Creep-rupture behavior of seven iron-base alloys after long term aging at 760/sup 0/C in low pressure hydrogen

Abstract

Candidate iron-base alloys for heater tube application in the Stirling automotive engine were aged at 760/sup 0/C for 3500 h in a low pressure argon or hydrogen atmosphere to determine the effect on mechanical behavior. The seven alloys evaluated were N-155, 19-9DL, 316SS, Nitronic 40, A286, Incoloy 800H, and RA330. Aging produced no appreciable changes in alloy grain size but did promote increased density and growth of precipitate particles in the grains and grain boundaries. Tensile properties were generally degraded by aging, with ductility and strength changes being influenced by grain structure. Aging also decreased creep-rupture strength, with coarse grain materials being more susceptible to strength loss. The presence of hydrogen during aging did not contribute significantly to creep strength degradation in the coarse grain alloys but did result in extensive strength losses in fine grain alloys. Based on current criteria for the Mod I Stirling engine, the N-155 and 19-9DL alloys were the only alloys in this study with strengths adequate for heater tube service at 760/sup 0/C.