Precipitation condition and effect of volume fraction on corrosion properties of secondary phase on casted super-duplex stainless steel UNS S32750

Abstract

Purpose The secondary phase decreased the corrosion resistance because of the segregation of Cr and Mo. Therefore, this paper aims to study the precipitation condition and the effect of secondary phase with volume fraction on corrosion behavior. Design/methodology/approach Secondary phase precipitated approximately from 375°C to 975°C because of saturated Cr and Mo at grain boundary by growth of austenite. Therefore, heat treatment from 800°C to 1,300°C was applied to start the precipitation of the secondary phase. Findings The secondary phase is precipitated at 1,020°C because of segregation by heterogeneous austenite. The growth of austenite at 1,000°C needs the time to saturate the Cr and Mo at grain boundary. When the volume fraction of austenite is 56 per cent (14 min at 1,000°C), the secondary phase is precipitated with grain boundary of austenite. The secondary phase increased the current density (corrosion rate) and decreased the passivation. That is checked to the critical pitting temperature (CPT) curves. The 1 per cent volume fraction of secondary phase decreased CPT to 60°C from 71°C. Research limitations/implications The precipitation of secondary phase not wants anyone. Casted super-duplex stainless steel (SDSS) of big size precipitates the secondary phase. This study worked the precipitation condition and the suppression conditions of secondary phase. Social implications Manufacturers need precipitation condition to make high-performance SDSS. Originality/value The corrosion resistance of SDSS is hard the optimization because SDSS is dual-phase stainless steel. The precipitation of the secondary phase must be controlled to optimize of the corrosion resistance of SDSS. Anyone not studied the precipitation condition of secondary phase and the effect of secondary phase with volume fraction on corrosion behavior of SDSS.

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

“…For the experiment, the microstructure was polished with diamond paste. After etching for 1min in 5 wt.%KOH electrolyte, it was confirmed using FE-SEM (Hitachi) [13]. The secondary phase was confirmed using X-ray diffraction (XRD), and the chemical composition was determined via EPMA and EDS after polishing.…”

“…2 of 13 fraction of phased and electrochemical properties [4,5,13,14]. The studies investigated the effect of the equilibrium volume fraction on the electrochemical properties but did not examine the effect of the secondary phase on the solution annealing process.…”

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

“…The complex chemical composition of duplex steels (especially of the third and fourth generations—referred to in the literature as Super- and Hyper-DSS [ 16 , 17 ]) promotes the release of undesirable and harmful secondary phases in the microstructures of these materials [ 18 ]. During their operation in corrosive environments, the presence of these phases increases the steel’s susceptibility to intercrystalline, pitting, and stress corrosion while also deteriorating its strength and welding properties [ 19 ]. The precipitation of the phases takes place within two temperature ranges, i.e., 300 °C–500 °C (α′, ε, π, G) and 600 °C–950 °C (M 7 C 3 , M 23 C 6 , Cr 2 N, CrN, σ, χ, γ’, R) [ 3 , 20 ].…”

“…On the other hand, the σ phase is formed primarily at the γ/α interface. This reduces not only the mechanical properties but also the corrosion resistance [ 19 ]. The presence of Cr-enriched precipitates in the microstructure of DSS steel leads to a depletion of the matrix in Cr in their immediate vicinity, thus creating conditions for pitting or intercrystalline corrosion.…”

Effect of Ferrite Proportion and Precipitates on Dual‐Phase Corrosion of S32750 Super Duplex Stainless Steel with Different Annealing Temperatures

“…In addition, higher alloy content significantly enhances the precipitation process of secondary phases. [2,[19][20][21] Improper heat treatment temperature can also result in undesirable secondary phase precipitation such as nitrides, [22][23][24][25][26] intermetallic compounds (σ phase and χ phase), [25][26][27][28][29][30][31][32][33] and carbides. [25,26,33,34] Among these undesirable secondary phases, σ phase and nitrides with high precipitation temperature and fast formation kinetics have been particularly noted since they can drastically deteriorate the mechanical properties and the corrosion resistance of DSSs.…”