Isothermal Austenite–Ferrite Phase Transformations and Microstructural Evolution during Annealing in Super Duplex Stainless Steels

Abstract

Super Duplex Stainless Steels (SDSSs) are composed of α-ferrite and γ-austenite grains, the simultaneous presence of which forms an optimal microstructure to achieve the best combination of mechanical and corrosion resistance properties. Moreover, international quality standards are strict about the phase fraction ratio. The purpose of this work is the achievement of a better description of the phase ratio evolution taking place during annealing at 1080 • C in the super duplex stainless steels F53-S32750 and F55-S32760. The experimental results show a damped sinusoidal trend in the α/γ phase ratio evolution with the increase of the soaking time of thermal treatment. This can be described by coupling both the competitive coarsening growth regime and the concept of the local equilibrium phase transformations, pointing out a good correspondence with the experimental data. Further, recrystallization phenomena also play a major role. Finally, the additivity character of the observed processes has been proven.

Hot-Dip Aluminizing on AISI F55–UNS S32760 Super Duplex Stainless Steel Properties: Effect of Thermal Treatments

“…In the annealed thermally treated samples, the secondary austenite precipitation has been trigged ( Figure 4D) and it is followed by the precipitates competitive growth with the ongoing of the thermal treatment. At 210 s/mm ( Figure 4E), a static recovery of the ferritic grains is reported to occur and, then, the competitive coarsening of austenitic grains leads the microstructure evolution until 360 s/mm ( Figure 4F) of holding time [26,36]. Moreover, the effect of the deformation by the cold rolling can be detected, observing the distortion of the austenitic grains, which is more marked in the samples experiencing higher strains ( Figure 4G,H).…”

“…The thermal generation of lattice defects can be observed in the maximum value of the aluminum diffusion rate associated to the maximum value of soaking time of the annealing thermal treatment [35,52]. The sample annealed for 210 s/mm undergoes a static recovery phenomenon of the ferritic grains, decreasing the number of its internal δ/δ boundaries and defects; then, also the aluminum diffusion rate is strongly reduced [26,36].…”

“…Afterwards, it has been executed an annealing thermal treatment at 1080 • C for 72, 210 or 360 s/mm of holding time. The corresponding specimens will be designated as: "A72", "A210" and "A360", respectively [26,27]. This thermal treatment reproduces the hot-working temperature, avoiding the precipitation of the detrimental secondary phases.…”

Transformation-Induced Plasticity in Super Duplex Stainless Steel F55-UNS S32760

“…The annealing thermal treatment produces the precipitation of γ-austenitic nuclei as Widmannstätten precipitates and via a martensitic-shear process within the α-ferritic matrix. Further, the γ-precipitates display different sizes, which result in becoming larger with the ongoing of the annealing thermal treatment, according to the well-known Ostwald ripening model [7,10,28,29]. The super-saturation of the α-matrix with γ-formers elements has been proven via SEM/EDX chemical analysis ( Table 3).…”

“…The steels not only inherit the mechanical properties of the completely ferritic or completely austenitic alloys, but they also exceed them. A factor of economic importance is the low content of expensive nickel, usually 4-7% compared with 10% or more in austenitic grades, as a result of which the life cycle cost of the DSSs is the lowest in many applications [4][5][6][7]. In DSSs, the two structure components b.c.c α-ferrite and f.c.c.…”