Influence of Secondary Cyclic Hardening on the Low Cycle Fatigue Behavior of Nitrogen Alloyed 316LN Stainless Steel
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n 316L-type stainless steel, secondary cyclic hardening (SCH) is evidenced in the temperature range that coincides with the occurrence of dynamic strain aging (DSA), i.e., from 573 K to 873 K (300 °C to 600 °C).[1,2] This temperature range also encompasses the sodiumcooled fast reactor (SFR) operating temperature [about 820 K (547 °C)] and temperatures during power transients. DSA by itself increases matrix hardening and causes drastic reduction in fatigue life.[3–6] Since SCH increases the degree of hardening (DOH) over and above that caused by DSA, it is vital to characterize the influence of SCH on cyclic deformation behavior and low cycle fatigue (LCF) life. The current article aims to identify the conditions for occurrence of SCH and also its consequences as a function of temperature [773 K to 873 K (500 °C to 600 °C)], strain amplitude (±0.25, 0.4, 0.6 pct), and nitrogen content (0.07 to 0.22 wt pct N) in 316LN SS. Such a comprehensive assessment on SCH has not been reported earlier. Four heats of 316LN SS containing 0.07, 0.11, 0.14, and 0.22 wt pct nitrogen are considered for the current G.V. PRASAD REDDY, Scientific Officer-E, and R. SANDHYA, Head, are with the Fatigue Studies Section, Indira Gandhi Center for Atomic Research, Kalpakkam 603102, India. Contact e-mail: [email protected], [email protected] M.D. MATHEW, Head, is with the Mechanical Metallurgy Division, Indira Gandhi Center for Atomic Research. S. SANKARAN, Associate Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India. Manuscript submitted May 20, 2013. Article published online October 16, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
study, with the basic composition (in wt pct) of: 17.5 Cr, 0.03 C, 12.0 Ni, 2.5 Mo, 1.75 Mn, 0.0055 S, and 0.017 P. They were solution annealed at 1363 K (1090 °C)/ 1 hour and water quenched and designated as N07, N11, N14, and N22, respectively. Total-strain controlled LCF tests were conducted at 3 9 10 3 s 1 at temperatures of 773 K, 823 K, and 873 K (500 °C, 550 °C, and 600 °C) (i.e., at ±50 K (±50 °C) to the SFR operating temperature). Some of the tests were interrupted during cycling to identify the microstructural evidence for the occurrence of SCH. SCH is the hardening observed in cyclic stress response curves after brief stabilization, as shown in Figure 1(a), where secondary hardening can be seen up to rapid stress drop concomitant with macrocrack propagation. As apparent from Figure 1(a), tendency to SCH increased with nitrogen content, and hence N22 showed much higher DOH during SCH. Interestingly, SCH either diminished or disappeared with the increasing temperature or strain amplitude or both (Figure 1(b)), particularly for nitrogen contents
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