25Cr-7Ni-1.5Mo-3W-xSi duplex stainless steel(DSS) alloys were cast in shell mold with different silicon contents from 0.47 to 1.2 wt%. The effect of Si content on the average fluidity length of the melt and the tensile property of the casting were investigated. The alloys having 0.7∼1.1 wt% Si showed good fluidity, and the strength of the casting slightly increased with increase of Si content.
25Cr-7Ni-1.5Mo-3W-xN duplex stainless steel(DSS) alloys were cast in shell mold with different nitrogen contents from 0.14 to 0.37 wt%. These DSS castings were isothermally heat treated in the temperature range of 950~1350℃ for 2 hours and then quenched in water. The effects of nitrogen contents and solution treatment temperature on the microstructure and mechanical properties of DSS casting were investigated and related to the change of the volume fraction of ferrite. The volume fraction of ferrite was significantly decreased and the solidification mode changed from ferrite single phase solidification mode to ferrite - austenite one by the increased nitrogen content. The increase of solution treatment temperature increased the volume fraction of ferrite and the size of austenite phase in ferrite matrix. The increase of the volume fraction of ferrite decreased the tensile strength and the elongation and increased the yield strength of DSS casting lineally.
The effect of misch metal(MM) on the microstructure of a duplex stainless steel(DSS) was investigated. Fe-25Cr-7Ni-3Mo-0.25N DSS ingot without MM, as a basic material was prepared in a 30-kW air induction furnace. Remelting and addition of MM were carried out in a vacuum induction furnace. Various amounts, from 0.022 to 0.078 wt% of MM were added to the 900g DSS melt in an Ar gas environment. The temperature of molten steel was kept at 1650℃ and was measured by two-color pyrometer. DSS samples with different MM contents were cast in a steel mold and 10mm thick castings were obtained. Each casting was solution-treated at 1050℃. The microstructures and the mechanical properties of hot-rolled specimens were compared.
The alloying experiment of nitrogen gas into the melt of DSS Fe-25Cr-7Ni-3Mo-2W by blowing on the top surface or bubbling through the melt was analyzed in terms of reaction rate constant and total nitrogen content. The rate constant k between nitrogen gas and the DSS melt is about $2.37 × 10^{-6}$ $mol cm^{-2} s^{-1} atm^{-1}$. The nitrogen solubility in the DSS melt is estimated to be 0.351 wt% at 1600℃. The nitrogen reaction rate of the DSS melt with nitrogen gas at 1600℃ can be expressed as:
\\ \begin{equation} \frac{d \left[ wt % N \right]}{dt} = 6.64 × 10^{-3} \frac{A}{W} \left( p_{N_2} –6.58 left[ wt % N \right]^2 10^{0.26 left[ wt % N \right]} \right) \end{equation}
The effects of casting thickness, nitrogen contents, cooling rate and Mn contents on the formation of nitrogen gas defects during solidification of 25Cr-7Ni-1.5Mo-3W Duplex Stainless Steels were quantitatively investigated. In case of a sand-mold, the formation of nitrogen gas defects wasn’t affected by the thickness of castings which ranged from 13 to 52mm and the critical initial nitrogen content for the formation of gas defects was 0.30 wt % N. In case of the molds made of a Stainless Steel(STS) and water-cooled Cu, the critical initial nitrogen contents did not change very much compared to the sand mold. The amount of nitrogen gas defects increased with initial nitrogen contents of castings. The segregation of nitrogen and alloying elements were calculated with Thermo-Calc software. The calculated data and the experimental results were used for the estimation of the critical nitrogen partial pressure at the onset of gas pore formation. The effect of Mn content on the formation of gas pores was also investigated.
