Influence of initial microstructure on microstructural stability of 20% cold worked Ti modified austenitic stainless steel

Abstract

The influence of initial microstructure at different solution annealing conditions on the stability of 20% cold worked microstructure of a Ti modified austenitic stainless steel has been studied. Solution annealed and 20% cold worked samples were isothermally annealed at different temperatures in the range 923-1123 K for 1-1000 h. Mechanical property measurements, ultrasonic velocity measurements, and optical metallography were carried out to assess the stability of the cold worked microstructure during annealing. The extent of recrystallisation was measured using quantitative metallography. The variation of hardness and velocity with annealing time exhibited a three stage behaviour for the conditions leading to complete recrystallisation. Based on the microstructural observations, the three stages identified are recovery, progress of recrystallisation and completion of recrystallisation. There is one to one correspondence between the hardness and velocity measurements in assessing the microstructural changes. However, variation in the velocity measurements with annealing time correlated well with the extent of recrystallisation compared to that of the variation of hardness with annealing time. With an increase in solution annealing temperature from 1173 K to 1473 K, more primary TiC precipitates dissolve into the matrix and there is an increase in the secondary precipitation of TiC during annealing of the 20% cold worked alloy. The increase in secondary precipitation of TiC retards recovery and recrystallisation of the cold worked microstructure effectively and hence enhances the stability of cold worked microstructure. Thus, an increase in the solution annealing temperature from 1373 K to 1473 K enhances the stability of cold worked microstructure in Ti modified austenitic stainless steel.