Fatigue of cold-work tool steels: Effect of heat treatment and carbide morphology on fatigue crack formation, life, and
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3/12/04
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Fatigue of Cold-Work Tool Steels: Effect of Heat Treatment and Carbide Morphology on Fatigue Crack Formation, Life, and Fracture Surface Observations KENZO FUKAURA, YOSHIHIKO YOKOYAMA, DAIEN YOKOI, NOBUHIRO TSUJII, and KANJI ONO The fatigue properties of two types of cold-work tool steels tempered at various temperatures were evaluated. The microstructure and fracture surface morphology were correlated to the fatigue behavior. Cold-work tool steels using this study were a conventional tool steel (JIS SKD11; 1.4C-11Cr-0.8Mo-0.2V) and its modified steel (M-SKD11; 0.8C-8Cr-2Mo-0.5V). The fatigue strength of the M-SKD11 steel increased 20 pct over that of the SKD11 steel for any number of cycles. This is attributed to the refinement of primary M7C3 carbides. These M7C3 carbides fractured during fatigue and were found at the sites of fatigue crack initiation. Change in crack initiation behavior was confirmed by acoustic emission testing. The S–N curves of the steels are similar to those of most structural steels. However, the subsurface fatigue crack initiation was dominant at lower alternating stresses. This study points to a general approach of carbide refinement that can be used for the enhancement of fatigue properties.
I. INTRODUCTION
IN recent years, metal working dies have been subjected to more severe environment, because of the necessity of working with high-strength materials for the automobile and electronics industries.[1,2,3] Widespread use of near-net-shape processing requires the maintenance of close tolerances throughout the life of the dies. Consequently, the life of typical cold-work tool steels has been noticeably shortened.[4,5,6] Fatigue life was also reduced from an increasing number of stress concentration sites, from high die pressures, and from chipping due to impact loading.[7,8] Therefore, a strong need exists to develop low-cost tool steels with high strength and toughness as well as high resistance to fatigue.[9] Numerous studies exist on fatigue properties of high-strength steels, such as high-speed tool steels, bearing steels, and coldwork tool steels. These generally show that nonmetallic inclusions initiate fatigue fracture.[8,10–15] However, studies on effects of carbides on fatigue behavior have been limited.[16,17,18] These reports suggest that fractured carbides or the debonding at the carbide-matrix interface can initiate fatigue, acting similarly to cracked inclusions. The roles of carbides should, thus, be explored in order to develop fatigue-resistant tool steels. We have completed an alloy development project for an improved cold-work tool steel. The goals of this project were to increase the strength, toughness, wear resistance, and resistance to temper softening. The last characteristic is important since surface hardening treatments such as carbonitriding and ion-nitriding are applied on tool-steel dies.[4,19] The starting KENZO FUKAURA, Professor, and YOSHIHIKO YOKOYAMA, Assistant Professor, are with the Department of Materials Science and Engi
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