Solidification cracking susceptibility of extra high-purity type 304 and 316 stainless steels was evaluated by the transverse-Varestraint test using arc and laser welding, in order to clarify the feasibility of "ferrite-free welding". The solidification brittle temperature ranges (BTRs) of 304EHP and 316EHP steels were slightly increased with a decrease in the amount of δ-ferrite in the weld metal, while they were considerably lower than those of commercial-purity steels with FA solidification mode during arc as well as laser welding. It follows that the ferrite-free welding of austenitic stainless steels could be accomplished by applying the extra high-purity steels. Theoretical approach to the BTR was made to elucidate the depression mechanism of solidification cracking susceptibility in extra high-purity steels. From the fact that solidification segregation of minor and impurity elements such as C, P and S was considerably depressed in the extra high-purity weld metal, the solidification cracking susceptibility would be reduced even when the amount of ferrite was extremely small or free, due to the inhibition of solidification segregation. Numerical analysis of the BTR suggested that the ferrite-free welding of type 304 and 316 stainless steels could be achieved at the compositional range of P+S+0.1C<80ppm in the weld metal.
