Behavior of model Fe-Ni-Cr alloys during stress-rupture in sulfidizing and oxidizing environments
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I.
INTRODUCTION
THE search for alternate energy sources has led to the use of high-temperature materials in increasingly aggressive environments which typically contain multiple oxidants, such as oxygen, sulfur, and carbon. The interaction between the metal and its gaseous environment (scale formation, internal precipitate formation, or inclusion of gas species in solid solution) can lead to either an increase or a decrease in creep resistance. 1 Furthermore, the creep process can influence the type and degree of environmental attack which occurs. 2 An understanding of the complex interactions which occur between creep processes and environmental interactions is thus of significant technological importance. In this paper the results of creep tests of four model Fe-Ni-Cr alloys performed in multioxidant environments are presented in terms of secondary creep rate, time to rupture, morphology of attack, and depth of attack. In addition, the interrelationships between creep and environmental attack are discussed, and the depths of attack are compared to predictions based on a combination of bulk and grain boundary diffusion.
II.
EXPERIMENTAL
A. Materials Four model Fe-Ni-Cr alloys hot rolled into 2.54-cm diameter rods were produced for this program. The composition of each alloy is listed in Table I. Alloys KC-1948, KC-1949, and KC-1950 were produced with a carbon content of 0.07 to 0.08. Alloy KC-1951 was produced as a low carbon (0.003C) version of KC-1950. Due to the considerable difference in grain size among the alloys and the observation that much of the attack in prototypic coal gasification environments takes place along grain boundaries, 3 an annealing treatment which yielded similar grain sizes for each alloy was determined. The annealing treatments employed for this purpose are listed below:
R . A . PAGE and J.E. HACK are Senior Research Metallurgists with Southwest Research Institute, San Antonio, TX 78284. Manuscript submitted January 11, 1983.
METALLURGICALTRANSACTIONS A
Alloy
Annealing Treatment
60Fe20Ni20Cr0.07C
5400 seconds at 1149 °C followed by water quench 60Fe20Ni20Cr0.003C 2700 seconds at 1093 °C followed by water quench 40Fe40Ni20Cr0.08C 3600 seconds at 1149 °C followed by water quench 40Fe30Ni30Cr0.08C 2700 seconds at 1204 °C followed by water quench These heat treatments resulted in relatively carbide-free microstructures with grain sizes of between 130 and 180/xm average diameter. To achieve a more stable microstructure during creep at 815 °C, the alloys were given a carbide precipitation treatment of 649 °C for 3600 seconds followed by 871 °C for 3600 seconds. This thermal treatment produced a fine dispersion of carbides on relatively smooth grain boundaries in all four alloys and a fine dispersion of matrix carbides in the 40Fe30Ni30Cr and 40Fe40Ni20Cr alloys. The above thermal treatments resulted in unusually low stress-rupture ductilities for the 40Fe30Ni30Cr and 40Fe40Ni20Cr alloys, i.e., 1.9 and 1.7 pct reduction in area, respectively, at 815 °C and a lifetime of less than 0.27
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