Correlation of substructure with time-dependent fatigue properties of aisi304 stainless steel
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I.
INTRODUCTION
THE elevated
temperature low-cycle fatigue (LCF) of materials has been a topic of concern to designers for years. With the advent of the Liquid Metal Fast Breeder Reactor (LMFBR), creep and environmental effects during LCF become even more important since many of the components operate at temperatures where the metal deformation is timedependent. Austenitic stainless steels, used extensively as structural components in the LMFBR, experience complex stress histories, resulting from static and cyclic loading, or a combination of both.l Several methods for treating this problem have been proposed, most relying on experimental data to construct empirical relationships such as Coffin-Manson type relationships, l-~ However, the equations lend no information or understanding to the mechanisms involved during the deformation. There is a considerable gap between macroscopic parameters such as stress and strain and their relations to substructural parameters. There have been numerous studies on dislocation substructures developed during low temperature deformation of structural materials, but fewer at elevated temperatures; S-~2 and very little work has been done on substructural studies of elevated temperature fatigue where hold times were imposed each cycle, t3 The objective of this study was to investigate the substructural features developed during time-dependent lowcycle fatigue of AISI 304 stainless steel. Various strain ranges, hold times, and temperatures were investigated, with particular emphasis given to tests conducted at 593 ~
II.
EXPERIMENTAL PROCEDURES
Specimens were fabricated from AISI 304 stainless steel, heat number 9T2796 (hereafter designated as ht. 796), a reference heat produced for LMFBR test programs. The chemical composition is given in Table I. Hour-glass shape specimens, 6.35 mm in diameter, were solution annealed in evacuated quartz tubes, backfilled with argon, for 30 rainA.M. ERMI and JOHN MOTEFF are, respectively, Graduate Student and Professor and Head, Materials Science and Metallurgical Engineering Department, University of Cincinnati, Cincinnati, OH 45221. Mr. Ermi is presently with Hanford Engineering Development Laboratory, Richland, WA 99352. Manuscript submitted December 4, 1980. METALLURGICALTRANSACTIONS A
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utes at 1092 ~ and aged for 1000 hours at 593 ~ The aging treatment results in a more stable microstructure in the specimens, producing carbide precipitates on the grain and twin boundaries and slight precipitation within the grains. It was found to have a beneficial effect on the fatigue life.in The grain size was determined to be 133/xm. All specimens were supplied by and tested at Argonne National Laboratory. The tests were performed in air in servo-controlled, hydraulically actuated fatigue machines in the axial-strain-control mode. Details of the strain measurements and additional testing procedures have been previously described by Slot et al. )s The test conditions included strain rates of 4 E-03 and 4 E-05 s -l, total strain ranges from 0.5 to 2.0 p
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