Precipitate alterations in the heat-affected zone of a Grade 100 microalloyed steel

Microalloy precipitate alterations (particularly dissolution) in the heat-affected zone (HAZ) of a Grade 100 steel, microalloyed by titanium, niobium, and vanadium and produced in the form of a plate with a thickness of 8 mm, was examined both theoretically and experimentally. For theoretical analysis of precipitate dissolution, pairs of effective peak temperature and holding time were extracted from the thermal cycles of welding, and were superimposed on the Ashby and Easterling non-equilibrium solubility curves for different fractions of precipitate dissolution. Intersections between the effective T–t curves and the non-equilibrium solubility curves gave critical pairs of effective peak temperature and holding time for dissolution of different fractions of a precipitate, which resulted in the establishment of precipitate dissolution profiles in the HAZ. Experimental analysis of precipitate alterations was carried out using carbon extraction replicas in a transmission electron microscope. The theoretical analyses were in agreement with experimental results, showing that it is the dissolution of small Nb-rich particles that paves the way to grain growth in the coarse-grained HAZ. Reprecipitation was generally suppressed in the low heat-input weld sample. There was some reprecipitation in the higher heat-input weld samples. Coarsening of TiN did not occur in the HAZ, due to the large size of these particles in the steel examined.