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Application of extra high purity austenitic stainless steel to weld overlay

超高純度オーステナイトステンレス鋼のオーバーレイ材への適用

井岡 郁夫; 鈴木 潤; 木内 清; 中山 準平*

Ioka, Ikuo; Suzuki, Jun; Kiuchi, Kiyoshi; Nakayama, Jumpei*

有害な不純物を100ppm未満に抑えた超高純度オーステナイトステンレス鋼(EHP合金)を開発した。EHP合金は、応力腐食割れ(SCC)の基点と考えられる粒界腐食に対して優れた抵抗性を有している。そこで、SCCを防止するためのオーバーレイ材料として、EHP合金の適応性を評価した。試験には、EHP合金の比較材として従来材を用いた。試験片は、各々の溶接金属部から加工した。粒界腐食試験は、1g/LのCr(VI)イオンを含む8規定の沸騰硝酸溶液中で行った。比較材は、EHP合金に比べ激しい粒界腐食が生じた。SCC感受性を評価するためのCBB試験を561K, 1000h,飽和酸素の高温水中で行った。比較材の割れと粒界腐食は、EHP合金に比べ発生数、サイズとも非常に大きかった。オーバーレイ材料として使用する場合でも、従来材に比べ、EHP合金は優れたSCC抵抗性があることが確認された。

An Extra High Purity austenitic stainless steel (EHP alloy) was developed with conducting the new multiple refined melting in order to suppress the harmful impurities less than 100 ppm. EHP alloy has great intergranular corrosion resistance. It is considered that intergranular corrosion becomes initiation of SCC. So, we try to apply EHP alloy to weld overlay materials to prevent from SCC. EHP alloy was melted by the new multiple refined method. The conventional weld metals were also prepared as comparisons. Specimens were machined from the welded metal of each material. Intergranular corrosion tests were performed in boiling 8 kmol/m$$^{3}$$ HNO$$_{3}$$ solutions containing 1 kg/m$$^{3}$$ Cr(VI) ions. The intergranular corrosion of conventional weld metals was severer than those of EHP alloys. Crevice Beam bending tests to evaluate susceptibility of SCC were carried out in high temperature water of 561 K with saturated oxygen for 1000 h. Cracks and intergranular corrosion of conventional weld metals were much more than those of EHP alloys. It was confirmed that EHP alloy had excellent SCC resistance in comparison with conventional materials when EHP alloy was used as a weld metal.

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