Investigation of die for pressure die-casting of Mg-alloy

A die was investigated for the pressure die-casting of Mg-alloy. The die was made of martensitic chromium hot-work steel grade and the surface was gas nitrided. Due to damaged surface the die was removed from the production line after 20.000 shots. Light microscopy and electron microscopy studies were performed. Mechanical properties as well as fracture toughness of the basic material were determined. The results of the investigations are presented. It was established that the base material properties correspond to declared properties. The corrosion phenomena in the form of pits were observed on all surfaces of the die. Metallography revealed that the nitrided layer consisted only of diffusion layer, without a γ′ compound layer. The depth of the pits was limited to the thickness of nitrided layer and in the pits the presence of Ca, Cl and S was detected. No cracks were observed in nitrided layer and all the edges of pits were rounded. The damages were caused by a combined mechanism of corrosion–erosion. The main reasons for the damages were not optimised nitride layer properties.