Effect of Ce addition on secondary phase transformation and mechanical properties of 27Cr-7Ni hyper duplex stainless steels

This study aims to investigate the effects of Ce addition on the transformation behavior of σ and χ secondary phases and corresponding mechanical properties of 27Cr-7Ni-2.5Mo-3.3W hyper duplex stainless steels. Four different steels containing various Ce content (0, 55, 110 and 450 ppm) were subjected to solid-solution heat treatment and subsequent isothermal annealing at temperature range of 873-1273 K for 1-1000 min. In the solid-solution heat-treated steels where secondary phases were not formed, tensile strength was slightly increased in the Ce added specimens, but Charpy impact toughness varied depending on the size and volume fraction of inclusion. Subsequent isothermal annealing resulted in the precipitation of secondary phases in all steels. However, when a small amount (55 and 110 ppm) of Ce was added, precipitation was considerably delayed which was attributed to the homogeneous distribution of Ce. On the other hand, in the high (450 ppm) Ce content steel, Ce-rich particles were formed resulting in extremely low Ce concentration in the matrix, which could not retard the precipitation of secondary phases. Time-temperature transformation diagram for the precipitation of either σ (1%) or χ phase (0.5%) was well matched with the 50% toughness reduction curve of the steel. It was concluded that optimum Ce concentration with its homogeneous distribution in the matrix is the most effective at retarding the formation of secondary phases and therefore to prevent the degradation of mechanical properties.