Thermomechanical and Isothermal Fatigue Behavior of 316LN Stainless Steel with Varying Nitrogen Content
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INTRODUCTION
316LN stainless steel (SS) with 0.06 to 0.08 wt pct nitrogen constitutes a vital structural material for the primary side components (main vessel, inner vessel, intermediate heat exchanger, etc.) and piping system of Indian prototype fast breeder reactor (PFBR). These components are often subjected to cyclic thermal stresses as a result of reactor start-up/shutdown operations, thermal transients during normal operation, thermal stratification, etc., in addition to the mechanical load variations. Moreover, the section thicknesses of some components reach up to 30 mm. From the viewpoint of cost and thermal stresses, it is necessary to reduce the section thickness which in turn demands the use of high-strength nitrogen-alloyed 316LN SS with increased resistance to high-temperature fatigue failure. Normally, design against fatigue failure under such G.V. PRASAD REDDY, Scientific Officer-E, A. NAGESHA, Scientific Officer-G, and R. SANDHYA, Head, Fatigue Studies Section, 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 and Materials Engineering, Indian Institute of Technology Madras (IIT Madras), Chennai 600036 India. M.D. MATHEW, formerly with the Mechanical Metallurgy Division, Indira Gandhi Center for Atomic Research, is now Dean of Postgraduate and Research Studies with the Saintgits College of Engineering, Kottayam, Kerala, India. K. BHANU SANKARA RAO, Ministry of Steel Chair Professor, is with the Department of Metallurgical and Materials Engineering, Mahatma Gandhi Institute of Technology, Hyderabad 500075, India. Manuscript submitted July 30, 2014. Article published online November 20, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
cyclic thermomechanical fatigue (TMF) loading is based on isothermal fatigue (IF) tests carried out at the peak temperature (Tmax) of the expected thermal cycle, which is assumed to lead to conservative life estimation. However, many engineering materials have been reported to display TMF lives inferior to IF lives, in contrast to the above design philosophy, due to temperature dependence of cyclic deformation and damage mechanisms in TMF.[1–4] The present study is focused on the understanding of TMF deformation behavior of 316LN SS alloyed with varying nitrogen contents of 0.07, 0.14, and 0.22 wt pct, in comparison to IF tests conducted at the Tmax of TMF thermal cycle. The study also aims to identify the optimum nitrogen content for enhanced TMF life and to check the above design philosophy. An increase in the nitrogen content in 316LN SS is reported to have beneficial influence on the fatigue life under IF cycling which is attributed to an increase in slip planarity and an associated increase in the slip reversibility.[5] Optimum nitrogen content corresponding to the maximum life in IF tests has been reported to vary from 0.12 to 0.14 wt pct N.[5–7] On the other hand, very limite
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