タップ用高速度工具鋼の曲げーねじり組合せ負荷下の強度特性

摘要

タップは，形状が細長く，刃先から柄部にかけてねじと溝部が設けられている．切削中は，先端に設けられた食付き部の全切れ刃で同時に加工するため，大きなねじれと下穴の曲がりや芯ずれによる曲げが同時に働く．従って，超硬合金より抗折力の高い高速度工具鋼が主に用いられている．近年，高強度を有し，耐摩耗性に優れている高速度工具銅は，切削工具以外に冷間鍛造型や機械部品の摺動部材への適用も増えてきている．%Combined loading tests were carried out on secondary hardened SKH51 to examine strength characteristics under combined bending and torsion loading. Tempering temperature at 560℃ after quenching temperature at 1200℃ 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.