Influence of oxidation on the tribological wear of hot work tool steels in sliding contact: implications for the forming process

Forging is accompanied by high temperatures, which results in oxidation of the charge material’s and the forging tools’ surface. Microscopic tests realized on the surfaces of post-service life tools demonstrated the presence of stick-ons originating from the forging material, which were accompanied by oxidation products. Their presence on the tools’ surfaces translates to their physico-chemical properties, which directly affect the tribological properties of the pair: tool-processed stock. The study presents the results of tribological tests performed on four hot work tool steels. The investigations were realized at the temperatures of 400 and 480°C with the use of the “ball-on-disc” test. In the case of the steels tested at 400°C, the recorded wear was higher than at the higher temperature. The wear resistance tests were complemented with microscopic tests of the friction face. It was stated that the presence of an oxide layer on the surface increases the steel’s resistance to sliding wear as well as affects the friction coefficient. This is connected with tribo-oxidation taking place at high temperatures as a result of mechanical forces (friction). The presence of oxides on the tool surfaces decreases the zone exposed to plastic deformations, which translates to the steel’s higher wear resistance. This oxide scale acts as a solid lubricant, protecting the contact surfaces from wear, which demonstrates the importance of preheating the dies before forging. On the other hand, we should consider the antagonistic impact of oxides, which can work as an abradant, leading to a possible damage of the forging surface and making it difficult to maintain its dimensional tolerances. The selection of steel for the forging tools should thus be based on a compromise between these two opposing phenomena.