Process Optimization for Friction-Stir-Welded Martensitic Steel

Advanced high-strength M190 steel sheets were joined by friction-stir welding under different
tool rotational and traversing speeds. The optical microstructure of the joints exhibited complete
martensite and partial martensite at the weld nugget depending on the cooling rate during
welding. The first heat-affected zone outside of the weld nugget revealed ferrite-pearlite phase
aggregate, and the second heat-affected zone showed a tempered martensitic structure. The
interplay of process variables in terms of peak temperature and cooling rate was studied to
observe their effect on joint efficiency under shear testing. The peak hardness at weld nugget was
close to the parent alloy at an intermediate cooling rate of 294 to 313 K/s. The lowest hardness
was observed at the first heat-affected zone for all welded joints. Joint efficiency was dependent
on relative quantity of ferrite-pearlite at first heat-affected zone. In that respect, the intermediate
temperature to the tune of ~1193 K to 1273 K (~920 C to 1000 C) at the weld nugget was
found to be beneficial for obtaining an adequate quantity of pearlite at the first heat-affected
zone to provide joint efficiency of more than 50 pct of that of parent alloy.