Enhanced Hardenability by Dissolution of Microalloy Carbonitrides in HAZ

Microalloy carbonitrides, such as Ti(C,N), are very effective to suppress the austenite grain growth at a low heat input welding in low carbon steel, but once they dissolve, the microalloys and nitrogen in the matrix might make the HAZ very brittle through the enhanced upper bainite transformation.  These double-edged-sword effects of microalloy carbonitrides are essential to understand the change of steel toughness at welding, but their both effects are hardly taken into account, simultaneously, in its chemistry design.

In this study, the dissolution and re-precipitation of some typical microalloy carbonitrides during the HAZ thermal cycles were estimated quantitatively by TEM/STEM observation experiment.  That was followed by the numerical calculation of kinetics of those particles.

The experimental study revealed that, when the weld heat input was somehow lower, a considerable amount of microalloys were released into the matrix as a result of the dissolution of their carbonitrides, without being accompanied by re-precipitation during the cooling leg.  That enhanced the hardenability of steel in the HAZ, bringing about upper bainite structure with concurrent mixture of martensite-austenite constituents.  But this hardenability enhancing was mitigating with increasing heat input because of the slower cooling rate after the peak temperature of the thermal cycle (Fig. 1)

Fig. 1 Amount of dissolved microalloys in the HAZ during welding

The theoretical or numerical supplementary work on this metallurgical process made it clear that the process was very dependent not only on the kind of microalloy but on the content of carbon and nitrogen, and on the size of the initial precipitates.  The quantitative effects of these factors were also discussed.