Effect of Ti/N Ratio on Grain Growth Inhibition in the Coarse-Grained Heat-Affected Zone of Low Carbon Steels Simulated for Ultra-High Heat Input and Tandem Electro-Gas Welding

In this study, the effect of the Ti/N ratio on the coarsening behaviour of (Ti, Nb)(C, N) precipitates and prior austenite grain size (PAGS) in the coarse-grained heat-affected zone (CGHAZ) during the welding process was investigated. Oscillation and tandem electro-gas welding (EGW) were simulated using the finite element method to calculate the thermal history of the CGHAZ. The calculated thermal history was applied to the Gleeble specimens to simulate the CGHAZ of tandem EGW. As the Ti/N ratio decreased to ~1.5, and the amount of N increased, finer and more (Ti, Nb)(C, N) precipitates existed in the as-rolled and CGHAZ specimens (peak temperature ~ 1335 °C), thereby producing a small PAGS. Comparing with the previous studies regarding to the isothermal kinetics conducted by isothermal heat treatment of Ti(C,N), the athermal kinetics (simulation of tandem EGW) of (Ti,Nb)(C,N) showed the insignificant growth of PAGS. Furthermore, Nb-rich precipitates were dissolved in the matrix above 1000 °C, and they did not contribute to the PAGS growth of the CGHAZ. Graphical abstract