, but are not referenced specifically in this paper. All of the known published data for SAE 52100 on the hardening response related to the prior microstructure 13 refers to material where cold deformation prior to hardening is not a specific variable. In bearing manufacture, for example, this type of steel is widely used in the cold worked condition, particularly as tube and wire from which components are produced by machining or forming, and in some cases for further cold forming processes such as cold heading in
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