The paper describes the mechanism of destruction of forging die used for plastic working processes by die forging on hammers. The test material was a matrix after exploitation for plastic processing using the hot forging method, made of WNLV grade material. Cyclical changes in temperature and stress in the matrix led to degradation of the structure to the extent that it ruptures during operation. The criterion determining in industrial conditions the degree of remaining operational usefulness is the assessment of material hardness performed by non-destructive methods of hardness measurement (Leeb method). Obtaining extended material characteristics after exploitation will allow to predict the time of safe work of the die, as well as to determine the moment in which the matrix can be repaired, e.g. by making a pad in the working layer or by machining the cavity. The research focused on the assessment of the structure and properties of the damaged matrix. The scope of the work included the assessment of the structure carried out by means of light microscopy, scanning electron microscopy and determination of mechanical properties in the impact test and static tensile test. The obtained results were referenced to the WNLV steel parameters given in the standard.Keywords: Degradation, tool steel, properties, die forging
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