Precipitates in HAZ of Ti-V-Nb microalloy steels

The dissolution,precipitation and grain coarsening of precipitates in the HAZ of submerged are welds deposited at 3 kJ/mm and 6 kJ/mm heat input on five Ti,V,Nb containing microalloy steels were examined.The Ti-bearing steel (0.014% Ti) showed little change in the TiN particle distribution on welding,being in agreemetn with the calculated result of very limited TiN solution during the weld thermal cycle.For low Ti-V steel (0.002% Ti,0.093% V) and low Ti-Nb steel (0.002% Ti,0.029% Nb),the V-rich particles and the Nb-rich particles in the base metal dissolved.During the heating leg of the cycle the Ti-rich particles formed,while on cooling,the V-rich and the Nb-rich particles precipitated again.Ti-V steel (0.014% Ti,0.08% V) and Ti-V-Nb steel (0.016% Ti,0.082% V,0.027% Nb) contained a wide distribution of particle compositions.On welding,the Ti-rich cuboidal precipitates did not dissolve while the V-rich and the Nb-rich spheroidal precipitates did dissolve.No reprecipitation was abserved on cooling in these two steels.The different behaviour of thc five steels was related to the dissolution of the precipitation in the basc metal and the available thermodynamic driving foree for reprecipitation on cooling.