Influence of Thermal Aging on Tensile and Low Cycle Fatigue Behavior of Type 316LN Austenitic Stainless Steel Weld Joint
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DUCTION
LOW-CYCLE fatigue (LCF) is an important design consideration in components operating at elevated temperatures. Austenitic stainless steels are extensively used in the construction of various components of sodium-cooled fast reactors (SFRs). Type 316LN austenitic stainless steel (316LN SS) containing 0.02 to 0.03 wt pct carbon and 0.07 to 0.08 wt pct nitrogen is the preferred structural material for the primary side components such as the main vessel, the inner vessel, and the intermediate heat exchanger of SFRs in view of its microstructural stability, coupled with an excellent combination of mechanical and chemical properties, good weldability, and compatibility with the liquid T. SURESH KUMAR is with the Homi Bhabha National Institute, Kalpakkam, Tamil Nadu, 603102, India. A. NAGESHA, J. GANESH KUMAR, and R. SANDHYA are with the Materials Development and Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, 603102, India. Contact e-mail: [email protected] P. PARAMESWARAN is with the Physical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, 603102, India. Manuscript submitted September 12, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
sodium coolant.[1] However, a low thermal conductivity and a high thermal expansion coefficient of the alloy result in significant amounts of thermal stresses during startup and shutdown operations and thermal fluctuations in liquid metal coolant of the reactor. The repetition of the above processes induces strain-controlled LCF loading. Fabrication of large components inevitably involves numerous weld joints that are microstructurally and mechanically heterogeneous. It is well recognized that the integrity of a welded structure is largely dictated by the properties of the weld joint. Fatigue-life evaluation of the weldments, though, has traditionally been based on the properties of the base metal, factored by an appropriate safety margin to account for the material and the geometric discontinuities. This is mainly due to the inadequate database available on creep, fatigue, and creep–fatigue interaction properties of the welds and weldments. Better understanding of the behaviors of welds would add to the confidence in design and associated safety factors. In SFRs, many primary side components inside the reactor vessel are inaccessible for in-service inspection, and hence, the resistance to surface microcrack initiation/ propagation is of utmost importance in the overall fatigue behavior of such components.
A significant problem encountered with the production of fully austenitic stainless steel welds is their tendency to hot cracking and microfissuring. In order to minimize these damages, the composition of the welding consumables is adjusted in such a way that a primary ferritic mode of solidification results, with a small amount of d-ferrite retained in the weld deposit.[2] The d-ferrite is only stable at high temperature while existing as a metastable phase in the austenitic matrix at room temperature. Up
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