Low Cycle Fatigue Behavior of 316LN Stainless Steel Alloyed with Varying Nitrogen Content. Part I: Cyclic Deformation Be
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TRODUCTION
PRIMARY side components (main vessel, inner vessel, intermediate heat exchanger, etc.) of Indian Prototype Fast Breeder Reactor (PFBR) are fabricated from 316LN austenitic stainless steel (SS) containing 0.06 to 0.08 wt pct nitrogen. The section thicknesses of these components reach up to 30 mm. From the perspective of cost and thermal stresses, it is necessary to reduce the section thickness of these components which in turn demands the use of high-strength nitrogen-alloyed 316LN SS with increased resistance to low cycle fatigue (LCF) failure (LCF being one of the design considerations). Nitrogen addition in austenitic SS is found to induce cyclic softening which has been attributed to the disordering of Cr-N short-range orders (SROs).[1] The interaction of these SROs with dislocations promotes planar slip, in addition to that caused by decrease in stacking fault energy (SFE) due to nitrogen addition.[2] Also, in the present study, cyclic stress–strain response is observed to be strongly influenced by dynamic strain aging (DSA) and secondary cyclic hardening (SCH), which occurred in the investigated temperature range [773 to 873 K (500 C to 600 C)] that encompasses the G.V. PRASAD REDDY, Scientific Officer-E, R. SANDHYA, Head Fatigue Studies Section, and M.D. MATHEW, Head, are with the Mechanical Metallurgy Division, Indira Gandhi Center for Atomic Research, Kalpakkam 603102, India. Contact e-mail: prasadreddy@ igcar.gov.in, [email protected] S. SANKARAN, Associate Professor, is with the Department of Metallurgical & Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India. Manuscript submitted January 11, 2014. Article published online July 9, 2014 5044—VOLUME 45A, OCTOBER 2014
PFBR steady state operating temperature [about 820 K (547 C)] and temperatures encountered during power transients. DSA causes localized planar slip and high matrix hardening that leads to drastic reduction in LCF life,[3,4] while SCH increases the degree of hardening over and above that caused by DSA.[5] Hence, it is utmost important to study the influence of these underlying phenomena on cyclic deformation behavior of nitrogen-alloyed 316LN SS. Accordingly, LCF tests were conducted to study the combined influence of nitrogen and total strain amplitude, in the temperature range 773 K to 873 K (500 C to 600 C), on LCF deformation and life in 316LN SS alloyed with different nitrogen contents of 0.07, 0.11, 0.14, and 0.22 wt pct. Several LCF studies have been conducted previously, on high nitrogen 316LN SS with an emphasis on the effect of nitrogen content on substructural evolution and fatigue life.[6–10] In the present study, the mechanisms that control the LCF deformation, life, and associated substructures are elucidated in a systematic way over a range of test parameters. This study is presented in two parts: part-I deals with the evolution of cyclic deformation behavior (current paper), and part-II details LCF life variation as a function of nitrogen, total strain amplitude, and temperature.[11]
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