Microstructural features of dissimilar welds between 316LN austenitic stainless steel and alloy 800

For joining type 316LN austenitic stainless steel to modified 9Cr-1Mo steel for power plant application, a trimetallic configuration using an insert p iece (such as alloy 800) of intermediate thermal coefficient of expansion ( CTE) has been sometimes suggested for bridging the wide gap in CTE between the two steels. Two joints are thus involved and this paper is concerned wi th the weld between 316LN and alloy 800. These welds were produced using th ree types of filler materials: austenitic stainless steels corresponding to 316, 16Cr-8Ni-2Mo, and the nickel-base Inconel 182(1). The weld fusion zon es and the interfaces with the base materials were characterised in detail using light and transmission electron microscopy. The 316 and Inconel 182 w eld metals solidified dendritically, while the 16-8-2 (16%Cr-8%Ni-2%Mo) wel d metal showed a predominantly cellular substructure. The Inconel weld meta l contained a large number of inclusions when deposited from flux-coated el ectrodes, but was relatively inclusion-free under inert gas-shielded weldin g. Long-term elevated-temperature aging% of the weld metals resulted in emb rittling sigma phase precipitation in the austenitic stainless steel weld m etals, but the nickel-base welds showed no visible precipitation, demonstra ting their superior metallurgical stability for high-temperature service. ( C) 2000 Elsevier Science S.A. All rights reserved.