Surface hardening of duplex stainless steel by low temperature active screen plasma nitriding

Abstract

The duplex stainless steel has high strength and excellent corrosion resistance. Since this steel has low hardness and wear resistance, the surface hardening treatment should be carried out under the wear environment. In this study, plasma nitriding has been applied for the improvement of wear resistance. However, traditional plasma nitriding at temperatures ranging from 723 to 823 K decreases the corrosion resistance of stainless steel. This is because the chromium in the steel reacts with nitrogen to form chromium nitride, and this decreases the concentration of chromium in the matrix, which is required for the formation of stable passive layers. Plasma nitriding carried out below 723 K forms a nitrided layer that is called the S phase. This phase improves the surface hardness without decreasing the corrosion resistance. Therefore, a low temperature plasma nitriding technique was used for treating stainless steel. Active screen plasma nitriding (ASPN) was carried out as the plasma nitriding process in addition to the conventional direct plasma nitriding (DCPN). ASPN has several advantages over the conventional DCPN, such as eliminating the edge effect, arcing, and carrying out hollow cathode discharge. The sample used was SUS 329J4L (X2CrNiMoCuN25-6-3). Nitriding was carried out at 673 and 723 K for 18 ks by DCPN and ASPN. The nitrided samples were then analyzed by various tests such as appearance observation, X-ray diffraction, SEM-EDX analysis, hardness testing, wear testing, and corrosion testing by measuring the pitting potential. After nitriding, the ASPN process improved the wear resistance and eliminated the edge effect when compared with the DCPN process. The S phase was observed on all the treated samples, and this phase was uniformly formed on both austenitic and ferritic phases of ASPN samples. Moreover, the corrosion tests showed that the pitting potential did not decrease with plasma nitriding.