Fracture toughness, strain hardening and life of AISI M2 high speed steel taps

Abstract

The fracture toughness and strain-hardening characteristics of AISI M2 high speed steel were measured after a variety of heat treatments. It was found that the fracture toughness is a sensitive function of heat treatment, depending on both the austenitizing and the tempering temperature. The fracture toughness exhibited an inverse correlation with hardness. The strain-hardening rate depended on heat treatment and plastic strain. At small plastic strains the strain-hardening rate follows the behaviour of the secondary-hardening reaction. The strain-hardening rate decreases with increasing plastic strain and approaches zero for specimens tempered at 560–580°C. Tapping experiments showed that it is advantageous to use a low austenitizing temperature and to temper below the secondary-hardening peak. A linear correlation exists between tap life and (K Ic / SHR) 2 HV where K Ic , SHR and HV are the fracture toughness, the initial strain-hardening rate and the Vickers hardness respectively. (K Ic / SHR) 2 HV is believed to correlate with the difficulty that a pre-existing crack has in propagating under tapping conditions.