Creep Rupture Properties of Some Nickel-Base and Nickel-Iron-Base Superalloys Subjected to Hot Corrosion

Abstract

Creep rupture tests of molybdenum-free and molybdenum-bearing Fe-42 Ni-15 Cr alloys were carried out at 800°C in static air for specimens, with or without a synthetic ash mixture coating composed of 90%Na₂SO₄ plus 10%NaCl, and the effect of hot corrosion on creep rupture properties was investigated in comparison with the case of a nickel-base superalloy Inconel 751. In comparison with Inconel 751, a decrease in the rupture strength of both Fe-42 Ni-15 Cr alloys, caused by hot corrosion, was noticeably suppressed. In particular, the Fe-42 Ni-15 Cr-3 Mo alloy exhibited the highest rupture strength in a hot corrosive environment due to the presence of molybdenum as a solid-solution strengthener which has no detrimental effect on hot corrosion resistance. In air, the creep rupture in all specimens occurred as a result of the growth and coalescence of a large number of grain boundary cracks formed in the interior of the specimens. In a hot corrosive environment, on the other hand, the rupture behavior differed completely between Incone 751 and the Fe-42 Ni-15 Cr alloys. In the former, a premature fracture occurred in a brittle mode by only a few aggressive intergranular penetrations of sulfides which propagated rapidly under applied stress. In the latter, however, fairly ductile fracture took place based on essentially the same behavior as in air, in which many interior cracks resulting from grain boundary sliding were responsible for rupture, as well as surface cracks stimulated somewhat by intergranular penetration of sulfides. These results emphasize that the creep rupture properties of these alloys subjected to hot corrosion is chiefly controlled by the behavior of aggressive intergranular penetrations. From the metallographic observation of the aggressive intergranular penetrations, it is also suggested that the superior resistance of the Fe-42 Ni-15 Cr alloys to a rapid propagation of such a penetration should be attributed to not only their lower nickel content but also to the decreased tendency of these alloys to form continuous chromium-depletion due to the precipitation of chromium-rich carbides at grain boundaries.