A comparative evaluation of low-cycle fatigue behavior of type 316LN base metal, 316 weld metal, and 316LN/316 weld join
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INTRODUCTION
THE 18-8 group of austenitic stainless steels, such as AISI type 316, 304, and their modified grades, finds applications as structural material for various components of the liquid-metal-cooled fast breeder reactor (LMFBR). The choice of these alloys is based on their excellent high-temperature tensile, creep, fatigue, and creep-fatigue strengths in combination with good fracture toughness and fabricability. With the current ability to control carbon content, the need for stabilization by either titanium and niobium additions to prevent intergranular corrosion or weld-decay has diminished considerably. The lower carbon content of 0.03 pct maximum in AISI 304L and 316L grades, although providing added resistance to sensitization, results in low usable strength that cannot be increased except by strain hardening. In this scenario, nitrogen-alloyed austenitic stainless steels have emerged as viable alternatives with enhanced high-temperature mechanical properties and lesser susceptibility to sensitization and associated intergranular corrosion. M. VALSAN, Scientific Officer, Materials Development Division, D. SUNDARARAMAN, Scientific Officer, Metallurgy Division, K. BHANU SANKARA RAO, Head, Mechanical Properties Section, Materials Development Division, and S.L. MANNAN, Head, Materials Development Division, are with Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India. Manuscript submitted February 7, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
The use of this alloy for LMFBR components, however, imposes some stringent requirements on the performance of the parent material as well as its welds. This arises due to cyclic thermal stresses on thick section components as a result of the temperature gradients that occur on heating and cooling during start-ups and shutdowns. Resistance to low-cycle fatigue (LCF), hence, is an important consideration in the design, operation, and safety of LMFBR components operating at high temperatures. As a result, the evaluation of elevatedtemperature LCF behavior of 316LN stainless steel has received much attention in recent years.[I-9] The primary objectives of these investigations have been to find out the effects of varying nitrogen contents on the LCF properties, to elucidate the mechanism of nitrogen interstitials and precipitates in LCF deformation, a n d to establish the composition of nitrogen-alloyed 316L steel that would give better LCF resistance than 316 steel. In these studies, nitrogen addition has been reported to be beneficial, and LCF life has been improved at both ambient temperature and 873 K. This beneficial effect of nitrogen has been found to saturate at approximately 0.12 pet of nitrogen. Further, aging has been found to reduce the fatigue life of alloys having higher nitrogen levels, especially at high strain levels. The LCF behavior of 316LN stainless steel welds has not been investigated. Some studies have been reported on the effect of nitrogen on the tensile properties of 16-8-2 welds I~~ and 20Cr-9Ni-3Mo stainl
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