Application of selected surface engineering methods to improve the durability of tools used in precision forging

The article presents a detailed analysis of the degradation phenomena and mechanisms of selected forging punches made of UNIMAX tool steel. Analyzed punches are used in the manufacture of a constant velocity joint boot forging (CVJB) applied in motorcars with front axle drive. The thorough analysis concerned the punches used in the fourth forging operation after a multi-operational process of forging at elevated temperatures, due to the lowest durability equaling only 4000 forgings. A comparison was made of 5 variants of surface thermo-chemical treatment including 2 types of nitriding (with a low and high potential) and 2 different coatings: CrN and AlCrTiN as well as a punch with and without additional thermo-chemical treatment. The performed complex analysis included a macroscopic analysis combined with scanning of the working surfaces, numerical modeling, microstructural tests, SEM microscopic tests, and microhardness measurements. The obtained results make it possible to select of the optimal variant of thermo-chemical surface treatment which improves the durability of these tools. In particular, the analysis included the manner and areas of wear of the punches as well as their resistance to the particular degradation mechanisms.