Study On Thermal Fatigue Property Of The Cast Hot-work Die Steel

Thermal fatigue is one of the main failure patterns for the cast hot-working dies and an important factor to limit the life of hot-working dies. Thus it is very important to improving thermal fatigue resistance of the cast die steels, in order to raise the life of dies and widen the application of the cast hot-wok dies. By using the self-restricting test method, the thermal fatigue properties of the cast die steels with different compositions of C, Cr, Mo, V and addition of micro-elements Ti, Nb under different tempering temperature were studied. The thermal fatigue characteristic of the cast hot-work die steel and the initiation and propagation mechanism of thermal fatigue cracks were also investigated. All of these provide a theoretical basis for alloy composition optimization and heat treatment process of the cast hot-work die steel.The results show that the thermal fatigue property of the cast hot-work die steel is markedly influenced by alloy elements. The optimal compositions of C, Cr, Mo, V and addition of Ti, Nb can improve the thermal fatigue resistance of the cast steel obviously. The cast steel containing 3%C possesses the highest resistance to thermal fatigue. Extra high C content would deteriorate the thermal fatigue property. As tempering at 440℃, Cr, Mo inα-Fe matrix can increase heat stability of the cast steels, but as tempering at 600℃, excessive Cr, Mo in steel would result in precipitation of Cr₂₃C₆, and Mo₆C. In the cases, the thermal fatigue resistance of the cast die steel should be reduced substantially. Whether tempering at 440℃or at 600℃, the thermal fatigue property of the cast steel with V/C=3.0 is the best because of the dispersed precipitation of VC in the tempered structure. It is found that the thermal fatigue of the cast hot-work die steel is high-cycle thermal fatigue. The main factor affecting thermal fatigue resistance is heat strength and thermal stability. The thermal fatigue cracks of the cast die steel initiate at the boundary between carbides and matrix or at the grain boundary. The propagation paths of the thermal fatigue cracks are intergranular or transgranular. After the optimization of alloy composition, the thermal fatigue resistance of the new cast hot-work die steel is obviously higher than that of H13 steel, and near to the thermal fatigue property of 8407 steel.