Microstructural Refinement of Bainite and Martensite for Enhanced Strength and Toughness in High-Carbon Low-Alloy Steel
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INTRODUCTION
CONVENTIONALLY quenched and tempered martensitic SAE 52100 steel is a very popular grade of high-carbon low-alloy steel for various structural applications, mostly as small, medium, and large ball and roller bearings for automobile applications and also for medium and heavy duty engineering components.[1] The strength of the martensitic matrix coupled with the high amount of primary alloy carbides provides resistance to both abrasive and adhesive wear encountered in bearing assemblies. However, high strain and strain rate loads of cyclic nature warrant greater tensile strength and toughness.[2] Recently, we have demonstrated that bainite-martensite duplex microstructure obtained through austempering followed by quenching route yields superior mechanical properties as compared to that developed by conventionally quenched and tempered martensitic structure in SAE 52100 steel without/ with prior cold deformation.[3,4] The present study aims to further increase the scope and extent of improving strength and toughness of this steel by aiming to refine the dimension (length or thickness) of bainitic sheaves and martensite plates through controlled cold deformation prior to austenitization and J. CHAKRABORTY, Research Scholar, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, W.B. 721 302, India, and Manager-Metallography, Scientific Services and Research and Development, Tata Steel, Jamshedpur 831 001, Jharkhand, India. P.P. CHATTOPADHYAY, Professor, is with the Department of Metallurgy and Materials Engineering, Bengal Engineering and Science University (BESU), Sibpore, Howrah 711103, India. D. BHATTACHARJEE, Director, is with Scientific Services and Research and Development, Tata Steel. I. MANNA, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology. Contact e-mail: [email protected] Manuscript submitted October 11, 2009. Article published online July 21, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
partial austempering followed by quenching to room temperature. Young and Bhadeshia[5] asserted that the presence of a soft phase embedded in a hard matrix is conducive to enhance toughness in steel. The conventional Hall–Petch relationship coupled with the effects of both substitutional and interstitial solid solution strengthening, work hardening, and second-phase spacing is summarized by the following equation: X 1=2 rss þ K1 ðL3 Þ1 þK2 qd þ K3 D1 r ¼ rFe þ rc þ ½1 where rFe is the strength of pure annealed iron; rc is the contribution P of solid solution strengthening due to carbon; rss is the sum of contributions to solid solution strengthening from all substitutional solutes; L3 is the ferrite plate thickness; qd is the dislocation density; D is the average distance between primary carbide particles; and K1, K2, and K3 are material constants. In order to reduce L3, an attempt has been made to decrease the prior austenite grain size by austenitization preceded by varying degrees o
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