The influence of stabilization with titanium on the heat-affected zone sensitization of 11 to 12% chromium ferritic stainless steels under low heat input welding conditions
The occurrence of sensitization during continuous cooling after welding was investigated for two low chromium ferritic stainless steels: a non-stabilized steel conforming in composition to EN 1.4003, and a corresponding grade stabilized with titanium to increase its sensitization resistance. These steels transform partially to austenite in the high temperature heat-affected zone (HTHAZ) adjacent to the fusion line during cooling, with the austenite subsequently transforming to martensite below the Ms temperature. In the non-stabilized condition these alloys are known to sensitize during continuous cooling after welding at low heat inputs levels (below approximately 0.5 kJ/mm). The rapid cooling rates associated with low heat input welds suppress austenite nucleation in the HTHAZ, resulting in almost fully ferritic microstructures. Chromium-rich carbides precipitate at the continuous ferrite-ferrite grain boundaries, resulting in chromium depletion. Slower cooling rates promote the formation of more austenite in the HTHAZ during cooling. The austenite absorbs excess carbon, preventing supersaturation of the ferrite and inhibiting carbide precipitation. Titanium stabilization does not prevent sensitization during low heat input welding, and may even be detrimental. The high temperatures experienced by the HTHAZ promote the dissolution of titanium carbides, and the titanium is retained in solid solution due to fast cooling. At lower temperatures chromium-rich M23C6 precipitates nucleate at the ferrite-ferrite grain boundaries, resulting in sensitization. The titanium in solid solution acts as a strong ferriteformer, increasing the ferrite content in the HTHAZ. The non-stabilized 1.4003 steel contained considerably more grain boundary martensite in the HTHAZ after low heat input welding than the titanium-stabilized grade, despite having very similar Kaltenhauser ferrite factors. The presence of grain boundary martensite in the non-stabilized grade increased the resistance to sensitization under low heat input conditions.