Static recrystallization and hot ductility of molybdenum- and nitrogen- alloyed austenitic stainless steels in association with two- and multistep deformation

Abstract

The kinetics of static recrystallization in connection with two-step deformations have been studied for an austenitic stainless steel of type DIN Wnr. 1·4439 (18Cr–12Ni–4·4Mo–0·2N) by hot compression testing. Experiments were performed on both wrought and cast materials to total strains in the range ε = 0·20−0·40 and at temperatures of 1050 and 1150°C. The results show that an increased pause time between the two deformation steps increases the time for achieving a given fraction recrystallized as compared to that for a single deformation performed with the same strain. The reason for this is an increased amount of softening with increasing pause time, i.e. a lower driving force for recrystallization. Using a model for recovery, the temperature and strain dependence of the fraction recrystallized could be calculated with high accuracy. A model has also been developed for calculation of the fraction recrystallized during multipass deformation and for continuous cooling conditions. The hot ductility was determined for AISI 316L, AISI 316LN, and DIN Wnr. 1·4439 at temperatures between 1050 and 1250°C. The results show a reduced hot ductility with increasing molybdenum and nitrogen contents in the steel. Experiments were also performed for DIN Wnr. 1·4439 to examine the effect of a pause time on the hot ductility. Increasing fractions recrystallized during the pause time improved the hot ductility up to about 50%. An analysis is presented of how hold times should be introduced into a rolling schedule to maximize the fraction recrystallized and thereby enhance the hot ductility.

MST/359