Effect of Structural Heterogeneity on In Situ Deformation of Dissimilar Weld Between Ferritic and Austenitic Steel

Low-alloy steel and 304LN austenitic stainless steel were welded using two types of buttering
material, namely 309L stainless steel and IN 182. Weld metals were 308L stainless steel and IN
182, respectively, for two different joints. Cross-sectional microstructure of welded assemblies
was investigated. Microhardness profile was determined perpendicular to fusion boundary. In
situ tensile test was performed in scanning electron microscope keeping low-alloy steel-buttering
material interface at the center of gage length. Adjacent to fusion boundary, low-alloy steel
exhibited carbon-depleted region and coarsening of matrix grains. Between coarse grain and
base material structure, low-alloy steel contained fine grain ferrite-pearlite aggregate. Adjacent
to fusion boundary, buttering material consisted of Type-I and Type-II boundaries. Within
buttering material close to fusion boundary, thin cluster of martensite was formed. Fusion
boundary between buttering material-weld metal and weld metal-304LN stainless steel revealed
unmixed zone. All joints failed within buttering material during in situ tensile testing. The
fracture location was different for various joints with respect to fusion boundary, depending on
variation in local microstructure. Highest bond strength with adequate ductility was obtained
for the joint produced with 309L stainless steel-buttering material. High strength of this weld
might be attributed to better extent of solid solution strengthening by alloying elements, diffused
from low-alloy steel to buttering material.