The Research On Intermetallics Precipitation Behaviors Of Super Ferritic Stainless Steel

Super ferritic stainless steel(SFSS) has performed excellent mechanical properties and superior corrosion resistance, which is a kind of nickel-saved ferritic stainless steels with low price. During the manufacture or employing process in the temperature range of 600°C~950°C, it would probably precipitate various secondary phases such as Laves phase, χ phase and σ phase. These secondary phases are usually brittle and hard, which would cause the embrittlement and reduce the plasticity or toughness. At the same time, the corrosion resistance of the steel would drop.The hot annealed sheet provided by the Baosteel has the chemical composition similar to the S44660 SFSS. By changing the aging temperature and holding time, the precipitation behaviors during the temperature of 600°C~950°C was investigated. The 800°C aged samples were tested by cyclic potentiodynamic polarization to study the relationship between the precipitates and corrosion properties. In addition, the research also explored the effects of aging on hardness.The results showed that the SFSS ferrite matrix kept stable during the temperature range of 600°C ~650°C because no intemetallics were found even when aging for 5h. During aging for 8min at 700°C~850°C, rod-like Laves phase precipitated in the grain interior. As aging for 5h at 750°C, 1h at 800°Cor 30 min at 850°C, σ phase would precipitate at the ferrite grain boundaries, indicating the formation of σ phase required shorter holding time with the increasing of temperature. With the prolonging of holding time, the amount of σ phase increased. During the short time aging, χ phase or σ phase could be found in the grain interior. When aging for 11 min at 900°C~950°C, Laves phase formed at the grain boundaries and in the grain interior.The cyclic potentiodynamic polarization tests of the 800°C aged samples were performed in the 1mol/L NaCl solution at room temperature. The results showed that holding time within 1h had little effects on the pitting potential. When aging for 5h, the pitting potential drop rapidly and lost the self-repaired capability. Large number of σ phase is the main reason leading to the deterioration of pitting resistance The hardness was influenced by the solution strengthening effects, dispersion strengthening effects and the growth of grains at elevated temperature.