Improvement of gigacycle fatigue properties by modified ausforming in 1600 and 2000 MPA-class low-alloy steels
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IN mechanical engineering, high-performance and highstrength steels are required to make machines and their components lighter, reduce CO2 emissions, and save natural resources. Gigacycle fatigue properties[1–16] are a key index of performance of high-strength steels in practical applications. High-strength steels often fail above 107 cycles due to internal defects in the materials, as shown in Figure 1,[4] whereas low- or medium-strength steels fail only from the surface. Fatigue failure in the superlong-life region is called gigacycle fatigue. Fatigue failure from internal defects is called a fish-eye fracture.[1–12] In the case of surface fractures only, the fatigue limit (W) has a well-known relationship to the tensile strength (B), as shown in Eq. [1]: W ⫽ 0.5B
[1]
In the case that fish-eye fractures occur, however, the fatigue limits are much lower than those predicted by Eq. [1], as shown in Figure 2.[4] The fatigue sites of fish-eye fractures are mostly nonmetallic inclusions[1–9] and, occasionally, internal facets.[10,11] The internal facet–originated type of fish-eye fracture is considered to be caused by nonuniformly quenched structures such as bainite.[11] Recently, Murakami et al.[6–9] Y. FURUYA, Researcher, and S. MATSUOKA, Sub-Group Leader, are with the 2nd Sub-Group of Strength and Life Evaluation Research Group, Materials Engineering Laboratory, National Institute for Materials Science, Ibaraki 305-0047, Japan. Contact e-mail: [email protected] Manuscript submitted January 23, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
reported that optically dark areas (ODAs) were observed by optical microscope observation around the inclusions on the fracture surfaces, and that ODA formations were related to hydrogen. The ODA sizes[6,7,16] show a close relationship with the number of cycles to failure and are about 2 to 3 times greater than the inclusion sizes in superlong-life regions. Murakami also suggested that ODAs should be regarded as defects when estimating fatigue strength, and that the defects grow with hydrogen assistance in proportion to the number of cycles. It means that the outer region of the ODA is a normal fatigue-crack-growth area and the inner region is an abnormal one. The abnormal fatigue-crack growth would need a large number of cycles and hydrogen assistance. Our research group is trying to improve the performance of high-strength steels that achieve high fatigue strength. The main target is the avoidance of fish-eye fractures leading to high fatigue limits of the surface-type fractures predicted by Eq. [1]. Improvement of surface-type fracture properties is also our target: we aim to achieve higher fatigue limits than those predicted by Eq. [1], such as W ⫽ 0.6B . Our research is based on the following two approaches: (1) controlling inclusion properties, and (2) applying modified ausforming. Deformable inclusions are lengthened by rolling, during which their diameter is reduced.[5] In high-strength steels, defect sizes have a strong influence on fatigue stre
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