Influence of re-profiling on the premature failure of hot forging dies

Re-profiling of hot forging dies is the most common method of die repairing in industry. Due to the surface to core hardness gradient caused by the limited hardenability of the die material, the surface layer after re-profiling possesses lower hardness than the first one, causing decrease of the tool lifetime in means of wear, but also can lead into total failure of the die due to other damaging mechanisms. Performed failure analysis of a hot forging die manufactured from 56NiCrMoV7 which included microstructure analysis, hardness profile, tensile and fatigue tests showed a 31 % decrease of fatigue properties when surface and core material is compared. Based on the experimental results it was calculated, that an optimal preventive action to increase the service life is to perform the re-heat treatment after every 10th re-profiling. The 16th re-profiling is critical; beyond this point, there is a significant risk of total die failure.