Inclusion-controlled fatigue properties of 1800 MPA-class spring steels
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I. INTRODUCTION
INCLUSIONS cause fish-eye fracture in fatigue of highstrength steels. The fish-eye fracture is an internally originating type of fatigue failure, which eliminates the fatigue limit of highstrength steels. Hence, inclusions decrease the fatigue strength of high-strength steels and cause giga-cycle fatigue,[1–9] i.e., fatigue failure in the long life region above 107 cycles. The major types of inclusions decreasing fatigue strength are oxides, nitrides, and sulfides. In most cases, the oxides are Al2O3 or Al2O3CaO inclusions, the nitrides are TiN inclusions, and the sulfides are MnS inclusions. The Al2O3 or Al2O3CaO inclusions are generally globular and the inclusionmatrix interface is debonded at the initial stage of fatigue.[10] Therefore, the Al2O3 or Al2O3CaO inclusions are similar to globular voids and only the size becomes the limiting factor in the fatigue strength of the steels. In contrast, the shape and properties, as well as the size, affect the fatigue strength in the case of the TiN inclusions, since the TiN inclusions are harmful in spite of the small size.[11] In early studies,[12,13] the main reason why the TiN inclusions were detrimental was believed to be due to high stress concentration because of their cubic shape. Recent research,[14] however, has shown that the elastic modulus of the TiN inclusions also plays an important role. In the case of TiN inclusions, the inclusionmatrix interface is hardly debonded, and in most cases, the TiN inclusions are cracked themselves before they initiate a fatigue crack. When the inclusions are tightly bonded to the matrix, the stress state around the inclusions depends on the
Y. FURUYA, Researcher, and T. ABE, Senior Researcher, Fatigue Group, and S. MATSUOKA, Deputy Director General, are with the Materials Information Technology Station, National Institute for Materials Science, Ibaraki 305-0047, Japan. Contact e-mail: [email protected] Manuscript submitted February 10, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
difference in the elastic modulus between the inclusion and the matrix.[15] The MnS inclusions, which are deformed during hot rolling, are generally believed to be less harmful. This type of inclusion becomes elongated during hot rolling and its effective size is reduced in the rolling direction (RD). Even elongated inclusions, however, cause fish-eye fractures if stress is applied in the transverse direction (TD), since the inclusion size is still large in that direction. In fact, Toyama et al.[16] have reported that elongated slender MnS inclusions caused fish-eye fractures in repeated compression fatigue tests using ring-shaped specimens of bearing steel. In the case of such deformable inclusions, anisotropy of fatigue strength has to be taken into account. Based on the preceding context, minimizing the inclusion sizes is an effective strategy to improve the fatigue strength of high-strength steels. To minimize the size of oxide-type inclusions, the application of an inclusion-control technique[17] could be one
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