高窒素ステンレス鋼溶接継手特性に関する研究 １ 高窒素ステンレス鋼溶接金属のブローホール抑制並びに凝固モードと耐食性の評価

Abstract

High nitrogen-bearing stainless steel (HNS) is attractive material due to its superior corrosion resistance, mechanical properties as well as resource saving type material. HNS of about 1 mass% in nitrogen content manufactured by a pressurized ESR was tried to joint by conventional arc welding. Because of the nitrogen supersaturation at molten state, blowholes were easily formed in the weld metal. A new welding method was proposed by using high Cr-Mo Steels as welding consumables which had relatively large nitrogen solubility at molten state in order to suppress the blowhole generation in the weld metal. The evaluation of pitting corrosion resistance of weld metal was also investigated with taking into account the solidification mode. The welding method by using high Cr-Mo steels as the welding consumables and by controlling the dilution rate to hold the nitrogen content in molten metal less than equilibrated solubility of molten metal with a nitrogen gas at 1×105Pa was found to be effective to suppress the blowholes generation in the weld metal. The critical pitting corrosion temperature (CPT) of the weld metal portion in 6%FeCl3 solution dropped compared with that of the base metal because of microsegregation of alloy elements. Controlling the composition of the weld metal so as to solidify with FA mode, the degree of CPT drop could be minimized compared with that of other solidification modes because of nitrogen segregation enriched in austenite in which Cr, Mo were relatively depleted. The alloy elements compositions at the portion where Cr and Mo contents were minimized in the weld metal could be estimated by Thermo-calc and compared with the results of EPMA analysis. Tensile strength of the weld metal portions were 700∼900MPa and those were about 20% lower value than that of the parent metal because of the reduction of nitrogen content.