Effect of stress-relief annealing on thermal fatigue cracks for die-casting die steels

The effect of stress-relief annealing at different stages of thermal fatigue tests on crack growth was investigated using a self-built thermal cycle setup. The results showed that annealing could limit crack expansion effectively. It reduced dislocation density and released the accumulation of residual stress. In addition, the strain accumulation in carbides was reduced by this process. It was also found that double annealing was even more effective at inhibiting crack expansion compared to single stress-relief annealing. After 1000 cycles, the maximum crack length was reduced by 31.1% and 45.2% for the samples after using the optimal single and double annealing processes, respectively. For single stress-relief annealing, earlier annealing provides more benefit in delaying crack expansion. However, effective double stress-relief annealing requires a suitable time interval between the annealing steps. Besides, after 800 cycles, surface hardness decreased significantly accompanied by an increase in the size and number of carbides, and cracks expanded predominantly along grain boundaries.