| A duplex surface treatment process of calorizing process followed by plasma nitriding was design to develop a process which improves wear resistance as well as oxidation resistance at high temperature. The surface properties of the duplex treated mild steel and STD61 steel were characterized and compared with those of both steels treated by single surface process of calorizing or plasma nitriding, in terms of microstructure, microhardness, wear resistance at room temperature and 500℃, and the oxdation behaviours at high temperatures. Duplex treated specimens were calorized at 1,050℃ for 5 hours and then were subsequently plasma nitrided at 500℃ for 5 hours. The pack cementization process on mild steel had created FeAl compound layer of approximately 120 ㎛ thickness on the surface. Much improved surface microhardness above 1200Hv, (0.1 ㎏) was measured after subsequent plasma nitriding and the nitriding depth was of the order of 80 ㎛. Although STD61 steel showed an decrease in microhardness in the Al-diffused layer upon calorizing, the subsequent plasma nitriding increased the hardness up to 1280Hv and the nitriding depth of over 80 ㎛ was obtained. The room temperature wear resistance of the duplex treated steels was slightly better than that of the nitrided one, but there was considerable improvement of the high temperature wear resistance at 500℃ in the duplex treated steels when both wear volume and weight change due to oxidation were considered. In addition the duplex treated steels showed an improved oxidation resistance than the plasma nitrided steels. |
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