Study on the microstructure and mechanical properties of hybrid laser plus MIG welded joints of 316LN stainless steel

Austenitic stainless steels are suitable choice for nuclear power plant components due to their excellent creep strength, desirable mechanical properties, and corrosion resistance. The mechanical behaviour of weld joints depends on the microstructure of the material. The weld thermal cycle significantly influences the evolution of microstructure in the weld joints. This research work presents a systematic investigation on microstructure and mechanical behaviour of hybrid laser + Metal Inert Gas (HLM) welded 11 mm thick type 316LN stainless steel joints. Changing the arc current at constant laser-arc source positions influenced the depth of penetration (-40%) due to the laser-arc interaction effect. Thus, the laser-arc source position (2-3 mm) and arc current were chosen in such a way as to achieve deep penetration. The average delta (8)-ferrite across the weld zone is found to be around 1.3 ferrite number. However, a substantial increase (-95 %) in 8-ferrite content was observed in the through-thickness direction of the fusion zone owing to the filler material addition. The through-thickness variation in the strength properties of the weld joint was estimated by using the ultra-sub size tensile test. The standard conventional uniaxial tensile tests were also performed and correlated with microstructure of the fusion zone. The impact toughness of the base material was 315.4 +/- 0.9 Joules and HLM weld 163.4 +/- 27 Joules. The 8-ferrite content, secondary dendrite arm spacing (-2.9 - 3.6 mu m), and variation in zone-wise microsegregation were the primary reasons for the mechanical properties of the HLM weld joint.

Automated summary

Austenitic stainless steels are ideal for nuclear power plant components due to their excellent creep strength, desirable mechanical properties, and corrosion resistance. The microstructure and mechanical behavior of weld joints are influenced by the weld thermal cycle. This research explores the microstructure and mechanical behavior of 316LN stainless steel joints welded using a hybrid laser + Metal Inert Gas (HLM) method. The depth of penetration was significantly affected by the interaction between the laser and the arc, leading to adjustments in the laser-arc source position and arc current to achieve deep penetration. The mechanical properties of the HLM weld joint were primarily influenced by the 8-ferrite content, secondary dendrite arm spacing, and microsegregation variation.