Analysis of wear of a gas nitrided H13 tool steel die in aluminium extrusion

Mechanisms responsible for the failure of an actual gas nitrided extrusion die were investigated. The compound layer is entirely removed during the extrusion process. The die channel surface is characterized by a heavy wear zone near the inlet with cavities as deep as 50 μm. The bearing channel had suffered only a mild wear beyond this heavy wear zone. The shear forces acting on the bearing channel during extrusion produced numerous surface cracks. The fracture strength of the tool steel is exceeded once these cracks reach a critical size. A wear fragment is then detached from the bearing surface. This process is encouraged further by the embrittlement across the diffusion layer due to the multiple nitriding operations. The brittle network of nitrides along the grain boundaries also plays a key role. The entrapment of the flowing Al alloy in the wear cavities promotes the fragmentation of the surface layer. Hard nitrided wear debris particles thus released provide a ploughing action and lead to the abrasion of the bearing channel in the extrusion direction.