Creep deformation and fracture behavior of types 316 and 316L(N) stainless steels and their weld metals
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I. INTRODUCTION
TYPE 316 austenitic stainless steel (SS) is widely used as a structural material for fast-breeder reactor components, due to its good high-temperature mechanical properties, compatibility with the coolant sodium, and adequate weldability. However, in general, austenitic SSs have relatively poor resistance to intergranular stress-corrosion cracking (IGSCC) in chloride and caustic environments. Type 316 SS welds exposed to marine environments have been reported to fail by IGSCC in the heat-affected zone, due to the combined influence of sensitization and the presence of residual stresses introduced during welding. A nitrogen-alloyed lowcarbon (0.03 wt pct maximum) version of this steel (316L(N) SS) has been chosen for the high-temperature structural components of the prototype fast-breeder reactor proposed to be built at Kalpakkam. (For this steel, nitrogen is specified in the range of 0.06 to 0.08 wt pct, in order to compensate for the loss in solid-solution strengthening due to the reduced carbon content.) Since very little information on the hightemperature behavior of this material is available in the literature, design of the components operating in the creep range has until recently been based on ASME code case N47,[1] with data available in the code on type 316 SS. It has been found that differences in chemical composition, microstructural stability, and thermomechanical history lead to wide variations in the high-temperature behavior of austenitic steels and their weld metals.[2] The safety factors G. SASIKALA, Scientific Officer, and S.L. MANNAN, Associate Director, Materials Development Group, and M.D. MATHEW, Scientific Officer, and K. BHANU SANKARA RAO, Head, Mechanical Metallurgy Division, are with the Materials Development Group, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, India. Manuscript submitted March 8, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
applied when designing for the allowable stresses in the welded components were supposed to take into account these differences as well. However, the design rules for welded components were later modified to take into account the weld metal creep rupture strength.[3,4] Accordingly, the allowable stresses in the weld are restricted to the lower value of the following. (1) The allowable stress in the base metal, which is based on 67 pct of the stress to rupture, 80 pct of the stress to cause onset of tertiary strain, and 100 pct of the stress to cause 1 pct total strain. (2) The product of 0.8 smin 3 R, where smin is the expected minimum stress to rupture for the base metal and R is the appropriate ratio of creep rupture strength of the weld metal to that of the base metal. The allowable deformation in the welds is restricted to half the deformation permitted for the base metal, since the ductility of welds at elevated temperatures is generally low. The strength-reduction factors proposed for 316 SS welds[4] were found to be adequate for a nuclear-grade type 316 SS used for high-temperature components in the fastbreed
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