The effect of environment on high-temperature hold time fatigue behavior of annealed 2.25 pct Cr 1 pct Mo steel
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I.
INTRODUCTION
THE annealed form of 2.25Cr1Mo steel has been used extensively for high-temperature applications in the petroleum, chemical processing, and power generation industries. The deformation experienced by steel components such as pressure vessels and piping in these industries is a combination of creep and fatigue. Fatigue of metals and alloys at elevated temperatures is strongly influenced by environmental interactions and cyclic waveform. High-temperature fatigue tests with hold periods performed in oxidizing environments may be related to the deformation experienced by these steel components in service. A significant amount of fatigue lifetime data for hightemperature tests in air with hold periods generated for annealed Fe2.25Cr1Mo[1–5] have shown that including a hold period at maximum compressive strain leads to a shorter fatigue life than the inclusion of a tension hold. The lifeshortening effect of a compressive hold has been attributed to an oxidation-fatigue interaction.[3,6] (Heat-treated, bainitic Fe2.25Cr1Mo does not show the same waveform dependence as the annealed ferritic 2.25Cr1Mo; in bainitic 2.25Cr1Mo, a tension hold is more damaging than a compression hold in both air and vacuum high-temperature environments.[7,8]) In the present investigations, high-temperature hold time tests were conducted in air and vacuum in order to study the oxidation-fatigue interaction in annealed Fe2.25Cr1Mo. The intent of testing in vacuum environments was to remove or limit the detrimental effects of oxidation in order to further understand the apparent environmental-fatigue interaction in Fe2.25Cr1Mo. When the oxidizing capability of the environment is reduced, the effect of the different hold patterns on fatigue lifetime changes. The effect of environment and hold type on surface damage or oxide scale R.L. HECHT, formerly Graduate Student with the Department of Materials Science and Engineering, Northwestern University, is Technical Specialist with the Ford Research Laboratory, Ford Motor Company, Dearborn, MI 48121-2053. J.R. WEERTMAN, Walter P. Murphy Professor, is with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108. Manuscript submitted December 28, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS A
crack patterns was studied at a selected set of conditions. Cross-sectional views of fatigue specimens were examined to determine the relationship between surface oxide cracks and cracks in the metal. Polished cross sections revealed crack propagation paths and evidence of creep damage for certain conditions.
II.
EXPERIMENTAL
The Fe2.25Cr1Mo material, provided by the Timken Company (Canton, OH) in bar form, is in the annealed condition and consists of approximately 60 pct ferrite and 40 pct pearlite. Its chemical composition appears in Table I. Hourglass-shaped specimens with a minimum waist diameter of 6.35 mm were machined with the longitudinal direction of the bar coincident with the ultimate loading axis. Prior to testing, specimens were polish
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