Effects of preheating on laser beam-welded NSSC 2120 lean duplex steel

Duplex stainless steels show sustainable alternative for the conventional austenitic grades, with higher strength, higher resistance against stress corrosion cracking, and lower purchase cost. Thus, duplex stainless steel gains more attention in construction, oil and gas, and chemical industries. Among duplex stainless steels, low nickel and low molybdenum alloyed lean duplex stainless steel are a cost-effective substitution of austenitic grades. However, keeping the balanced ferrite/austenite phase ratio in the weld metal can be challenging, mostly for autogenous and low heat input welding processes. In our research, a newly developed NSSC 2120 lean duplex stainless steel grade was welded autogenously by fiber laser welding process. Different welding parameters and preheating temperatures were used during the experiments. The welds were evaluated by microscopic and metallographic techniques, and also by electrochemical corrosion measurements. The welding parameters and the preheating temperature greatly influenced the weld shape and the austenite content in the weld metal. It was found that the focus point distance from the sheet surface had significant effect on the weld geometry. Changing the focus point distance to + 2 mm, the penetration depth increased from 4.96 to 5.53 mm, and increased the austenite content by 2.6%. Due to the preheating the welds became wider and shallower, e.g., from 4.96 to 4.08 mm penetration depth, while the cross-section are increased from 5.10 to 6.12 mm2 at the same sample. The preheating resulted in more intergranular austenite formation, which meant maximum 4% increase in the weld metal. The increasing austenite content resulted in increasing pitting corrosion resistance in 3.5% NaCl electrolyte, the highest increase was 90 mV.