高窒素ステンレス鋼溶接継手特性に関する研究 ２ 高窒素ステンレス鋼溶接熱影響部における窒化物析出挙動と耐食性

Abstract

High nitrogen-bearing stainless steels (HNS) are attractive materials because of their superior localized corrosion resistance, mechanical properties and also resource saving type compositions. HNS in which nitrogen content reached to about 1 mass% were manufactured by pressurized ESR, and an attempt was made to weld them. In the heat affected zone (HAZ) of GTAW, only nitride precipitation or nitride precipitation and ferrite growth were observed. The precipitated nitrides were determined to be Cr2N containing a small amount of Mo and Fe by TEM/EDS analysis. The pitting corrosion potential and crevice corrosion potential in the artificial seawater of the HAZ, in which Cr2N precipitated, degraded comparing with the base metal. The time-temperature-precipitation (TTP) curves of HNS were obtained. It was found that for 18 Cr-14 Ni-2.5 Mo-0.7N steel, in the heating temperatures ranging from 1073K to 1473K, the critical pitting corrosion temperature (CPT) in the 6% FeCl3 solution decreased after heating 1 s, and for 23 Cr-4 Ni-2 Mo-1 N steel, CPT decreased raging from 1173K to 1373K after heating 2 s. The TTP curves of the HNS were estimated by taking account of the incubation period and the precipitates-growing period. The former period was derived from the kinetics theory of solid-solid nucleation at grain boundary, and the latter period was estimated on the assumption that the growing rate of Cr2N was controlled by one dimensional diffusion of Cr inside the grains. The estimated total growing period was roughly consistent with the experimental period at which CPT began to decrease. CPT decrease was successfully predicted by using additivity rule (linear summation rule of the thermal cycle during welding). Cr2N precipitation could be prevented by CO2 laser welding due to its high cooling rate. The CPT of the CO2 laser welded joint of the HNS with high Cr-Mo steel insert metal was 348K, and its tensile strength was 920 MPa.