Predictive Analysis of Low Cycle Fatigue of Tool Steel in Warm Working Conditions

Synopsis

Cyclic deformation behaviour and fatigue life of hardened and tempered high speed tool steel SW7M under low cycle fatigue conditions are studied at temperatures ranging from 20 to 650°C. The stress response during isothermal cycling shows three stages, primary hardening or softening, pseudostabilized behaviour, and secondary softening. Cyclic stress-strain curves are described mathematically and modulus of cyclic elasticity is introduced as an important parameter in calculating stress-strain field in warm working tools. Fatigue lives vary considerably over the range of test temperature and initial hardness of specimen. High fatigue strength at 350 and 500°C is explained by dispersion hardening of matrix. A considerable decrease in fatigue strength is observed in 650°C. Heating and cooling devices adequate for controlling working temperature of tool are very important to obtain the highest low cycle fatigue resistance of tool steel. SEM observations on the fracture surfaces of specimens reveal that the fracture proceeds by complex fracture mode.