On Characterizing a Composite Microstructure in 316LN Stainless Steel Weld Metal and a New Damage Micromechanism During
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uctural components of fast breeder reactors are fabricated using 316LN stainless steel (SS).[1] In addition to assessing the base metal creep strength, it is also essential to evaluate the creep properties of weld joints as welding is the most resorted technique for fabricating huge components. The weld metal region is often considered as the weakest link in structural components. Delta ferrite in the weld metal which is intentionally prescribed in the microstructure to prevent hot cracking is the primary source for instability during elevated
V.D. VIJAYANAND, S.D. YADAV, P. PARAMESWARAN, P.K. PARIDA, and G.V.P. REDDY are with the Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. Contact e-mail: [email protected] K. LAHA is with the AUSC-Mission Directorate, Bharat Heavy Electricals Limited, Noida 201 301, India. Manuscript submitted January 4, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature exposure.[2] Apart from the presence of delta ferrite, the heterogeneity in the microstructure is enhanced due to the morphological and dislocation-substructural variations within the weld metal. Modifications in the morphology of delta ferrite and dislocation substructure occur in a multipass weld joint when the previously deposited passes are subjected to thermal cycling. Since there is a sharp gradient in the peak temperature attained across the weld-pass interface during the thermal cycle, these modifications are limited to a narrow region adjoining the weld-pass interface. A comprehensive understanding pertaining to the generation of the heterogeneous microstructure can help to evolve new micromechanisms which govern the microstructural instability in the weld metal during elevated temperature exposure. In the current study, systematic characterization of 316LN SS weld metal was carried out employing light optical microscopy, transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD) to understand the evolution of microstructural variation and its influence on creep cavitation during elevated temperature exposure. The chemical composition (in wt pct) of the base metal steel was 0.025C, 12.15Ni, 17.57Cr, 2.53Mo, 1.74Mn, 0.2Si, 0.004S, 0.017P, 0.14N and that of the electrode was 0.051C, 11.2Ni, 18.45Cr, 1.94Mo, 1.37Mn, 0.48Si, 0.005S, 0.024P, 0.1N. Shielded metal-arc welding process was used to fabricate the weld joint. Cross section of weld creep sample of 10 mm diameter and 50 mm gauge length containing both the weld metal and base metal was extracted from the weldment. Creep test was carried out at 923 K with an initial applied stress of 140 MPa. The rupture life of the joint was 5625 hours. Microstructural observations using optical microscope was carried out after electrolytically etching the polished samples with 60 pct HNO3 and 40 pct H2O solution at 2 V for 30 seconds. TEM examination was carried out on thin foils extracted from relevant locations in the weld metal. The extracted TEM coupons were dual jet polished using ele
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