Fatigue Strength of Ultrafine Ferrite-Cementite Steels and Effects of Strengthening Mechanisms
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nement is one of the most effective methods for strengthening steel. This process increases the strength without concomitantly increasing the ductile-to-brittle transition temperature. Several researchers have recently achieved grain refinement that yields ferrite grains below 1 lm, starting from bulk steel.[1–3] Our institute has also established a submicron grainrefinement process using multipass warm-caliber rolling.[4] On the other hand, fatigue strength is one of the key properties that permit the practical use of ultrafinegrained steels, since it is a frequently required characteristic of mechanical components. Research into fatigue strength thus needs to be focused on ultrafine-grained steel. The fatigue strength of steel shows a close relationship to its tensile strength. The fatigue-limit ratio, which is the ratio of the fatigue limit to the tensile strength (fatiguelimit/tensile strength), is roughly 0.5 for steel. However, fatigue-limit ratios are affected by the details of the steel’s microstructure. For example, National Research Y. FURUYA and T. HANAMURA, Senior Researchers, and S. TORIZUKA, Group Leader, are with the National Institute for Materials Science (NIMS), Ibaraki 305-0047, Japan. Contact e-mail: [email protected] S. MATSUOKA, Professor, formerly with the National Institute for Materials Science (NIMS), is with Kyushu University, Kyushu, Fukuoka 819-0395, Japan. S. SHIMAKURA, Researcher, formerly with the National Institute for Materials Science (NIMS), is with the Hitachi Construction Machinery Co., Ltd., Hitachi, Ibaraki 300-0013, Japan. Manuscript submitted March 12, 2007. Article published online October 30, 2007 2984—VOLUME 38A, DECEMBER 2007
Institute for Metals (NRIM) fatigue data sheets[5,6] report that the fatigue-limit ratio of tempered martensite steel is 0.53, but for ferrite-pearlite steel is 0.43. A review of the research on fatigue strengthening of ferrite-pearlite steel shows the effects of strengthening mechanisms to be well established.[7–9] The major strengthening mechanisms discussed in past studies were pearlite, precipitation, solid-solution, dislocation, and grain-refinement strengthening. In these strengthening mechanisms, pearlite strengthening resulted in a lowering of the fatigue-limit ratio.[7–11] This is why ferritepearlite steels show low fatigue-limit ratios in the NRIM fatigue data sheets. Fatigue cracks are initiated in the ferrite grains, because they are softer than the pearlite grains. However, the ferrite grains are not reinforced by pearlite strengthening. Pearlite strengthening merely increases the pearlite volume fraction, leading to proportionally less fatigue strength improvement than tensile strength improvement. Similar results are reported for dual-phase steel,[12,13] in which the fatigue-limit ratios fell when the martensite phase was increased up to a 50 pct volume fraction. In contrast, precipitation and solid-solution strengthening effectively reinforce ferrite-pearlite steel without lowering the fatigue-limit ratio,[7–9,14] because the ferrit
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