Hot friction and wear behaviour of plasma nitrided and low pressure carburized hot work tool steel sliding against 6082 Al alloy

摘要

High temperature friction and wear behaviour of a hot work tool steel sliding against AA6082 alloy were investigated by means of a customary test configuration, which provides the conformal contact between a die block and an AA-disc. Uninterrupted tests in air were carried out. The friction behaviour is characterized by limited fluctuations just after the beginning of test, finally evidencing a transition to a more unstable contact regime after progressive transfer of Al to the die surface. The tendency towards Al pick up was evaluated by measuring the mass increase of die samples and their capability in delaying the formation of hard Al oxide particles, which are very detrimental for the surface finishing of the extrudate. The efficacy of different surface treatments, i.e. plasma nitriding and low pressure carburizing, was studied. Furthermore, samples with different level of roughness, corresponding to conventional machining (0.7-0.8 μm) and mirror polishing (0.02μm), respectively, were produced. Finally the influence of hardness was also evaluated by considering two different hardness of the base steel (56 HRC and 44 HRC). In general, surface hardening improves the load bearing capability of the die surface, causing an extended permanence in the stable friction regime. Plasma nitrided samples (1125 HV 0.1) evidenced a better behavior than case hardened (670 HV 0.1) and base steel (56 HRC, 620 HV 0.1), respectively. Overtempered samples (44 HRC, 500 HV 0.1) confirm this trend. In view of the experimental error, a lower surface roughness did not improve the tribological properties.