The effect of nitrogen concentration in the shielding gas on microstructure and mechanical properties of autogenously plasma arc welded lean duplex Allegheny Ludlum ATI 2003® material was studied. Six single-pass plasma arc butt welds were produced for the evaluation. One weld was performed using 100% argon shielding and backing gas, while the other five utilized varying additions of nitrogen in the shielding and backing gas, from 1 to 5%, with argon as the primary gas. 100% argon was used as the plasma orifice gas for all welds. The coupons were labeled 0 through 5, which correlated with the concentration of nitrogen. The heat input was consistent for each weld and was representative of a typical value for production plasma arc welding of lean duplex stainless steel. Mechanical testing and evaluation was completed in accordance with typical customer requirements and industry standards. Each weldment was tested mechanically and analyzed microstructurally to investigate any correlations with the nitrogen additions in the weld shielding gas. Mechanical tests consisted of transverse and longitudinal tensile testing per ASME 2010 Section II, Part A, SA-370, Charpy impact testing per ASME 2010 Section II, Part A, SA-370 at -40° C, and micro-hardness testing per ASTM E384 with a test force level of 500 grams-force (gf). The microstructural analysis included ferrite testing, utilizing both the Fischer® Ferritescope and PAXit® software, and optical microscopy with focus on austenite formation and morphology, precipitate formation, and grain size comparison.
The mechanical tests from this study revealed that only specific coupons met the requirements provided in ASME Boiler and Pressure Vessel Code. The transverse tensile tests revealed that all the coupons met the minimum requirements for the tensile strength of 620 MPa (90 ksi). Conversely, the elongation for each of the coupons except coupon 5, with 5% nitrogen in the shielding gas, fell short of the 25% elongation minimum specified for the base material. Charpy impact tests disqualified coupons 0 through 2, which were unacceptable due to lateral expansions less than the minimum of 0.38 mm (.015 inches) per ASME Section VIII. Vickers micro-hardness testing was found to be optimum and below the ATI 2003® base metal requirement of 293 BHN per ASME Section II, Part A, SA-240. During ferrite testing, both using the Fischer® Ferritescope and the PAXit® software, it was determined that coupons 2 through 5 had optimum ferrite values of between 40 and 60%, for each the fusion zone, HAZ, and base metal. Conversely, Coupons 0 and 1 had values that fell outside this range. Neither secondary austenite precipitation nor formations of detrimental second phases were detected in any of the weld microstructures. In conclusion, 5 percent nitrogen added to the shielding gas had a beneficial effect on autogenously plasma arc welded ATI 2003® Lean Duplex Stainless Steel and resulted in optimum mechanical properties and microstructure of the weldment. Alternatively, autogenous plasma arc welding with 100% argon as the backing, shielding, and orifice gas resulted in unacceptable mechanical properties and an unbalanced ferrite-austenite ratio.