Transition in Failure Mechanism Under Cyclic Creep in 316LN Austenitic Stainless Steel
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e 316LN austenitic stainless steel (SS) is the currently favored structural material for the primary side components in sodium cooled fast reactors (SFR). Cyclic creep (ratcheting) is the progressive directional accumulation of strain taking place under stresscontrolled asymmetric loading. In SFRs, high amount of strain accumulation due to cyclic creep in service can thin down the structural components in the primary sodium circuit, thereby leading to buckling. It is wellknown that cyclic creep is a consequence of fatigue and creep damage, both occurring in tandem. It has been pointed out by earlier researchers that the presence of very high mean stress (rm) can lead to a drastic reduction in the fatigue life following a changeover in the deformation mode from fatigue to creep.[1–3] However, the reasons behind this transition are far from clear, necessitating a thorough investigation on the cyclic creep behavior of the alloy. Such an investigation would not only pave the way for a better understanding of the phenomenon of cyclic creep but would also help in establishing appropriate empirical relationships so
ARITRA SARKAR, Scientific Officer-D, A. NAGESHA, Scientific Officer-F, and M.D. MATHEW, Head, are with the Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603102 Tamil Nadu, India. Contact e-mail: aritra@ igcar.gov.in; [email protected] P. PARAMESWARAN, Scientific Officer-G, is with the Materials Synthesis and Structural Characterization Division, Indira Gandhi Centre for Atomic Research. R. SANDHYA, Head, is with the Fatigue Studies Section, Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research. Manuscript submitted November 12, 2013. Article published online April 12, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
that the aforesaid transition in the deformation mechanisms could be clearly distinguished and the deleterious consequences thereof can be eliminated. This becomes extremely important at high temperatures wherein more complex deformation mechanisms are encountered. The current study aims at investigating the cyclic creep behavior of a type 316LN SS, as a function of rm, stress amplitude (ra) and temperature. Dynamic strain aging (DSA) is a common feature in 316LN SS during hightemperature deformation in the range from 823 K to 873 K (550 °C to 600 °C), as reported by several investigators.[4,5] DSA is also known to have a pronounced influence on the cyclic creep behavior of the alloy.[6,7] However, the role of DSA on the transition in deformation behavior under cyclic creep remains by far unexplored. The chemical composition (in wt pct) of type 316LN SS used in the current study was as follows: Fe-0.025C17.5Cr-12.1Ni-2.53Mo-0.14N-1.74Mn-0.0041S-0.017P. The material was subjected to solution annealing at 1363 K (1090 °C)/1 hour followed by water quenching. Cyclic creep tests were carried out in the temperature range from 823 K to 923 K (550 °C to 650 °C) employing different combinations of rm and ra under a uniaxial stress-controlled mode. Under cyclic
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