Strain aging and load relaxation behavior of type 316 stainless steel at room temperature
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I.
INTRODUCTION
THE load
relaxation test has been an important experimental tool in the development of the state variable theory originally proposed by Hart. 1,2 For establishing the low homologous temperature plastic equation of state, room temperature load relaxation data on types 316 and 304 austenitic stainless steels have largely been used. 2.3.4It has been shown in these studies that the low homologous temperature relaxation curves, which in the logarithmic stress-strain rate presentation have a concave upward shape, can be translated along a straight line to overlap each other and, furthermore, can be described analytically by = fi*(r)[(~ - ~ * ) / G ] M,
11]
where ¢r and k are the applied stress and the nonelastic strain rate, respectively, tr* is the hardness parameter, which measures the strength of barriers to dislocation motion, b*(T) is a dislocation glide parameter, M is a constant, and G is the shear modulus. Parameters M and b*(T) are to be measured for each material. For the verification of an equation of state behavior it is, of course, desirable that the state of the material is maintained constant during any measurement; i.e., the state of the material does not change because of such processes as recovery or strain aging. Repeated relaxation tests have often been performed to demonstrate that such parameters as fi*(T) and tr* remain constant during a load relaxation run. In a repeated relaxation test the specimen is reloaded to a stress slightly below the stress of the initial load relaxation run, and another load relaxation run is performed. If the state of the material remains unchanged during the initial S.-P. HANNULA, formerly Research Associate in the Department of Materials Science and Engineering, Cornell University, Ithaca, NY, is Senior Research Scientist with Metallurgy Laboratory, Technical Research Centre of Finland, 02150 Espoo, Finland. M. A. KORHONEN, formerly Visiting Assistant Professor in the Department of Materials Science and Engineering, Comell University, Ithaca, NY, is Supervising Assistant with the Department of Mining and Metallurgy, Helsinki University of Technology, 02150 Espoo, Finland. C. -Y. LI is Professor with the Department of Materials Science and Engineering, Comell University, Ithaca, NY 14853. Manuscript submitted June 17, 1985. METALLURGICALTRANSACTIONS A
and reloading relaxation runs, the data from the reloading runs will retrace those from the initial run after a short initial transient. 5 By performing repeated load relaxation tests it has recently been demonstrated that strain aging can also affect the load relaxation behavior of type 316 stainless steel. 6 Although there is evidence that strain aging indeed occurs in austenitic stainless steels, 7'8'9 to the authors' knowledge no systematic studies have been performed to investigate aging at room temperature (RT). In a range of austenitic alloys based on the iron-nickel-carbon system, however, the existence of strain aging at RT is well established. 1° The presence of carbon was determined to be e
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