Effect of powder feeding mode on the stability and nitrogen distribution of high nitrogen steel welding process

The effect of powder feeding mode on the stability and nitrogen distribution of the high nitrogen steel (HNS) welding process was evaluated. The N loss of the joint was serious while the tensile strength was reduced to about 63.4 % in the absence of any effective measure. Four different configurations of powder feeding were designed to study the feasibility of powder feeding in the laser CMT hybrid welding process. And the powder feeding process was analyzed by arc shape and current-voltage curves which were collected by the Acuteye V4.0 welding observation system at 4000 frames/s. The most stable configuration was to fill the powder behind the arc. But non-uniformity was observed in the microstructure and properties in these configurations. The difference of ferrite content and tensile strength between the arc zone and laser zone reached 4.9 % and 150 MPa, respectively. A multi-stream powder feeding assisted laser-CMT welding method was proposed to improve the powder distribution and arc stability. The powder distribution was similar to the theoretical range and narrowed the ferrite content gap between the arc and laser zone to 1.3 %, and also the arc became stable even when a proportion of N2 was used. The tensile strength of the HNS welded nitride by CrN powder could reach 1198 MPa, which was 24.5 % higher than that of the non-nitride joint. However, the combination of N2 and powder nitriding needs to be explored in the future work.