Isothermal dissolution kinetics of a phase in a superduplex stainless steel UNS S32760

The σ phase is a Cr- and Mo-rich intermetallic, which can arise after relatively long holding times at temperatures ranging from 923 K to 1223 K as well as after cooling from high temperatures or in the heat affected zone produced by welding. In duplex and superduplex stainless steels (DSS & SDSS), this phase is known to dramatically decrease toughness and corrosion resistance of the alloy [1,2]. However, while the kinetics of σ phase precipitation is widely studied in literature [2-5], and different models have been developed to foreseen its formation according to the material composition and cooling rates [6-8], much less attention has been paid to its dissolution kinetics during a solution heat-treatment. In this paper, the kinetics of σ phase dissolution in a SDSS (UNS S32760) was studied both at 1323 K and 1353 K and the main results obtained are summarised as follows: 1. By using the Avrami and Wang et al. [16] models, the dissolution rate was observed to have the same expression of the precipitation rate (Eq. (3)). 2. A semi-empirical dissolution model was proposed in which the Wang et al. equation was generalized by an impingement factor of the form yc+1 (Eq. (5)). 3. The dissolution kinetics of σ phase in the analysed SDSS is well described by the proposed model with the impingement exponent c and exponent n equal to 1 and 1.82, respectively (Fig. 5). While the n exponent was found to depend on prior ageing conditions and nearly temperature independent, the k parameter was found to depend both on temperature and prior ageing conditions (Tab 3). Since any undissolved precipitates and unremoved chemical non-uniformities in the parent phase during homogenization greatly affect the precipitation process upon cooling, the results obtained in this work can be very useful both for the optimisation of the solution heat treatment parameters and for the heat-treatment numerical simulation models.