MICROSTRUCTURAL CONTROL OF STAINLESS STEEL DURING WELDING AND SURFACE MODIFICATION USING NITROGEN

点击翻译按钮获取中文摘要

Nitrogen absorption and desorption by stainless steels during welding and surface modification processes affect the microstructure and various properties, because the nitrogen content decides the ferrite/austenite phase ratio and precipitation phenomena. The control of nitrogen during the processes is very effective to improve the properties, but there is very little knowledge of the basic behavior of nitrogen in molten and solid stainless steels. The purpose of this project is to make clear the nitrogen absorption and desorption during arc and laser melting and welding processes, and to show the effects of nitrogen on the microstructure, corrosion and mechanical properties. The results are summarized as follows ;(1) Quantitative equations to predict the nitrogen contents of iron alloy weld metals were derived on the basis of thermodynamics using equilibrium and experimental data. The application of these equations to stainless steel weld metals showed a good agreement with the experimental nitrogen contents.(2) Nitrogen absorption of stainless steels during laser welding was much smaller than during are welding. The nitrogen behavior during laser processing was discussed thermodynamically.(3) The microstructures of duplex stainless steel weld metals were controlled by nitrogen addition into welding atmosphere. The nitrogen addition improved the mechanical and corrosion properties. Observation of high temperature transformation in the weld metals indicated that sigma phase precipitation starts later in weld metals than in the base metal, and depends strongly on the delta/gamma interfacial energy.(4) The cryogenic mechanical properties of nitrogen-strengthened austenitic stainless steel JJ1 weld metal was improved by nitrogen addition into welding atmosphere.