Duplex Steels: Part I: Genesis, Formation, Structure

Abstract

The ferritic-austenitic duplex steels are equipped with a mechanical-technological combination of properties, which is advantageous compared to the features of stainless completely ferritic or completely austenite steels. The duplex steels crystallize by fully ferritic or ferriticaustenite solidification with the austenite precipitation due to the solid solution reactions during the further cooling. To adjust the ferrite-austenite ratio, the steels must be heat treated by temperatures above the field of precipitation stability, followed by water quenching. The temperature and the time of the heat treatment effect the element distribution according to their higher solubility in the ferritic or austenitic phases. The typical microstructure of the duplex stainless steels can only be realized due to deformation and recrystallisation processing.

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

“…Indeed, the presence of γ/γ internal interfaces and the twinning of interfaces into γ-austenite crystalline grains are caused by the coarsening phenomena that have taken place ( Figure 10A). However, the formation of the typical lamellar microstructure at high temperatures is induced by the α/γ interface energy, which is lower than the α/α and γ/γ grain boundary energies [6,20]. The frequency of occurrence of a peculiar kind of CSL grain boundary is inherently associated with the newly recrystallized grains and forms a preferred crystallographic orientation relative to the parent grains.…”

“…Indeed, the presence of γ/γ internal interfaces and the twinning of interfaces into γ-austenite crystalline grains are caused by the coarsening phenomena that have taken place ( Figure 10A). However, the formation of the typical lamellar microstructure at high temperatures is induced by the α/γ interface energy, which is lower than the α/α and γ/γ grain boundary energies [6,20]. …”

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

“…A consequence of this super-saturation is the segregation of γ-stabilizers elements, resulting in γ-austenite thick plates located at α-grain boundaries, which interconnect the γ-austenitic grains. Another result of the super-saturation is the presence of the early stages of the α-intragranular γ-nuclei nucleated during the cooling path; both Widmanstätten grain boundaries saw-teeth structures and Martensitic Shear products nuclei can be detected ( Figure 2) [8,27]. 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.…”

“…In DSSs, the two structure components b.c.c α-ferrite and f.c.c. γ-austenite lies as crystals of the same size statistically distributed next to each other [8]. However, adjustment of the two-phase microstructure of duplex stainless steels is complicated because a balanced phase ratio does not only depend on alloy components [9].…”

Influence of carbon diffusion on microstructure and wear behaviour of duplex stainless steel surface layers on lamellar grey cast iron

“…The appeareance of the austenite and remaining δ-ferrite varies a lot depending on the chemical composition and cooling rates. The ratio of chromium and nickel equivalent (Cr eq /Ni eq ) is used to describe the microstructure that results from high cooling rates, e. g. up to 800 K s -1 in laser welding [8,[19][20][21][22]: * Cr eq /Ni eq > 1.95: The melt solidifies purely to δferrite. Upon cooling, δ-γ-transformation may occur.…”