Microstructural characteristics of fusion zone in continuous wave fiber laser welded Nb-modified δ-TRIP steel

Welding processes were accomplished by means of a 600 W Continuous Wave (CW) fiber laser instrument in bead on plate manner with 60, 80 and 100 J mm −1 heat inputs. δ-TRIP Steel samples were similar in chemical composition and only contained different Nb (40Nb: 0.040 & 72Nb: 0.072 wt.% Niobium) amounts. Elevation of heat input resulted in changes in microstructure and hardness properties, which are desirable for welders to guarantee safe joints in engineering structures. An increase in the heat input can lead to reduction of the austenite in weld metals of both steel sheets. The reduction of the austenite could be attributed to the increase in δ-ferrite volume fraction which was induced by availability of sufficient time for segregation of aluminum and formation of more dendrite structured δ-ferrite at lower cooling rates. Increase in δ-ferrite content results in decreasing austenite content required for transformation of austenite to martensite. The direct consequence of a decrease in the martensite volume fraction is reduction of hardness. It is shown that an increase in the heat input from 60 to 100 J mm −1 results in reduction of hardness difference between weld and base metals by 40% and 37% for 40Nb and 72Nb steels respectively.