Effects of composition and microstructure on constitutional liquation in a niobium-bearing austenitic stainless steel

A systematic study of persistent weld heat-affected zone constitutional liquation cracking in Nb-bearing Type 347 stainless steel with a focus on both compositional and microstructural effects has been conducted. The study employed a series of custom-designed and melted alloys comprising the matrix of a design experiment of composition, with thermomechanical processing that permitted the superimposition of microstructural variations and size and distribution of niobium carbide particles.; Constitutional liquation of NbC and associated susceptibility to cracking during welding were fully characterized by Varestraint and Gleeble(TM) hot ductility tests. The kinetics of constitutional Equation of second phase NbC particles during rapid heating were studied via Gleeble-based quench tests.; It has been confirmed that HAZ hot cracking susceptibility in Type 347 stainless steel is attributable to localized melting of austenite grain boundaries decorated with NbC that undergo constitutional liquation. Furthermore, it has been shown that HAZ liquation cracking in Type 347 stainless steel is not only affected by chemical composition, but is also affected by the microstructure, specifically the size and distribution of second phase NbC particles.; Increasing Nb and N levels have a negative effect, while increasing C level has a positive effect on the alloy's liquation cracking resistance. However, significant scattering of crack resistance data, as a function of Nb/C ratios, indicated that a Nb/C ratio of 10 alone, as specified by most industrial standards, seems inadequate to ensure HAZ liquation cracking resistance. Size, distribution, and dissolution kinetics of the NbC particles exhibit major microstructural effects, because the melted grain boundaries in both the fusion-welded and simulated HAZs are found to be always associated with partially liquated NbC particles. While the quantity of NbC in the as-received condition is mainly a linear function of Nb content in the alloy, the size and amount of particles in the cracked regions in the HAZ are affected by the dissolution kinetics of these particles. Alloys with a Nb/C ratio closer to the stoichiometry of 7.7 contain NbC particles that are more stable and thus slower to dissolve.