LAUSR

Abstract The temperature field and thermal cycling characteristics of a cast-steel matrix die of automobile crankshaft were predicted. The hot forging die was divided into three temperature regions, i.e., surfacing temperature fluctuation region (0–3 mm in thickness), near surfacing temperature gradient region (3–20 mm in thickness) and matrix temperature balanced region (above 20 mm in thickness), and their temperatures were distributed in high, medium and low-tempered temperature zones respectively. The influences of temperature distribution on the microstructure and mechanical properties of the forging die before and after service were studied. The tempered martensite of strengthened layer decomposed and the coarse grain appeared after service. The protruding part of the ribbed slab was easy to propagate fatigue crack, leading to significant decreasing of tensile strength and impact properties. The tempered martensite and lower banite increased in transition layer, the mechanical properties under high temperatures decreased obviously. The strengthening of hardened structure in weld zone was reduced and the coarse grain structure disappeared, this enabled the performance of the weld zone was more stable than the cast-steel matrix layer and the transition layer.