Thermally activated relaxation behaviour of shot-peened tool steels for cutting tool body applications

Surface residual stress, microstrain and domain size were determined using X-ray diffraction in shot-peened low-alloyed and hot-work tool steels subjected to varying temperatures (200–600 °C) and times (0.5–50 h). The stress relaxation was directly related to microstrain decrease and domain size increase.The Zener–Wert–Avrami function was used to model the relaxation behaviour. The mechanism responsible for the relaxation behaviour is proposed to be diffusion-controlled as the activation energies for stress and microstrain relaxation were close to the activation enthalpy for lattice self-diffusion of alpha-iron.The relaxation rate varied over time and temperature. At temperatures up to 300 °C, the relaxation rate was insignificant in all the steels. In the temperature range of 300–400 °C and 400–550 °C for low-alloyed and hot-work tool steels, respectively, the stresses decreased at a similar rate with increasing time and temperature. At the tempering temperatures of the steels, the stress relaxation was controlled by microstructure transformations. A stress relaxation stabilization was found in one of hot-work steels between 450 and 500 °C at annealing times longer than 3 h.