Effect of thermal aging on the microstructure and mechanical property of 410S ferritic stainless steel

The low chromium ferritic stainless steel, 410S, as a candidate control rod drive mechanism (CRDM) material used in nuclear power plants, was subjected to accelerated thermal aging at 400 degrees C in air up to an exposure period of 10,000 h (similar to 417 days). A series of analytical techniques including optical microscope (OM), electron back scatter diffraction (EBSD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and 3D atom probe tomography (3D-APT) were used to characterize the evolution of the microstructure with thermal aging time, whilst a couple of tests (i.e., hardness, Charpy impact and tensile tests) were employed to link the changes of mechanical properties at different thermal aging times with the microstructure. After <= 3000 h thermal aging treatment, Cr-rich alpha' phase formed near the ferritic grain boundaries, leading to minor change in the hardness and strength of 410S but sharp decrease in impact energy. When the thermal aging time increased to 10,000 h, the formation of Cr-rich alpha' phase spread from near the ferritic grain boundary to the inside of ferritic grain and the growth of carbides precipitated led to the slight increase in the hardness and strength but the decrease in the impact energy and plasticity. Under these circumstances, the decrease of free substitutional atoms (i.e., Cr) caused by the formation of Cr-rich alpha' phase and the growth of carbides changed the dynamic strain aging type from type B to type A.