Abstract:
The study comprised of the investigation into the sensitization
characteristics of AISI 409 titanium stabilized ferritic stainless steel
during low heat input welding. AISI 409 is a fully ferritic stainless
steel used in catalytic converters and tubing for automotive exhaust
systems due to the supposition that sensitization does not occur
during low heat input welding. Two plates of 2 and 4 mm were
tested for sensitization during low heat input welding. The study
confirmed that chromium carbide (M23C6) precipitation occurs on
the grain boundaries in the heat-affected zone with consequent
depletion of Cr adjacent to the grain boundaries during welding. In
the 2 mm plate sensitization occurred in the heat input range of 0.1
kJ/mm to above 0.25 kJ/mm and in the 4 mm plate sensitization
occurred in the heat input range of 0.2 to 0.9 kJ/mm. Ti stabilization
is ineffective at the heat inputs used in this investigation due to the
rapid cooling rate through the temperature region where TiC precipitates.
The presence of N was found to be detrimental since it
consumes Ti on cooling, forming TiN, effectively lowering the
amount of Ti available for TiC formation during rapid cooling after
welding. Annealing for 5 minutes at 725°C does not improve the
sensitization characteristics of the welded plate. There is a linear
increase in grain size as heat input increases. The resistance to
pitting corrosion decreases in the sensitized, welded plate in the
heat-affected zone adjacent to the fusion line of welding.
