Thermal fatigue resistance of plasma duplex-treated tool steel

Thermal fatigue represents one of the most important stresses encountered in technical applications like aluminum pressure die casting and hot forging. The resulting damage is the well-known heat checking phenomenon, characterized by nucleation of a fine cracks network on die and insert surfaces. Usually, many other detrimental stresses, i.e. wear, erosion–corrosion by the molten aluminum and oxidation act during service, thus suggesting the introduction of protective surface treatments. The beneficial effect of nitriding, PVD-coating and, more recently, of their combination in duplex systems, in counteracting these stresses has been proved. On the contrary, the coating influence on thermal fatigue resistance is not yet clearly established. In this paper the influence of plasma nitriding, multi-layer PVD coating (i.e. CrN, ZrN) and duplex treatment (plasma nitriding and PVD) on the thermal fatigue resistance of AISI H11 hot work tool steel was studied. For this purpose a specific experimental apparatus, based on induction heating and water cooling, has been realized. Particular attention was given to plasma nitriding, used as pre-treatment in duplex plasma surface engineering. The role of compound layer, diffusion layer and coating(s) is discussed on the basis of the damage observed during the test.