Solidification behaviour of laser welded type 21Cr-6Ni-9Mn stainless steel

The solidification mode and microstructures were characterised for various processing parameters for laser welding 21Cr-6Ni-9Mn stainless steel. Two heats with varying nitrogen content showed both primary ferrite and primary austenite solidification. Weld ferrite content varied from 1 to 11 vol.-%, and decreased as travel speed increased. Base metal nitrogen content affected both solidification mode and weld ferrite content. Nitrogen loss from the weld pool was found to range from 10 to 45%, and decreased with increasing travel speed. Solidification mode was dependent on both chemical composition and processing parameters. The solidification mode shifted from primary ferrite to primary austenite as travel speed increased due to increased undercooling. Solidification mode varied within welds with constant nitrogen content, and the change in solidification mode was attributed to changes in undercooling along the weld cross-section. It is proposed that the variation of solidification mode with undercooling is affected by both the solidification rate and thermal gradient.