System study of the formability, nitrogen behaviour and microstructure features of the CMT wire and arc additively manufactured high nitrogen Cr-Mn stainless steel

Abstract Large-scale high nitrogen steel (HNS) components with complex facade promote the application of Wire and Arc Additive Manufacturing (WAAM) technology in this area. However, the lack of fundamental research restricts its development. In this study, a self-made HNS wire with 0.99 wt.% nitrogen content (N%) was systematically studied in the aspects of the additive seam formability, nitrogen behaviour, along with microstructure characterizations. A designed Tanguich experiment with three factors (cold metal transfer (CMT) models, wire feeding speed and oxygen content in shielding gas) and four levels were used to reveal the suitable parameters for AM and the rule of nitrogen behaviour in the molten pool. For the semi-quantitative calculation of the N%, a modified kinetic model was proposed with the prediction accurate less than 12 %. Besides, the nitrogen pore and microstructure characterizations under different CMT models were studied. Results showed that for fewer pores in a smaller size, the molten pool volume should be in the range of 200 ∼ 300mm3. The austenite cell substructures in the microstructure were mainly found in the low heat input additive seams with a diameter around 9 μm. The microhardness test showed that for the AM of HNS, the first few layers can also be used with no need to remove. When the N% is lower than 0.8 %, positive relativity between N% and microhardness can be expected. Besides, the fluctuation of microhardness was found to be connected with the irregular columnar grains, Mn Oxides and Cr nitrides in the additive seams.