Experimental investigation on the influence of surface engineering on thermal fatigue of a hot work tool steel

Thermal fatigue is an important life-limiting factor in die casting moulds. It is observed as a network of fine cracks on the surface exposed to thermal cycling. The crack network degrades the surface quality of the tool and, consequently, the surface of the casting. Surface engineered materials are today successfully utilised to improve the erosion and corrosion resistance. However, their resistance when exposed to thermal cycling is not fully understood. In this work, surface treatments (boriding and Toyota diffusion to give CrC) and physically vapour deposited (PVD) coatings of CrN, as single-layered or duplex-treated (nitriding followed by PVD coating), on hot work tool steel specimens were compared with the untreated reference material by experimental simulation of thermal fatigue. The test is based on induction heating and surface strain measurements by a non-contact laser speckle technique, which enables studies of the surface strain during thermal cycling. Thermal fatigue cracking of a surface engineered tool steel is influenced by the modification of the mechanical properties of the substrate which occurs during the engineering process. With the exception of duplex-treatment, all variants of surface engineering show a tendency to decrease the resistance to thermal fatigue cracking as compared to the reference steel. However, the fact that the duplex-treated PVD CrN coating increased the resistance to thermal fatigue cracking as well as reduces the density of cracks as compared to the single-layered CrN coating, the potential to improve the life and performance in for example die casting applications still prevails.