Effect of niobium and carbon on weld solidification cracking susceptibility of ferritic stainless steels

• Ferrite fraction had little influence on the brittle temperature range (BTR). • Increasing niobium content widened the BTR in ferritic stainless steels. • Higher carbon content suppressed Nb segregation and narrowed the BTR at fixed Nb. • Competitive segregation between Nb and C governs solidification cracking behavior. Niobium is often added to welding consumables for ferritic stainless steels to improve properties, such as weld-decay resistance. However, its addition may increase solidification cracking susceptibility due to significant solidification segregation resulting from its low partition coefficient. Carbon also affects on the cracking susceptibility by solidification segregation and the formation of NbC. This study examined Fe–18 mass%Cr alloys with fully ferritic solidification while varying the ferrite fraction and contents of niobium and carbon to elucidate the effect of secondary-phase formation and elemental segregation on cracking susceptibility. The dependence of brittle temperature range (BTR) on the ferrite fraction of 27 to 100% was minimal, regardless of the niobium addition. Increasing niobium content increased the BTR, whereas at fixed niobium content of 1.0%, increasing carbon content from 0.04% to 0.10% decreased the BTR. Competitive segregation between niobium and carbon, and carbon enrichment suppressed niobium segregation while promoting NbC formation during solidification. Therefore, the decrease in the BTR with increasing carbon content is attributed to NbC precipitation, which increases the solidus temperature, and the segregation mitigation. These results highlight that controlling solidification cracking in ferritic stainless steels requires the simultaneous consideration of competitive segregation behaviour and secondary-phase formation.