When austenitic stainless steels such as SUS304 are welded, intergranular corrosion can occur in the heat-affected zone a short distance from the weld line (weld decay). Which combination of cause and countermeasure is correct?

Answer: C. In zones heated to about 500 to 800°C, Cr carbides precipitate at grain boundaries, leaving Cr-depleted zones. Use SUS304L or Ti- or Nb-added steels

In the heat-affected zone a short distance from the weld line, there is a band exposed for some time to temperatures of about 500 to 800°C. There, Cr carbides such as Cr₂₃C₆ precipitate at grain boundaries, and the Cr concentration right next to the boundaries falls to a level that cannot sustain passivity, so corrosion proceeds along the grain boundaries. This is called sensitization. Countermeasures are to use a low-carbon steel (SUS304L) with less of the carbon that forms the carbides, to use stabilized steels (SUS321 or SUS347) in which carbon is tied up as Ti or Nb carbides, and to give a solution heat treatment after welding to dissolve the carbides again. Austenitic stainless steels do not become martensite on quenching, and Cr evaporation is not the main cause either.