An Investigation of the Melt-out and Heat Checking Phenomena in Surface Modified Hot Die Steel for Aluminum Die Casting

摘要

For hot die steel, failure is mostly caused by heat checking and melt-out on its surfaces which are in contact with molten metals. In the present research, resistances to melt-out and heat checking of surface modified H13 hot die steels, such as gas nitriding (GN) and micro-blasting (MB), were investigated. The evaluation of melt-out behavior was carried out by measuring the mean depth from the original surface after immersion. To examine the thermal fatigue resistance, a cyclic thermal shock system consisting of induction heating and water spray quenching was constructed. The value of Lm is proposed as the index representing the susceptibility to crack initiation and propagation. The melt-out depth was the lowest for the GN treated surface. MB-GN and MB-GN-MB treated specimens also showed good resistance to melt-out. In the case of GN treatment, while the white layer was completely melted out, the diffusion layer still existed even after immersion for 43 hours. This implies that nitriding significantly reduced the rate of melt-out. From the total crack number, it is seen that crack initiation was reduced to the extent of half after surface treatment. This result means that the residual compressive stress and nitrided layer were beneficial to crack initiation resistance.