Solidification microstructure of laser welded stainless steels

The solidification microstructures of pulsed Nd:YAG laser or CW CO2 laser welds were studied in commercial SUS 310S and 304 (corresponding to AISI Types 310S and 304) austenitic stainless steels and in other several austenitic and duplex stainless steels. In the pulsed laser welding of Type 310S, neither grain growth nor recrystallization grain-refining was observed in the heat-affected zones (HAZ), and planar and cellular growth occurred epitaxially in weld metals from adjacent, incompletely-melted grains in the HAZ. According to the data relating a primary dendrite arm spacing to the cooling rate in Type 310S weld metals, the average local cooling rates within the single laser shot welds were extrapolated to be so rapid as to range from 5×104 to 5×106°C/s depending mainly on the pulse energy. In the case of Type 304, the pulsed laser weld metals were almost fully austenitic in contrast to GTA or CO2 laser weld metals containing about 5% residual delta(δ)-ferrite. It was significantly confirmed that almost fully austenitic structure of Type 304 could be produced with extremely high cooling rates obtained by CW CO2 laser welding process with low heat inputs at high traverse speeds. From the microstructural observation of several materials, it was further revealed that the solidification microstructures of pulsed laser welds were not consistent with the prediction from the Schaeffler diagram. In other words, the steels containing the calculated contents of less than 8% δ-ferrite or more than 30% δ-ferrite exhibited fully austenitic or fully ferritic microstructure, so that the compositional region indicating a duplex microstructure was very narrow in the case of pulsed laser weld metals. The disappearance of ferrite in the laser-welded microstructure was interpreted in terms of the change in the solidification process due to extremely high cooling rates accompanied with large undercooling. In addition, the change from duplex to fully ferritic structure was attributed to the suppression of the solid-state, ferrite-to-austenite transformation.