Effect of cold rolling on the precipitation of Custom 455 stainless steel during aging at 480°C

Abstract

This work studies the effects of cold rolling on the evolution of nanoscale precipitates and associated strengthening in the precipitation hardenable Custom 455 stainless steel during aging at 480°C by atom probe tomography, transmission electron microscope, and mechanical property testing techniques. The quantitative and correlative analyses of how cold deformation-induced dislocation structure affected the nucleation, morphology, and composition of Cu-rich precipitates and other phase precipitates in Custom 455 during aging were achieved for the first time by comparing the steel samples with and without cold rolling. The results showed that the cold rolling by 75% deformation enhanced the level of response to age hardening at a shorter time, giving rise to a peak value of yield strength of 1790 MPa and a fairly high elongation (EL) of 6.6%, which is 120 MPa higher than the standard value of 1670 MPa for the Custom 455 stainless steel used in industry. In comparison with the solution-treated sample, the cold-rolled sample had a higher number density of spherical Cu-rich and Cu/Ni/Ti/Si clusters when aged for 5 min, due to the introduction of a high density of dislocations and other defects by cold rolling that served as nucleation sites for the Cu-rich and Ni/Ti/Si-rich clusters. With increasing aging time to 0.5 h, the Cu-rich precipitates and Ni₃Ti phase precipitates grew larger, showing the highest number density of precipitates during the aging process, and the G phase particles were formed. After being aged for 4 h, the sizes of Cu-rich precipitates, Ni₃Ti phase, and G phase particles increased for both the solution-treated and cold-rolled samples, while their number densities decreased. The orientation relationship between the three precipitates and the matrix obeys: (111)_Cu//(0004)_Ni3Ti//(022)_G//(011)_M and [0−11]_Cu//[−12−10]_Ni3Ti//[100]_G//[100]_M.