Abstract
The behaviour of carbo-nitride particles in HSLA steels when subjected to a weld thermal cycle corresponding to heat inputs of 2, 5, 10 or 20 kJ/mm has been studied by analytical electron microscopy. Three types of response were observed: 1. (a) complete dissolution followed by reprecipitation on cooling (in a Nb-V containing steel), 2. (b) partial dissolution accompanied by precipitate coarsening (in two Ti bearing steels) 3. (c) dissolution followed by reprecipitation at the peak temperature of the thermal cycle (in Ti-Nb bearing steels). The latter behaviour was shown to be dependent upon the reheat temperature used during manufacture and was only found in a steel reheated to 950°C; this treatment promoted the formation of Nb-rich particles having a narrow size range in the base metal. The rates of dissolution observed in the experiments were supported by numerical calculations, which gave dissolution times of the order of 1 s or less at the peak temperature (1350°C) of the weld thermal cycle. The changes in the particle size distributions in the Ti and Ti-Nb bearing steels and the composition changes observed for (Ti, Nb) carbo-nitride particles were explained by modelling the precipitation behaviour in the austenite phase field.
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