2相ステンレス鋼溶接部の耐孔食性に及ぼすB添加の影響

抄録

A study has been performed to determine the effect of boron on the pitting corrosion resistance in HAZ of 25Cr-6Ni-2Mo duplex stainless steel. The effect of boron on the hot ductility of this steel has also been investigated. Boron had an effect to improve the pitting corrosion resistance of HAZ, but conversely to deteriorate it with being added more than about 100 ppm. Scanning electron microscopic observation for the specimens corrosion-tested for various periods demonstrated that the most susceptible site for the pit initiation in HAZ of the boron free alloy was the lamellar phase which consisted of the cellular M₂₃C₆ and the austenite, precipitated on the austenite/ferrite boundary during welding. The austenite existing among the cellular M₂₃C₆ in a lamellar phase was depleted in chromium content, which resulted in the preferential attack of this phase in a corrosion environment. Boron addition made the volume ratio of M₂₃C₆ to the austenite in a lamellar phase to decrease, resulting in the increase of the chromium content of the austenite in a lamellar phase to the level more than that required for passivation (12wt%). Consequently, the lamellar phase in the boron containing steel became more resistant, thus the pitting corrosion resistance in HAZ of the steel was improved. Hot ductility of 25Cr-6Ni-2Mo duplex stainless steel was also improved by addition of boron, especially for the alloy with high nitrogen content which had essentially the shortness of the hot workability.