Studies on autogenous laser welding of type 304B4 borated stainless steel

• Autogenous CO2 laser welding studies carried out on 3 mm thick SS 304 B4.
• Effect of laser power, welding speed, beam mode and welding speed was studied.
• Welds were analysed in as welded and post weld heat treated condition.
• Results are reported and discussed in detail.

304B4 Borated austenitic stainless steel is widely used in the nuclear industry due to high neutron absorption efficiency. In the present investigation, autogenous bead-on-plate (BoP) laser welding studies were carried out on 3 mm thick 304B4 grade stainless steel using a 3.5 kW slab CO2 laser. Influence of variables such as laser power, welding speed, shielding gas and laser beam mode on microstructure and mechanical properties were studied. Dye penetrant testing, macrostructural analysis, bead geometry measurements, microhardness survey, and microstructural analysis in both as-weld and post-weld heat treated conditions were carried out. The macrostructural and bead geometry analyses of the welds have shown that the welds were free from cracks in the fusion zone (FZ) and also in the heat affected zone (HAZ) for all the welding parameters studied. The Gaussian mode has given a very narrow weld width compared to donut mode. During welding use of helium and nitrogen has reduced the width of the FZ and HAZ. The as-weld micro hardness was more than double the base metal, and the peak hardness was shifted from the centre to the fusion boundaries with the increase in heat input. The PWHT has reduced the hardness of both the FZ and HAZ. In summary, usable laser welding parameters for welding 3 mm thick 304B4 grade stainless steel have been identified.