高温腐食を受けるNi基耐熱合金のクリープ破断特性におよぼす環境因子の影響

抄録

In order to investigate the effect of environmental factors such as the corrosive species and temperature on the creep rupture properties of a nickel-base superalloy subjected to hot corrosion, creep rupture tests have been performed of Inconel 751 specimens with and without coating of different compositions of Na₂SO₄-NaCl salt mixtures in a temperature range between 1023 and 1173 K. The effect of hot corrosive environment on the creep rupture strength and the rupture behavior was found to differ considerably, depending on temperature and salt composition. Adding only a small amount of NaCl to the coating salts results in the marked degradation of the creep rupture strength along with the completely altered rupture behavior as compared with the case in air, in which the stress-enhanced preferential intergranular penetration of sulfides, oxides and occasionally chlorides causes directly a premature intergranular fracture in a brittle manner prior to the formation of creep-induced internal grain boundary cracks. In particular, the detrimental effect of NaCl bearing environment is more pronounced in the lower temperature region below about 1073 K, even if it is lower than the melting point of salt, where chromium oxy-chlorides play the most important role in the intergranular attack behavior. However, an aggressive intergranular attack associated with the oxy-chloride formation tends to decline with increasing temperature because of higher volatility of oxy-chlorides as well as NaCl. In the higher temperature region above about 1073 K, an intergranular attack proceeds mainly by a successive sulfidation-oxidation process, while NaCl appears to play a catalystic action to promote the Na₂SO₄- induced aggressive attack. Further increased temperature to about 1173 K results in the rather mitigated degradation of corrosion rupture properties along with the rupture behavior essentially similar to that in air because of the oxidation-dominating intergranular attack.