Duplex stainless steels consisting of ferrite and austenite in similar volume fraction have good combinations of mechanical properties and corrosion resistance. In addition to ordinary duplex stainless steels, hyper duplex stainless steels with high pitting resistance equivalent (PRE = wt%Cr + 3.3×(wt%Mo + 0.5×wt%W) + 30×wt%N) have been developed for use in severely corrosive environments. Currently, extensive researches are being conducted to further improve mechanical properties and corrosion resistance of hyper-duplex stainless steels.
The effect of Ce, Si and Ce-Si addition on the microstructural evolution and corresponding pitting corrosion resistance of the hyper duplex stainless steels is investigated, at firstly. In solid solution-treated base steel, pitting corrosion was initiated at manganese oxy-sulfide inclusions. Addition of Ce or Si suppressed formation of these inclusions and thereby improved the pitting corrosion resistance of the steel. However, in Ce and Si-added steel, formation of Ce-rich oxy-sulfide decreased pitting corrosion resistance by micro-galvanic effect.
Generally, the secondary phasese forming by subsequent isothermal annealing at 850°C decreased ductility but increased hardness, but the steel was softend by generated regions where alloying elemets are depleted. About 3 volume percent of the secondary phases reduced hardness withough loss of ductility. Therefore the low-cycle fatigue life did not reduced comparing with solid solution heat-treated steel because fatigue crack is hardly propagated although fatigue crack is easily initiated.
The pitting corrosion resistance and Charpy impact toughness of steels was considerably deteriorated by secondary phases due to its inherent brittleness and generation of regions in which alloying elements are depleted. Addition of Ce was effective on retarding the precipitation of the sigma and chi phases and therefore on preventing the degradation of both Charpy impact toughness and pitting corrosion resistance. However, in both Si-containing and Si-Ce-containing steels, formation of sigma phase was accelerated and resulted in severe degradation of toughness and pitting corrosion resistance. Addition of both Ce and Si elements resulted in inhomogeneous distribution of Ce by formation of Ce-bearing oxy-sulfides, and accelerated the precipitation of the sigma phase. It was concluded that addition of Ce alone was effective on improving corrosion resistance and on retarding precipitation of secondary phases, but addition of both Ce and Si formed Ce-bearing oxy-sulfides inducing the pitting initiation sites and promoting formation of the secondary phases, and accordingly deteriorated pitting corrosion resistance significantly.
The optimum chemical composition is obtained by investigating of the effects of Ce addition on the transformation behavior of sigma and chi 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 600 ? 1000°C for 1 - 1000 min. 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 sigma (1%) or chi 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.