Investigation On Pitting Corrosion Behaviors Of2304Duplex Stainless Steel Plasma Arc S Welded Joints

As the development direction of corrosion resistance alloy family in recent years, duplex stainless steels (DSS) attract more and more attentions in academic and industrial circles. Due to the composition of ferrite (a) and austenite (y) phase in microstructure, duplex stainless steels have combined the merits of Ferrite Stainless Steel and Austenitic Stainless Steel. Their outstanding properties strongly rely on the a balanced two-phase microstructure with approximately equal ferrite phase and austenite phase and no or less formation of harmful secondary phases. However, welding process modifies the ideal microstructure of duplex stainless steel and deteriorates the perfect properties especially the pitting corrosion resistance in high chlorine ion environment. As the main failure form of duplex stainless steel, pitting corrosion depends on pitting resistance equivalent number (PREN) value of the weaker phase in multiphase alloys. Complicated deformation process of welding makes the pitting behavior become more complex. Pitting performance of weld is not only determined by material, but also by welding parameters. Plasma arc welding is a common method of automatic welding, which causes significant effects on microstructure and properties of double phase stainless steel, especially the corrosion properties.Using critical pitting temperature measuring method and standard solution immersion method, microstructure and corrosion resistance of2304duplex stainless steel and its plasma arc welding have been studied. And the study of the occurrence of pitting corrosion mechanism and rule, Laws and mechanisms of inntergranular corrosion and pitting, alloy composition for a series of local corrosion resistance of alloy structure evolution and discussion on the basic principles of the organization. Ferrite phase in the high temperature HAZ has been revealed as the weakest phase in the weakest zone. The degradation of pitting corrosion in the welded joints is due to the lower chromium and molybdenum content in ferrite phase of the high HAZ resulted from the insufficient diffusion of alloying elements in the non-equilibrium welding process, compared with that in the base metal. Shot time post weld heat treatment (PWHT) has been carried out to restore the balanced microstructure and pitting corrosion resistance. The influence of PWHT on microstructure and pitting corrosion resistance has been studied and the optimized PWHT system has been determined. For the harmful precipitation chromium nitride (Cr2N) of duplex stainless steel, this paper uses the combination of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to study the precipitation behavior of O2N in the weld and the effect on O2N of heat treatment after welding, points out morphology of CrzN, and clarifies that the sub metastable pitting may be born in peripheral chromium depleted zone of Cr2N. This series of work for the development and application of double phase stainless steel has been of great value and significance.