Effects of N2 content in shielding gas on microstructure and toughness of cold metal transfer and pulse hybrid welded joint for duplex stainless steel

Abstract

The influences of N2 content in shielding gas on microstructure and impact toughness of different zones in cold metal transfer and pulse (CMT-P) hybrid welded joint of duplex stainless steel (DSS) were systematically studied. The results showed that the N2-supplemented in shielding gas significantly facilitated austenite formation (weld root: 39.9%→41.2%, weld filler: 40.5%→43.7%, and heat affected zone (HAZ): 36.4%→39.6%). However, when the N2 content in the shielding gas exceeded 4%, there was no significant change in austenite content because of reaching the solubility limit of N atoms. In addition, γ2 precipitated both in the weld root and HAZ but not in the weld filler, which cannot be inhibited by N2 addition in the shielding gas. Besides, a great number of Cr2N and dislocations in addition to γ2 formed in the HAZ, and the content of Cr2N and dislocation significantly increased with the increase of N2 content in the shielding gas. Furthermore, the HAZ exhibited the lowest toughness in comparison with other zones. In addition, with the increase of N2 content in the shielding gas from 0% to 6%, the toughness increased first and then decreased, and reached the maximum when 4% N2 added to the shielding gas as (weld root: 122.0 J/cm2, weld filler: 135.0 J/cm2, and HAZ: 91.8 J/cm2). According to detailed microstructure analysis and toughness level, Ar+4% N2 was recommended as the shielding gas to join DSS by using CMT-P welding technique.