Effect of Isothermal Holding at 750 °C and 900 °C on Microstructure and Properties of Cast Duplex Stainless Steel Containing 24% Cr-5% Ni-2.5% Mo-2.5% Cu

Abstract

Changes in the microstructure and selected mechanical properties of two-phase ferritic-austenitic cast steel containing 24% Cr-5% Ni-2.5% Mo-2.5% Cu after isothermal holding at 750 °C and 900 °C are presented. The choice of the two temperatures of isothermal holding was dictated by the precipitation of brittle phases within a range of 600 °C–950 °C, while the holding time depended on the casting cooling time in the mould. Changes in the microstructure were studied by the SEM-EDS and XRD techniques. As a result of the decomposition of the eutectoid ferrite, a σ phase that was rich in Cr, Mo, and Ni and a secondary γ2 austenite with Widmannstätten morphology were formed. Compared to the austenite, the chemical composition of the secondary γ2 austenite showed depletion of Cr and Mo. In the ferrite, the presence of Cr2N nitrides was also detected. After a holding time of 3 h at 900 °C, these phases increased the hardness of the tested cast steel to approximately 275 HV10. At the same time, the UTS value was recorded to decrease with the increasing temperature based on the tensile test results. At 750 °C, the value of UTS was 250 MPa for 1 h of holding and 345 MPa for 3 h of holding. These values decreased after increasing the temperature to 900 °C and amounted to 139 for 1 h holding and 127 MPa for 3 h holding. It was also found that the elongation values at 750 °C ranged from 7–10%, while they amounted to 35–37% at 900 °C. A fracture analysis of the tested cast steel showed that in the prevailing part, the fractures were made of ductile nature with an arrangement of dimples that is typical for this type of fracture. Non-metallic inclusions that are typical for cast steel (i.e., oxides and nitrides) were also found in the area of the fractures.

Effects of the Volume Fraction of the Secondary Phase After So-Lution Annealing on Electrochemical Properties of Super Duplex Stainless Steel UNS S32750

“…Solution annealing is a heat treatment to optimize the corrosion rsistance [4][5][6]. However, SDSS, in which the secondary phase is precipitated, does not have an optimized corrosion resistance [10][11][12]. The secondary phase is transformed into ferrite, and austenite should be transfromed into ferrite [16].…”

“…Although SDSS is composed of austenite and ferrite, the secondary phase is known to precipitate owing to the segregation of Cr at 975℃ [10][11][12]. To confirm the secondary phase, heat treatment was performed from 900 to 1050℃.…”

“…When the secondary phase is precipitated inside, hot temperature cracking occurs, decreasing the corrosion resistance. Although significant research efforts have been devoted to suppress the precipitation of secondary phases, no studies have been conducted on the effects of the secondary phase on solution annealing [10][11][12].…”

Effects of the Volume Fraction of the Secondary Phase after Solution Annealing on Electrochemical Properties of Super Duplex Stainless Steel UNS S32750

“…Although SDSS is composed of austenite and ferrite, the secondary phase is known to precipitate owing to the segregation of Cr and Mo at 975 • C [10][11][12]. To confirm the secondary phase, heat treatment was performed from 900 to 1050 • C to check the inside microstructure.…”

“…In order to obtain excellent corrosion resistance of SDSS, the ratio of the austenite phase and the ferrite phase should be close to 1:1, and appropriate heat treatment temperature conditions are required to control the ratio of the two phases. Although significant research efforts have been devoted to suppressing the precipitation of secondary phases, no studies have been conducted on the effects of the secondary phase on solution annealing at 1100 • C [10][11][12].…”