Innovation in thermal cycling aging compared to isothermal aging for precipitation hardening stainless steel

Abstract

The cyclic thermal process can assist and accelerate the kinetics of phase transformation. Conventional UNS S17400 grade stainless is characterized by a martensitic microstructure. After solution treatment, the steel was aged by thermal cycling between 600 degrees C and 25 degrees C and quenched in water in each cycle, completing under the self-designed system. The nano precipitates of very fine copper particles and larger NbC particles were found by using transmission electron microscopy (TEM). The fraction and quantity of high angle grain boundaries (HAGBs) after 36 cycles were the highest among the three numbers of thermal cycles. The peak hardness also occurred after 36 cycles and was attributed to the finest grains, high fraction of HAGBs, and the largest local microstrain. The microtwins and the reverted gamma were formed by the thermal cycling process. The estimated fraction value of reverted gamma was very low, below 0.1, with a calculated precipitation rate about 12.6 s-1 at t0.5.