マルテンサイトおよびマルテンサイト/ベイナイト基地組織球状炭化物鋳鉄の疲労挙動

抄録

The objective of the present study is to understand the fatigue behaviour and fracture mechanisms of cast irons with spheroidal vanadium carbides dispersed within martensitic matrix microstructure. At first, rotary bending fatigue tests have been performed using smooth specimens of cast irons with two different matrix microstructures: fully martensitic microstructure and martensitic/bainitic microstructure. It was found that there existed a considerable scatter, but the former showed higher fatigue strength than the latter. Fatigue cracks invariably initiated from casting defects in both cast irons, and thus their fatigue strengths depended strongly on casting defects. Then, additional fatigue tests have been conducted using a cast iron with fully martensitic microstructure whose castability was improved by varying chemical composition. Fatigue cracks were still generated from casting defects, but the fatigue strength was significantly high with a small scatter because the casting defect sizes were very small compared with the cast irons described above. The √area parameter model was applied to predict the fatigue limit of those cast irons. Although the maximum casting defect sizes estimated from the statistics of extremes were considerably smaller than the sizes of the casting defects from which the crack initiated, the √area parameter model gave a reasonable prediction of fatigue limit.