A Comparison of Mechanical Properties and Hydrogen Embrittlement Resistance of Austempered vs Quenched and Tempered 4340
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INTRODUCTION
A. Practical Significance and Study Purpose
Q uenched and tempered (Q&T) processing of medium-carbon steels is the most common form of hardening and strengthening heat treatment. As a result, many researchers have extensively characterized the common properties and problems of Q&T steels, e.g., environmental embrittlement susceptibility under certain conditions. The use of austempered steels is less common, but these steels offer advantages over Q&T steels in some applications that require limiting distortion and residual stress. Early work also showed that austempered steels with lower bainite microstructures offer improved Charpy impact toughness over their Q&T counterparts with predominantly martensitic microstructures. Figure 1, from the ASM Handbook, shows that austempered steels exhibit moderately (but significantly) lower Charpy V-notch (CVN) impact ductile-to-brittle transition temperatures (DBTTs) and higher upper shelf energies (USEs) at comparable strengths.[1] Since austempered steel has greater toughness than Q&T steel in the absence of embrittlement, it is JOHN M. TARTAGLIA, Engineering Manager and Senior Metallurgical Engineer, and KRISTEN A. LAZZARI and GRACE P. HUI, Engineering Associates, are with Stork Climax Research Services, Wixom, MI 48393. Contact e-mail: [email protected]. KATHY L. HAYRYNEN, Technical Director, is with the Technologies Division, Applied Process, Inc., Livonia 48150, MI. Manuscript submitted January 23, 2007. Article published online February 1, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
reasonable to investigate whether austempered steel also has greater toughness when subjected to environmental conditions that deleteriously affect Q&T steel. After conducting a comprehensive literature search, the present authors found some articles that suggested that austempered steels might have greater hydrogen embrittlement (HE) resistance than their Q&T counterparts, but the comparisons were limited, for various reasons. For example, Chuang et al.[2] reported that the fatigue crack growth rates of Q&T 4140 steels were greater than those in austempered samples; however, the experimental steel they used was exceptionally dirty (0.093 pct S) and they conducted their Q&T heat treatments in the blue-brittle range of 4140 steel, which caused their Q&T samples to be temper embrittled in the absence of hydrogen exposure. Reynolds and Hayes[3] reported that austempering exhibited less HE than Q&T heat treatment in the bend testing of spring steel and Lantsman et al.[4] reported similar results in a brief note. The present study represents the first straightforward comparison, in a widely available and applicable open publication, of the HE resistance of a low-alloy steel that was heat treated by two different methods: austempering vs quench and tempering. A secondary benefit of this study is that other properties of austempered steel were determined and openly published for the first time. B. Heat-Treatment Effects on Microconstituent Formation and Toughness It is worthwhile
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