Abstract: The lifetime of hot-work tools is often limited by the development of thermal shock cracks. To analyze the influence of near-surface materials states on the damage process during thermal shock loading, isothermal annealing experiments and thermal shock tests with hot-work tool steel AISI H11 in hard turned, electroeroded, shot-peened or deep rolled surface conditions were performed. Cyclic thermal shock tests were carried out by heating and subsequent quenching of specimens. Crack depths and crack densities were investigated after 100, 1000, 10000 and 30000 cycles at microsections using optical microscopy. Complementary, residual stresses were determined by X-ray diffraction-(XRD) stress analysis. Under identical thermal loading conditions characteristic cracking behaviour was observed at different surface types examined. Neither deep rolling nor shot-peening results in decreasing crack lengths compared to hard turned surface conditions. While the residual stress state during simple annealing is proportionally stable, almost all initial residual stresses relaxed during a few hundred thermal shock cycles. Additional Martens-hardness measurements and FWHM values of XRD analyses indicate that the work hardening induced by mechanical surface treatment is not stable under thermal shock loading.

Key words: Thermal shock, Residual stress, Hot-work tool steel