Strain-aging in highly worked 316L stainless steel
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The room temperature burst pressure of 316L stainless steel burst discs exhibited increases of about 10% over 90 days. This increase may be associated with a strain-aging phenomenon requiring the presence of carbon since tensile property instability in worked austenitic stainless steels has been reported.[1–5] The cold worked material directly beneath the score root on the burst disc could undergo the strain aging process, thus causing the observed increase in burst strength. Characterization and analysis were therefore undertaken to identify the controlling phenomena in the small heterogeneous volume that controls rupture of the burst disc. Optical metallography and magnetic measurements confirmed the presence of martensite. Nanoindentation hardness measurements were correlated with finite element simulation of the as-formed mechanical properties. A representative portion of the microstructure was then recreated through cold rolling, and subjected to real-time and accelerated thermal aging treatments and mechanical activation analysis. Saturation of strengthening was observed, and a low temperature martensite reversion anneal was found to prevent or reverse the aging process. The results are consistent with previous observations of strain aging, although in this instance the effects are observed over a 10,000-fold greater aging time. Aging mechanisms are discussed, incorporating the phenomenologies of activation enthalpy and aging kinetics. A model explaining the sensitivity of aging rate to extreme cold work-induced dislocation densities and cold work-induced vacancy content is proposed.
Keywordscold work stainless steel strain aging
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