High power laser welding of Nb microalloyed steel plates

Laser welding studies on 12 mm thick Nb microalloyed steels were done using a 25 kW CO2 laser at welding speeds of up to 3 m min(-1) with the aim of identifying the influences of Nb and carbon on laser weldability and weld properties. Welds were examined for solidification flaws, penetration characteristics, microstructure and mechanical properties. Solidification cracking did not appear to be influenced by carbon or Nb in the ranges studied when welding at power levels between 22 and 25 kW. The area fraction of martensite was shown to increase with decreasing energy input and increasing carbon content. Weld metal toughness was improved by reducing carbon content in the range 0.08 - 0.05% C but was little influenced by Nb. Weld metal and heat affected zone hardness levels were reduced, as expected, by reducing carbon content. The results show that increasing Nb content leads to higher strength material without significant loss of toughness, while lowering carbon content can improve toughness without loss of strength. Thus low carbon Nb microalloyed steels with about 0.05% C can achieve a good combination of parent plate and laser weld properties and appear to be suitable for high power laser welding situations.