Effect of Rare Earth Cerium Replacement of Nickel on Cryogenic Toughness of 7Ni Steel

The effect of replacing Ni by two different contents of Ce on the cryogenic toughness of 7Ni steel is studied for revealing the significant role of Ce element in the toughening mechanism. The results show that the close impact absorb energy at −150 and −196 °C of 7Ni steel can be acquired with 0.0026 wt%Ce replacing 0.5 wt%Ni. As the Ce content increases to 0.0265 wt%, the cryogenic toughness of the experimental steel deteriorates. Except for the inclusions of steel increase at the number and size, the 265Ce–6.5Ni steel toughness deterioration also is attributed to the enrichment of Ce in the steel substrate, which together leads to stress concentration under impact loading. The appropriate amount of Ce toughening mechanism is attributed to the formation of the fine and dispersed inclusion particles belonging to the soft phase, which can increase the plastic deformation in the crack propagation process and ensure that the 7Ni steel has excellent cryogenic toughness.