EVALUATION OF BACKFILLED SOLIDIFICATION CRACKS IN AUSTENITIC STAINLESS WELDS IN RELATIONSHIP TO EVALUATION OF HOT CRACKING

This paper describes the significance of the backfilling phenomenon in solidification cracks on the evaluation of the weldability of stainle ss steels. Backfilling of solidification cracks was systematically eva luated in terms of: 1) Primary solidification mode (austenitic or ferr itic). 2) Augmented strain levels (2 & 4% Varestraint Test). 3) Type o f stainless steel (316 and 347). 4) Location in the weld and length of solidification cracks. The extent of backfilling was found to be a fu nction of the location of the crack in the weld and the solidification crack length, such that the shorter and narrower the crack, the great er is the potential for backfilling. The greatest extent of backfillin g occurs at the center of the weld where the temperature gradient is m ost shallow. Weld solidification in a primary ferritic mode has a grea ter potential for backfilling as compared to weld solidification in th e primary austenitic mode. The applied strain level does not appear to significantly influence the degree of backfilling. The concept of cap illarity can be successfully used in explaining the backfilling phenom enon. In austenitic stainless steel the backfilled region is rich in c hromium and/or nickel compared to the surrounding area depending on th e primary solidification mode. Silicon is also segregated in the liqui d during solidification and thus was detected in the backfilled solidi fication cracks. The formation of a Nb rich liquid, along the fusion z one grain boundaries during the solidification process, which forms a NbC-austenite eutectic, can play a role in the backfilling of cracks i n Type 347.