DUPLEX PLASMA SURFACE-TREATMENT PROCESS ON MILD-STEEL AND HIGH ALLOYED TOOL STEEL

Duplex plasma surface treatment by calorizing and plasma nitriding was carried out on mild steel and high alloyed tool steel to improve thei r elevated temperature wear properties. Effects of calorizing on the p lasma nitriding behavior of the steels and the characteristics of the duplex-treated specimens were analyzed using SEM, EDS, XRD, and a micr ohardness tester. Calorizing on mild steel created a FeAl compound lay er of approximately 120 mu m thickness, resulting in the improvement o f the surface hardness by a factor of 2. Subsequent plasma nitriding i ncreased the surface microhardness above 1200 Hv (0.1 kgf) and the nit riding depth was of the order of 80 mu m. The high alloyed tool steels showed a decrease in microhardness in the Al-diffused layer upon calo rizing. The subsequent plasma nitriding, however increased the hardnes s up to 1280 Hv and the nitriding depth of over 80 mu m was obtained. The room temperature wear resistance of the duplex-treated steels was slightly better than that of the nitrided one, but there was considera ble improvement in the high temperature wear resistance at 500 degrees C in the duplex-treated steels when both the wear volume and the weig ht change due to oxidation were considered.