83 Practical Failure Analysis Volume 1(2) April 2001 Strain-Aging in Highly Worked 316L Stainless Steel S.L. Robinson, B.C. Odegard, Jr., N.R. Moody, S.H. Goods, M. Chiesa, and B.A. Meyer (Submitted 3 January 2001; in revised form 11 January 2001) 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. S.L. Robinson, B.C. Odegard, Jr., N.R. Moody, S.H. Goods, and M. Chiesa, Sandia National Laboratories, PO Box 969, Livermore, CA 94551-0969; and B.A. Meyer, Los Alamos National Laboratory, Mail Stop G780, Los Alamos, NM 87545. Contact e-mail: slrobin@sandia.gov. Keywords: cold work, stainless steel, strain aging PFANF8 (2001) 2:83-92 © ASM International Introduction An instability has been observed in the burst pres- sure of 316L stainless steel burst discs. Figure 1 shows the time dependence of burst pressure for the burst discs examined and tested at room temperature. Fig- ure 2 shows the rupture disc cross section, in which a ligament about 0.005 cm thick is formed by a scoring tool in a 0.0175 cm thick 316L hemispherical membrane. The carbon content of the burst discs was less than 0.03 wt.%. Ductile rupture initiates directly opposite the score root, several microns below the back surface of the disc, transitioning to rapid shear fracture at less than half thickness. A burst pressure increase requires an increased ligament strength, and approximately 90 days is required for Fig. 1 A typical burst pressure-time trace for scored 316L burst discs showing the time-dependent increase in pressure Fig. 2 Burst disc cross section. The top surface is the inner, pressurized surface. Rupture initiates at the bottom surface.