Abstract:In this present study, the influence of solution annealing temperature on microstructural properties of a forged Super Duplex Stainless Steel (SDSS) was investigated by SEM-BSE (Scanning Electron Microscopy-Backscattered Electrons) and SEM-EBSD (Scanning Electron Microscopy-Electron Backscatter Diffraction) techniques. A brief solution treatment was applied to the forged super duplex alloy, at different temperatures between 800 • C and 1100 • C, with a constant holding time of 0.6 ks (10 min). Microstructural characteristics such as nature, weight fraction, distribution and morphology of constituent phases, average grain-size and grain misorientation were analysed in relation to the solution annealing temperature. Experimental results have shown that the constituent phases in the SDSS alloy are δ-Fe, γ-Fe and σ (Cr-Fe) and that their properties are influenced by the solution treatment temperature. SEM examinations revealed microstructural modifications induced by the Cr rich precipitates along the δ/γ and δ/δ grain boundaries, which may significantly affect the toughness and the corrosion resistance of the alloy. Solution annealing at 1100 • C led to complete dissolution of σ (Cr-Fe) phase, the microstructure being formed of primary δ-Fe and γ-Fe. The orientation relationship between δ/δ, γ/γ and δ/γ grains was determined by electron back scattering diffraction (EBSD). Both primary constituent phase's microhardness and global microhardness were determined.

No abstract

In this present study, the influence of isothermal aging temperature and duration on microstructural and mechanical properties of a hot-deformed UNS S32750 super duplex stainless steel (SDSS) alloy was investigated by SEM-EBSD (scanning electron microscopy-electron backscatter diffraction) and tensile testing techniques. An isothermal aging treatment, at temperatures between 400 and 600 °C and treatment duration between 3 and 120 h, was applied to a commercial UNS S32750 SDSS alloy. Microstructural characteristics of all thermomechanical (TM) processed states, such as distribution and morphology of constituent phases, grain’s modal orientation (MO), and obtained mechanical properties were analysed correlated with the TM processing conditions. The obtained experimental results show that the constituent phases, in all TM processed states, are represented by elongated γ-phase grains within the δ-phase matrix. The R-phase was observed in the case of aging treatment performed at 600 °C for 120 h. Within the δ-phase matrix, dynamically recrystallized grains were identified as a result of applying hot deformation and isothermal aging treatments. Also, it was observed that aging treatment parameters can significantly influence the mechanical behaviour exhibited by the UNS S32750 SDSS alloy, in terms of elongation to fracture and absorbed energy during impact testing.

No abstract

Super duplex stainless steel alloys are characterized by a high content of chromium, nickel, molybdenum and nitrogen. In these steels, the main role of chromium, nickel and molybdenum alloying elements is to increase corrosion resistance. Increased mechanical properties are obtained by adding nitrogen, which promotes structural hardening. The microstructure of super duplex stainless steels consists of a mixture of ferrite and austenite phases, while other phases are also formed, such as: sigma, chi, chromium nitride, carbides and secondary austenite. All secondary phases are considered deleterious phases due to their negative effect on corrosion resistance and mechanical properties. The precipitation formation mechanism of the sigma phase assumes depletion of chromium and molybdenum from the matrix, worsening the alloy properties. The aim of the present work is to show the microstructural changes occurring during a short duration (10 minutes) solution treating of an UNS S32760/1.4501/F55 alloy, three temperatures being considered: 800 °C, 900 °C and 1000 °C. Scanning electron microscopy (energy dispersive spectroscopy and electron backscattered diffraction) was used for quantifying the alloying elements distribution, to distinguish between the observed phases, and to better understand the evolution of each phase.