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

不锈钢在焊接和表面改性过程中对氮的吸收和解吸会影响其组织和各项性能，因为氮的含量决定了铁素体/奥氏体的相比和析出现象。过程中氮的控制对改善性能非常有效，但人们对氮在熔融和固态不锈钢中的基本行为知之甚少。本项目的目的是明确电弧和激光熔化和焊接过程中氮的吸收和解吸，并显示氮对微观组织、腐蚀和力学性能的影响。结果总结如下：(1)在热力学的基础上，利用平衡和实验数据，推导出铁合金焊缝金属氮含量的定量方程。将这些方程应用于不锈钢焊缝金属，结果与实验氮含量吻合较好。(2)不锈钢在激光焊接过程中的氮吸收比普通焊接过程中要小得多。从热力学角度讨论了激光加工过程中氮的行为。(3)在焊接气氛中加入氮气可控制双相不锈钢焊缝金属的组织。氮的加入改善了材料的力学性能和腐蚀性能。对焊缝金属高温转变的观察表明，焊缝金属中sigma相的析出开始晚于母材，且与δ / γ界面能密切相关。(4)在焊接气氛中加入氮气改善了氮强化奥氏体不锈钢JJ1焊缝金属的低温力学性能。

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.