DEVELOPMENT OF DUPLEX HIGH TEMPERATURE GAS NITRIDING AND LOW TEMPERATURE PLASMA NITRIDING SURFACE TREATMENTS FOR UNS S31803 DUPLEX STAINLESS STEEL

摘要

Specimens of a UNS S31803 duplex stainless steel were (i) single low temperature plasma nitrided and (ii) high temperature gas nitrided, at 1200°C and low temperature plasma nitrided. Plasma nitriding treatments were carried out at 400°C by 20 h, in a 75%N2+25%H2 atmosphere, duplex surface treatment. The microstructure of the as received material is composed by ferrite and austenitic stringers, aligned in the rolling direction. Light optical microscopy (LOM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron-backscattered diffraction (EBSD) were used to characterize the microstructure. The duplex surface treatment led to the formation of 550 μm thick fully austenitic layer at the surface of the specimens, by transformation of ferrite stringers, and the subsequent plasma nitriding led to the formation of a homogeneous expanded austenite layer, 1144 HV. The single plasma nitrided (LTPN) layer was composed by expanded ferrite (an) and expanded austenite (yn), with microhardness of 1509 HV and 1362 HV, respectively.