AN EXPERIMENTAL AND THEORETICAL-STUDY OF HEAT-AFFECTED ZONE AUSTENITEREFORMATION IN 3 DUPLEX STAINLESS-STEELS

Three duplex grades, one molybdenum-free, one 22Cr type, and one super duplex grade, have been subjected to weld simulation treatments, and the resulting microstructures have been quantified by automatic image analysis techniques. Substantial differences between the duplex grades were observed with an increased ability to reform austenite with incr eased alloying content. A theoretical model has been applied, based up on the paraequilibrium concept elaborated by Hillert, and the paraequi librium compositions of individual phases were calculated as a functio n of temperature using the THERMOCALC database. A model based on Cahns theory of grain boundary nucleated reactions: has also been utilized to calculate the kinetics of the reaction. By using this model, the gr ain size effects could be included in the treatment. The results of th e calculations were compared with experimental data, and the experimen tal results were reproduced using the same parameter set for the three materials, with the exception of the diffusion coefficient values whi ch had to be adjusted. This adjustment has in a later study been verif ied experimentally. The results validate the model used and the physic al relevance of using the paraequilibrium model. The appropriateness o f a paraequilibrium approach is also supported by experimental evidenc e from weld metal compositions. It is shown that the nitrogen content of the alloys plays an important role, and a higher nitrogen content r esults in more efficient austenite reformation. This implies that the alloy nitrogen compositions should lie close to the upper specificatio n limits for these materials and nitrogen losses should be avoided on welding since: the material properties, both-mechanical and corrosive, are strongly related to the austenite-ferrite phase ratio.