Influence of Delta-ferrite on Sensitization of the Austenitic Stainless Steel Weld Metal

Sensitization behavior of the delta-ferrite containing stainless steel weld metal was investigated using Type 304 and Type 308 alloys with the carbon content ranging from 0.04% to 0.09%, respective-ly. The weld metals heat treated at 923 K for times varying between 1 sec and 100 hr were tested for intergranular corrosion susceptibility in the acidified copper sulfate solution. The delta ferrite contain-ing weld metal with lower carbon content indicated rapidly healing of sensitization after the short heat treatment, whereas ones with higher carbon content than a critical value behaved as a fully austenitic steel regardless the existence of ferrite without showing the sign of rapid healing. SEM observation revealed that M23C6 precipitated preferentially at the ferrite-austenite boundary and also at the austenite-austenite boundaries although its amount at the latter boundaries was much less than the other in the both weld metals which indicated rapid healing of sensitization and behaved as a fully austenitic steel. A mathematical model is developed for the sensitization of the ferrite containing austenitic stainless steel weld metal. The degree of sensitization calculated using this model has shown a good agreement with the experimental corrosion test results. The calculation also demonstrated that for a given carbon content there exist the critical values for not only the total amount but the thickness of ferrite, above which the weld metal develop the rapid healing of sensitization.