Influence of Strain Rate and Temperature on Tensile Deformation and Fracture Behavior of Type 316L(N) Austenitic Stainle
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AISI type 316 austenitic stainless steel is widely used as a major structural material for sodium-cooled fast reactor (SFR) components in view of its favorable hightemperature mechanical properties, compatibility with liquid sodium coolant, and adequate weldability. However, austenitic stainless steels, in general, exhibit poor resistance to intergranular stress corrosion cracking.[1] The problems associated with stress corrosion cracking of austenitic steels become all the more serious, when the fabrication site, storage of components, and/or the power plant are located in the coastal region.[1] In view of this, a nitrogen-alloyed low carbon (0.03 wt pct max.) version of this steel, designated type 316L(N) austenitic stainless steel has been chosen for the high-temperature structural components of the prototype fast breeder reactor (PFBR) currently in an advanced stage of construction at Kalpakkam, India.[1] The carbon content ranging from 0.02 to 0.03 wt pct is preferred to minimize the susceptibility toward sensitization either during fabrication or during service, and improve stress corrosion cracking resistance. Addition of nitrogen ranging from 0.06 to 0.08 wt pct is intended to increase high-temperature mechanical properties comparable to that of normal grade 316 SS. The beneficial influence of controlled addition of nitrogen on tensile, creep, and low cycle fatigue properties has been demonstrated in type 316L stainless steel.[2–5] It has been suggested that the increase in strength in nitrogen bearing B.K. CHOUDHARY, Scientific Officer-H, is with the Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102 Tamil Nadu, India. Contact e-mail: bkc@ igcar.gov.in Manuscript submitted July 13, 2012. Article published online September 17, 2013 302—VOLUME 45A, JANUARY 2014
stainless steels is derived through one or a combination of following several mechanisms as (i) solid solution strengthening, (ii) decrease in stacking fault energy, (iii) precipitation hardening, (iv) formation of interstitialsolute complexes, (v) clustering, and (vi) order strengthening.[6–9] Tensile, creep, and low cycle fatigue properties are important considerations for the design of SFR structural components operating at high temperatures. Elevated temperature tensile properties form the first step toward characterization of materials performance for high-temperature application. In general, austenitic stainless steels exhibit a decrease in strength values with increase in temperature from 300 K (27 C) to 523 K (250 C) followed by plateaus/peaks in flow stress/ strength values at intermediate temperatures ranging from 523 K to 923 K (250 C to 650 C) and rapid decrease beyond 923 K (650 C) at high temperatures.[3,10–13] The variations in tensile ductility with temperature also display three distinct temperature regimes consisting of a decrease in the values from 300 K (27 C) to 523 K (250 C) followed by a minima or a minimum at intermediate temperatures and a rapid increase at high temperatures.[3,10
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