Accelerated fracture due to tritium and helium in 21-6-9 stainless steel
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I.
INTRODUCTION
AUSTENITIC stainless steels have excellent hydrogen compatibility and generally show ductility superior to ferritic steels ~ and lesser fracture mode changes at similar hydrogen pressures. Susceptibility to hydrogen embrittlement in stainless steels has been linked to stacking fault energy through its effect on planarity of slip and deformation twinning.tS] As in ferritic steels, hydrogen compatibility is also dependent upon yield s t r e n g t h , t6] The principal time dependence associated with hydrogen-induced property changes is that associated with permeation of hydrogen into the material. However, an additional time dependence results from exposure to tritium, the radioactively unstable isotope of hydrogen. Tritium decays to 3He with a half-life of 12.33 years, and the accumulation of this helium is now known to cause time-dependent changes in the ambient temperature tensile properties of the low-strength stainless steel alloys AISI 304L and 316. [7,8,9] Flow stress increases, and ductility losses and fracture mode changes o c c u r 17-93 because helium is essentially insoluble in metals, precipitating as small clusters and bubbles which can be nucleated homogeneouslyt71 or on existing microstructural defects. [71 An understanding of helium effects on the tensile properties of high-strength austenitic stainless steels is desirable because of the known reduction of hydrogen compatibility with yield_strength level, t6j The nitrogen-strengthened stainless steel 21Cr-6Ni-9Mn (NITRONIC* 40) was chosen for this investigation of *NITRONIC is a trademark of Armco Steel Corporation, Baltimore, MD.
helium effects on high-strength austenitic stainless steels, S.L. ROBINSON and G.J. T H O M A S , Senior Members of Technical Staff, are with Sandia National Laboratories, Livermore, CA 94551-0969. Manuscript submitted April 17, 1990. METALLURGICAL TRANSACTIONS A
because it develops relatively high yield strengths when forged at intermediate temperatures, t~~ Furthermore, hydrogen embrittlement of 21-6-9 has been well characterized, 15,6,8,1~ allowing the effects of hydrogen and helium to be separated. An additional experimental benefit is that the 21-6-9 alloy absorbs up to 67 pet more tritium than other austenitic stainless steels 03'~4'~5] under equivalent charging conditions, allowing higher 3He concentrations to be readily obtained. II.
EXPERIMENTAL PROCEDURES
In this study, tritium was thermally charged into highenergy-rate forged (HERF) 21-6-9 stainless steel tensile specimens which were then freezer aged for selected times to allow the accumulation of 3He while limiting diffusive tritium losses. After different aging times, the specimens were tensile tested to failure at ambient temperature. The relationships between helium precipitation, plastic flow and fracture properties, and the deformation-induced microstructure were analyzed. The composition of the 21Cr-6Ni-9Mn alloy studied is given in Table I. With the given nitrogen concentration of 0.29 wt pct, a stacking fault energy of 33 J / m E i
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