安定形オーステナイトステンレス鋼における炭化物の挙動とナイフラインアタック現象 ＩＩＩ ナイフラインアタック現象の機構の解明ＩＩ

Abstract

In previous reports, it was made clear that knife line attack phenomenon in stabilized austenitic stainless steels was closely related with the precipitation of M23C6 type carbide at the austenitic grain boundaries in HAZ near the weld interface during sensitizing-heat treatment.However, knife line attack phenomenon in SUS 321 could not be explained fully by the precipitation of M23C6 at the austenite grain boundaries.In this report, the further investigation on the mechanism of knife line attack phenomenon in austenitic stainless steels, especially in SUS 321 was carried out, in order to clearify this phenomenon in detail.From experimntal results, the following facts were identified on the mechanism of knife line attack phenomenon.(1) In SUS 347, the precipitation of M23C6 type carbide at austenite grain boundaries in HAZ near the weld interface by sensitizing heat treatment after welding is the main cause of knife line attack.(2) In SUS 321, δ ferrite phases precititate at the region heated higher than about 1100°C in HAZ, and these phases occupy about 10 volume % at this region.(3) After sensitizing heat treatment, the corrosion resisting property of δ ferrite phases in HAZ deteriorates and these phases accelerate the intercrystalline corrosion at this region.(4) The new mechanism of knife line attack in SUS 321 is that δ ferrite phases at austenite grain boundaries accelerate the intercrystalline corrosion, in addition to the deteriorating effect of M23C6 typpe carbide on the intercrystalline corrosion resistance.