Hydrogen effects on the tensile properties of 21-6-9 stainless steel
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INTRODUCTION
THE adverse
effects of hydrogen on the mechanical properties of austenitic stainless steels have been the subject of numerous experimental investigations.l-l~ The susceptibility of austenitic steels to hydrogen damage has been attributed, in part, to a' martensite, 7'8'9co-planar dislocation motion,1'3-6 twinning 2 impurity distribution at grain boundaries 9 surface effects, 4,5,7,11 and the morphology of second phase precipitates. 11'~2,13Other metallurgical factors clearly play important roles in the degradation process, and it is unlikely that any single embrittlement model will be able to rationalize, much less explain, the relative roles of the important metallurgical variables. 1z.13,14However, a reasonably strong case can be made to show that, other things being equal, the susceptibility of austenitic steels to hydrogen damage is enhanced by co-planar dislocation motion. The effects of high energy rate forming, ~5 nitrogen content, ~'16 dispersed phases, ~7 and other factors on hydrogen compatibility have been explained through considerations of the effects of these factors on the tendency for co-planar dislocation motion. Several rationalizations are used to explain the importance of dislocation dynamics: (1) co-planar motion enhances the efficiency of hydrogen transport by dislocations;3'4'5(2) dislocation pileups increase the magnitude of local stress concentration at grain boundaries and other dislocation blocks; 6 (3) strain-induced transformations are more common in austenitic steels with low-stacking fault energies 2'7'8 and produce martensite which is more susceptible to hydrogen, 7'8'9 and (4) alloys with low-stacking fault energies are more likely to twin 2 and thereby increase their hydrogen susceptibility. Furthermore, hydrogen appears to enhance the tendencies for co-planar dislocation motion in several alloys which have a wide range of stacking fault energy. Each of these factors may play an important role in embrittlement under specific test conditions. Thus, AN'TON J. WEST, Jr. is with Sandia National Laboratory, Livermore, CA 94550. McINTYRE R. LOUTHAN, Jr. is with Virginia Polytechnic Institute, Blacksburg, VA 24061. Manuscript submitted December 16, 1981.
METALLURGICAL TRANSACTIONS A
for any given type of austenitic stainless steel, the susceptibility to hydrogen damage will be influenced by mechanical and thermal history, as well as minor changes in composition and perhaps surface finish. 2's'7'12 Any realistic model of the embrittlement process must therefore be compatible with large-scale variations in susceptibility to hydrogen damage in any given alloy system. A rather large data base is necessary to develop an embrittlement model, and if material variables are to be major factors affecting compatibility, experimental factors such as hydrogen charging technique, test environment, and test technique need to be constant. No such data base was apparent for hydrogen effects on austenitic stainless steels. Therefore, a large scale test program for the austenitic stainless
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