Plasma post oxidation of nitrocarburized hot work steel samples

Abstract

Pulse plasma oxidation of untreated, salt bath nitrocarburized and plasma nitrided/nitrocarburized samples made of hot work steel grade H11 was investigated. A continuous multi-step plasma processing was performed consisting of a half an hour plasma nitriding at 520 °C followed by 3.5 h nitrocarburizing at the same temperature and by a subsequent post-oxidation in a low current density H 2O glow discharge during 45 min at 450 °C. The salt bath nitrocarburizing was carried out at a temperature of 580 °C for 210 min and was followed by the oil quench. In order to compare the oxide layer growth on the base material and on the ε-Fe 2(N,C) 1− x layer the salt bath treated sample was half cut and plasma post-oxidized in a 80% H 2O−20% H 2 glow discharge for 45 min at the temperature of 480 °C. In plasma processing pulse power supply was used with 57 μs pulse duration and 3 μs pulse pause. The surface structures were analyzed by optical microscopy, SEM and XRD, while Vickers microhardness test was used for measuring the surface microhardness and microhardness distribution over the sample cross sections. On the nitrocarburized surfaces XRD analyses revealed the existence of a superficial magnetite oxide layer and the presence of a ε phase and a small amount of γ′ phase, while a hematite Fe 2O 3 layer was formed on untreated sample surface.