Fatigue Properties of Nitrided Ultrafine Ferrite-Cementite Steels under Rotating Bending Fatigue Testing
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GRAIN refinement is a logical way of strengthening steel, because this process achieves high strength without concomitantly increasing the ductile-to-brittle transition temperature. Using the latest techniques, grain sizes can be refined to below 1 lm. More specifically, several research studies have succeeded in grain refinement that yields submicron ferrite grains, starting from bulk steel.[1–3] Our institute has also established a submicron grain-refinement process using multipass warm caliber rolling.[4,5] This process yields ultrafine-grained steel bars that are thick and long enough to machine into specimens for mechanical tests. The microstructure of this ultrafine-grained steel is not ferrite-pearlite but ferrite-cementite, so this ultrafine-grained steel is termed ultrafine ferrite-cementite steel in this report. Although many kinds of mechanical properties are required of structural materials, fatigue strength is one of the most important, because it is a frequently required characteristic of mechanical components. This has prompted research into fatigue strength in ultrafine ferrite-cementite steel.[6–8] Previous studies have comY. FURUYA and H. HIRUKAWA, Senior Researchers, and S. TORIZUKA, Group Leader, are with the National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. Contact e-mail: [email protected] S. MATSUOKA, formerly Deputy Director General, with the National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan, is Professor, Department of Mechanical Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. H. KUWAHARA, Vice President, is with the Research Institute for Applied Science, Tanaka Ohicho, Sakyo-ku, Kyoto 606-8202, Japan. Manuscript submitted December 27, 2007. Article published online May 7, 2008 2068—VOLUME 39A, SEPTEMBER 2008
pared ultrafine ferrite-cementite steel with conventional tempered martensite and ferrite-pearlite steels.[9,10] Although tempered martensite steel showed higher fatigue strength than ferrite-pearlite steel, the fatigue strength of ultrafine ferrite-cementite steel matched that of tempered martensite, as seen in Figure 1. The reason for this high degree of fatigue strength is that the microstructure of ultrafine ferrite-cementite steel is not only ultrafine but also uniform. On the other hand, surface modification to harden the surface is frequently conducted in steels destined for machine structural use. Surface modification enhances both fatigue strength and wear resistance. It is therefore necessary to investigate the potential for surface modification of ultrafine ferrite-cementite steel. Typical surface modification methods are carburizing, induction hardening, and nitriding. Carburizing is, however, very difficult to apply to ultrafine ferrite-cementite steel. In this process, steels are held at high temperatures, above 1173 K, for a long time. At this temperature, transformation into austenite occurs, making it almost impossible to preserve the ultra
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