タップ用高速度工具鋼の曲げ—ねじり組合せ負荷下の強度特性

概要

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Combined loading tests were carried out on secondary hardened SKH51 to examine strength characteristics under combined bending and torsion loading. Tempering temperature at 560°C after quenching temperature at 1200°C was chosen as heat treatment condition. The tests were conducted using torsion testing machine with a special chucking device which enabled to change the stress ratio of bending and torsion. The stress-strain curves were derived from equivalent torsional moment, and the yield and fracture surfaces were analyzed by applying Hill's quadratic function. The yield and fracture surfaces lay inside Tresca's surface. The direction of plastic strain increment did not obey the normality rule. It was revealed by SEM and TEM that bending strain included displacements due to crack and interfacial debonding of carbide. Static toughness increased with increasing shear stress component. The variation of static toughness with stress ratio was divided into three regions on the basis of fractographic fracture aspects; the brittle, the brittle-ductile transition and the ductile regions.