Effect of copper on the strength of AISI 316 stainless steel
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INTRODUCTION
THEREare several well-known approaches for modifying type 316 austenitic stainless steels to achieve microstructural stability and mechanical strength at elevated temperatures, l~ One of these is to make additions of refractory elements such as Ti, Nb, or V. These stabilizing elements form MC type carbides which, when the steel is given the "proper pretreatment, precipitate predominantly in the grain matrix and produce a stable matrix precipitate-dislocation network, l~l MC type carbides are more thermodynamically stable than M23C 6 type carbides since they have a higher melting point and a lower free energy of formation.J2] In the normal range of service temperatures for type 316 steel the free energy of formation of MC type carbides is sufficiently negative to ensure a relatively small ripening rate compared to other carbides found in these steels. Hence MC type carbides form a more stable distribution of fine precipitates, t2] A second approach for achieving elevated temperature strength is to add copper to the steel. This approach is utilized in 17/14 CuMo steel, p] In type 316 steel, copper additions are known to improve creep strength; the exact strengthening mechanism, however, is unclear. [4'5'6] Possible explanations of the copper effect are solid solution hardening, modification of precipitation reactions, or a combination of solid solution hardening and modification of precipitation reactions. Copper is also known to increase the stacking fault energy of the austenite lattice in austenitic steels, t4l Factors involved in the pretreatments which also influence the strength of modified type 316 stainless steels at elevated temperatures include (l) temperature and duration of solution treatment, which affects the grain size, solute segregation, and the available carbon content; (2) quench rate, which affects solute segregation; (3) cold work, which introduces a dislocation substructure available for the nuR.A. CAROLAN, Postdoctoral Research Associate, and CHE-YU LI, Professor, are with the Department of Materials Science and Engineering, Cornell University, Ithaca, NY, 14853-1501. P.J. MAZIASZ and R.W. SWINDEMAN are Members, Research Staff, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 3783 I. J. A. TODD, Assistant Professor, and J. C. REN, Graduate Student, are with the Department of Materials Engineering, University of Southern California, University Park, Los Angeles, CA 90089-0241. Manuscript submitted July 15, 1987. METALLURGICAL TRANSACTIONS A
cleation of carbides; and (4) duration and temperature of stabilization aging, during which MC carbides precipitate. This paper examines the effect of both stabilizing additions and copper addition on type 316 stainless steel. Mechanical properties were evaluated by using the load relaxation test. This test provides information on both the short term (high strain rate) tensile strength and long term (low strain rate) creep strength of the material. Transmission electron microscopy (TEM) was used to correlate mechan
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