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Abstract
A lean duplex stainless steel, LDX2404, was DC plasma nitrided under a range of treatment conditions. The microstructure characterisation evaluation of the treated samples revealed that a dense, super-hard surface layer can be produced by low-temperature (<450 °C) plasma treatments. The original austenite phase became S-phase and the ferrite phase was supersaturated with nitrogen and ε-Fe3N nitride precipitated from it. When plasma nitriding was carried out at above 450 °C, chromium nitrides precipitated in the surface nitrided layer. Compared to the untreated samples, the surface hardness of the lean duplex stainless steel (DSS) is increased up to four times. The dry wear resistance increased when increasing the treatment temperature. In contrast, the low-temperature treated samples showed the best performance in the electrochemical corrosion and corrosion-wear tests; the performance of the high temperature (>450 °C) plasma nitrided samples was found to be significantly worse than that of the untreated material.
Highlights
Duplex stainless steels (DSSs) are the materials of choice for the offshore gas and oil, petrochemical and chemical industries
The transmission electron microscope (TEM) sample was prepared in cross-sectional view by FIB/SEM (FEI Quanta 3D) and the surface layer structure formed during the plasma nitriding in the original austenite and ferrite grains was observed by JEOL2100 TEM equipment (JEOL Ltd., Akishima, Japan)
Plasma nitriding of lean duplex stainless steel LDX 2404 has been conducted at temperatures ranging from 390 to 450 ◦ C for 10 h in a gas mixture of 25%N2 and 75%H2
Summary
Duplex stainless steels (DSSs) are the materials of choice for the offshore gas and oil, petrochemical and chemical industries. When the treatments were carried out at conventional temperatures (450 to 600 ◦ C), the corrosion resistance of stainless steels severely deteriorated This was caused by the chromium nitrides precipitation, which depleted the chromium content in the matrix, preventing the formation of a dense and continuous oxide layer on the surface [2,3]. A new low-temperature plasma surface alloying technique has been developed to achieve combined improvements in hardness, wear resistance and fatigue properties for austenitic stainless steels without reducing their corrosion properties due to the formation of a so-called S-phase, a nitrogen/carbon. The reported research mainly investigated the surface layer structure, hardness and corrosion resistance of duplex stainless steels after plasma nitriding. The microstructures of the surface nitrided layers were characterised and their surface hardness, corrosion resistance, dry wear resistance and corrosion-wear properties were evaluated
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