Ferrite number (FN) is a crucial parameter for austenite steel welding product, since it has significant influences on the crack sensitivity and ductility at cryogenic temperature. Normally, the FN is measured using a Feritscope or predicted according to WRC-1992 and the results generally match. This investigation detected the FN of 304L stainless steel multi-pass GTAW joint protected by mixed shielding gas with different N₂ additions; the measured FN did not match with the FN predicted through WRC-1992 as the N₂ content was more than 1%. The microstructure showed that a fully austenitized zone formed at the inter-bead of the joint shielded with Ar–1.0%N₂ and Ar–1.5%N₂, which resulted in the inaccuracy of the FN predicted through WRC-1992. The formation mechanism of the fully austenitized zone was proposed based on the nitrogen solubility and equilibrium phase transformation map. It is suggested that traditional prediction diagrams cannot give an accurate prediction of the FN during the multi-pass process shielded with N₂ addition.

铁素体数（FN）是奥氏体钢焊接产品的关键参数，因为它对低温下的裂纹敏感性和延展性具有重大影响。通常，FN是使用Feritscope测量或根据WRC-1992进行预测的，结果通常是匹配的。这项研究发现了304L不锈钢多道GTAW接头的FN受不同N ₂添加的混合保护气保护。N ₂含量超过1％，因此测得的FN与通过WRC-1992预测的FN不匹配。显微组织表明，在用Ar–1.0％N ₂和Ar–1.5％N ₂屏蔽的接头的焊缝之间形成了一个完全奥氏体化区，这导致通过WRC-1992预测的FN不准确。根据氮的溶解度和平衡相变图，提出了完全奥氏体化区的形成机理。建议传统的预测图不能在N ₂加成屏蔽的多程过程中给出FN的准确预测。