Thermal fatigue cracking of surface engineered hot work tool steels

Abstract

Thermal fatigue cracking is an important life-limiting failure mechanism in die casting tools. It is observed as a network of fine cracks on the surfaces 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 applied to improve the erosion and corrosion resistance. However, their resistance against thermal fatigue is not fully explored. In this work, a selection of hot work tool steel grades was surface modified and experimentally evaluated in a dedicated thermal fatigue simulation test. The surface modifications included boriding, nitriding, Toyota diffusion (CrC), and physical vapour deposition (PVD) of coatings (CrC, CrN and TiAlN), both as single-layers and deposited after nitriding (duplex treatment). Untreated specimens of each tool steel grade were used as references. The test is based on cyclic induction heating and internal cooling of hollow cylindrical test rods. The surface strain is continuously recorded through a non-contact laser speckle technique. Generally, all surface treatments decreased the resistance against surface cracking as compared to the reference materials. The reason is that the engineering processes influence negatively on the mechanical properties of the tool materials. Of the processes evaluated, duplex treatment was the least destructive. It gave a lower crack density than the reference steel, but the diffusion layer is more susceptible to crack propagation. In addition, the single-layered CrN coating showed almost comparable thermal fatigue cracking resistance as the reference material. Finally, the resistance against thermal crack propagation of surface engineered tool steels is primarily determined by the mechanical properties of the substrate material.