Study On The σ-phase Precipitation And Deformation Of SAF2906 Duplex Stainless Steel

Compare with ordinary duplex stainless steels. SAF2906 duplex stainless steels show better corrosion resistance especially for the local pitting resistance and stress corrosion cracking in high chloride environments. They are more widely used in marine. The study on the σ-phase precipitation and deformation of SAF2906 duplex stainless steel is limited because of the chemical compositions such as chromium, nickel and molybdenum are different from inchoate duplex stainless steels. That is why develop a new understanding of sigma phase in such a material.In the present study, Samples of SAF2906 duplex stainless steel were aged at different temperatures after high temperature solution heat treatment. The morphology was then studied by means of scanning electron microscopy, X-ray diffraction and transmission electron microscopy; Thermodynamic model of SAF2906 duplex stainless steel was established in order to calculate the thermodynamic parameters; specimen was treated by hot compress, thermal expansion and microstructures were observed:Samples were tested on an isothermal tensile machine for studying its plastic elongation after be treated by different Heat treatment and cold rolled for studying the superplasticity. Precipitation behavior and deformation of SAF2906 duplex stainless steel were study systematically in this thesis.The results of study on precipitation in SAF2906 duplex stainless after solution and aging treatment indicate that:the σ-phase formed at d-ferrite/c-austenite interfaces and in the d-ferrite, but some σ-phase regions formed within austenite; The chromium content of σ-phase rose as the aging temperature increased; Compared with the SAF2205/2507, chromium content of the σ-phase in SAF2906 was higher by up to 70% under the same conditions. So chromium became the major element in σ-phase instead of iron.Thermodynamic model of SAF2906 duplex stainless steel was established on the basis of solid solution thermodynamics in order to calculate the driving force of high-Cr σ-phases. The results show σ-phases precipitate hardly as increasing of chromium content, and the change of Gibbs energy in high-Cr σ-phase was negative, showing this kind of σ-phase can be precipitated; Simulation experiment was carried out by Thermo-Calc. The results was that compared with the SAF2205/2507. the area of austenite in SAF2906 was smaller and expanding in σ-phase. The difficulty of σ-phase nucleation in SAF2906 reduced. TTT and CCT curves were draw by jmat-pro software. The curves showed that range of nucleation temperature expand in SAF2906; the stress-strain curve show the stress of specimen increased after deformation reached higher 70%.The microstructure of materials in thermo compression was observed. Fluctuation of volume fraction of ferrite/austenite happened at the early stages of compression when the deformation was below 60%. When the deformation was higher than 70%. fraction of every phase became steady. The fraction of σ-phase was between 1% and 5%. The boundaries of precipitation were obvious when the deformation was below 70%. The shape of σ-phase changed after the deformation was 80%.SAF2906 duplex stainless steel is tested on an isothermal tensile machine for studying its plastic elongation after be treated by different solutions and cold rolled. The results indicate that:The elongation of specimen decreased to 670% from 1430% as the temperature increased from 1100℃ to 1350℃ with rolled on 85% at a strain rate of 1×103s-1. The elongation of SAF2906 duplex stainless steel could be enhanced by increasing rolling reduction rate, and the elongation of the specimen solution-treated at 1100℃, rolled on 85% reached to 1430% at a deformation temperature of 960℃ and a strain rate of 1×103s-1. But under the same condition, elongation was 800% as rolled on 80%. The volume ratio of δ-ferrite phase to y-austenite phase varies with the solution temperature. Precipitation of σ-phase controlled the growth of other phases and refined grains. So σ-phase was conducive to slippage of grains and increased the elongation of SAF2906 duplex stainless steel.