Influence of Ti/(C+6/7N) ratio on the recrystallization behaviour of a cold worked 15Cr-15Ni-2.2Mo-Ti modified austenitic stainless steel

The influence of Ti/(C+6/7N) ratio on the recrystallization behaviour of 15Cr-15Ni-2.2Mo-Ti modified austenitic stainless steel was studied. The alloys were given a thermo-mechanical treatment which involved solution annealing at 1343 K for 0.5 h followed by 20% cold work. The cold worked samples were aged at 873–1173 K for times up to 1, 100 and 1000 h. Recrystallization behaviour was studied by carrying out hardness and tensile property measurements supported by optical metallography of microstructural changes on the aged samples. Using a thermodynamic model given in the literature the amount of primary Ti(C,N) precipitates, which remain undissolved after solution annealing treatment, and secondary TiC precipitates, which form during post deformation aging, were estimated. The amount of primary coarse precipitates increased with an increase in the Ti/(C+6/7N) ratio of the alloys and they facilitated earlier onset of recrystallization. The amount of fine TiC precipitates, which retarded recovery and recrystallization was maximum in the alloy with Ti/(C+6/7N) ratio of 3.67 compared to other two alloys. The formation of a maximum amount of TiC precipitates in the matrix along with the formation of grain boundary precipitates in the alloy with Ti/(C+6/7N) ratio of 3.67 provided the best resistance against recrystallization of the cold worked microstructure. The alloy with Ti/(C+6/7N) ratio of 3.67 exhibited higher temperature and time for recrystallization compared to other two alloys.