Failure analysis of forging die insert protected with diffusion layer and PVD coating

• Failure of plasma nitrided and PVD hard coated hot-forging die insert in drop forging hammer
• Metallographic investigation of the die surface showing surface spalling
• Cracks appearing in the surface coating and diffusion layer
• Crack modes and lengths depend on the local thermomechanical loading.
• Local thermomechanical loading determined by computational analyses

The forging die used in a hot forming operation on a hydraulic drop hammer is often exposed to high thermal and mechanical loadings where the service life of the die insert is mainly limited by wear and surface cracking. The surface of the die should thus be appropriately hardened through surface preparation and treatment to reach the required mechanical properties for improved wear and cracking resistance.This paper summarizes an investigation of the plasma nitrided and PVD hard coated hot-forging die insert performance on a drop hammer in real industrial environment. The depth of nitrided diffusion layer varied between 190 and 210 μm while the depth of hard PVD coating was approximately 2.5 μm. The metallographic investigation of the die insert surface for penultimate forging operation showed significant surface spalling, caused by a number of cracks appearing in the surface coating and underlying diffusion layer at some critical regions after only 6500 testing forging strokes. Comprehensive supplemental computational analyses have been conducted to estimate the thermomechanical loading in these critical regions and better understand the surface failure process. In the investigated case the PVD-coating of hot-forging die insert contributed to a significant decrease of its fatigue resistance due to a combination of the die design and high local thermomechanical loading caused by rapid energy transfer of forging force to the workpiece in the penultimate forging operation on a hydraulic drop hammer. Optimised shape design of the die insert to reduce the critical loading which causes premature damage of the PVD layer in combination with improved plasma nitriding process is suggested to extend the service life of the die insert.