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In this study, the role of Nb addition on the corrosion behavior of low-alloy steel exposed to H 2 S solution is studied by means of microstructure, macro/microsurface morphology characteristics, type of corrosion product, cross-sectional structure and elemental distribution. The addition of Nb is a route to refine grains and improve corrosion resistance. The surface formation process of iron sulfides changes from homogeneity to dispersion at the initial corrosion stage when Nb is added to steel. However, the transformation process and crystal structure of iron sulfides are not affected by the addition of Nb. The elemental distribution of corrosion products is not affected by Nb addition although thick and compact corrosion products are formed on the specimen surface of Nb steel. A schematic model is proposed to clarify the difference of the corrosion mechanism in atomic and molecular level. • Niobium addition improves corrosion resistance of low alloy steel in H 2 S solution. • Role of niobium addition on the characteristics of iron sulfides is weak. • A schematic model of corrosion mechanism is fundamentally proposed. [ABSTRACT FROM AUTHOR]

The microstructural evolution, precipitation, and mechanical properties of 27Cr-4Mo-2Ni steels were investigated by using SEM/EBSD and TEM/STEM technique, and the RT impact test. Experimental results demonstrate that the elongated grains are gradually transformed to recrystallization grains after high temperature annealing. Both sigma and Laves phase were found nucleated at grain boundaries and/or sub-grain boundaries during annealing below 1100 °C except for Nb(C, N) and TiN particles. Recrystallization of 〈111〉 orientated deformation regions were impede due to the pinning effect of Laves phase particles at sub-grain boundaries accompanied by the missing of γ-fiber texture. The recrystallization grains are still kept elongated along the rolling direction for the hindrance of fine Nb(C, N) formed at initial hot rolled grain boundaries. Formation of Laves and sigma phase particles shows great influence on the mechanical properties, such as the decrease of the tensile elongation and the impact toughness, and the emergence of brittle fracture during the tensile or impact test. Solution treatment should be strictly controlled above 1050 °C with water quenching to avoid the embrittlement of the test steels. • Laves and sigma phase nucleated at grain/sub-grain boundaries during solution annealing. • Prior-formation of Laves phase particles impeded the recovery and recrystallization of the deformed microstructure. • Precipitation of sigma and Laves phase particles at boundaries led to the embrittlement. [ABSTRACT FROM AUTHOR]

Hot-rolled 430 ferritic stainless steel samples were annealed at 840–1150 °C together with water or air cooling to study their microstructural evolutions, mechanical properties and corrosion resistances via a scanning electron microscopy (SEM), electron back scattering diffraction (EBSD), tensile tests and immersion tests. The experimental results demonstrate that after annealing below 880 °C, the microstructure consists of ferrite grains and (Fe, Cr) 23 C 6 particles at the grain boundaries, whereas abundant martensitic phase is observed at the grain boundaries after annealing above 950 °C. The ferrite grains coarsen as the annealing temperature increases. Martensitic transformation after high-temperature annealing results in a sharp increase in both the tensile strength and Vickers hardness and a substantial decrease in the elongation. Both the formation of (Fe, Cr) 23 C 6 particles and martensitic transformation at the grain boundaries are detrimental to the corrosion resistance in Cl-rich environments due to the Cr depletion in the ferrite among the (Fe, Cr) 23 C 6 particles. The annealed samples with water quenching show higher hardness and better corrosion resistance than that with air cooling. Annealing at intermediate temperatures (900–950 °C) and then cooling in air is an appropriate method for fabricating 430 ferritic stainless steel with good mechanical properties and high corrosion resistance. [ABSTRACT FROM AUTHOR]

Abstract A 27Cr-4Mo-2Ni super ferritic stainless steel was hot rolled and aged at 800 °C for times ranging from 10 min to 4 h to study the microstructure, precipitation and their effects on mechanical properties by using SEM, EBSD and TEM techniques. Experimental results demonstrate that a great deal of substructure, especially shear bands is obviously observed within elongated ferrite grains after hot rolling. After aging at 800 °C, Laves, chi and sigma phases are identified, and precipitation of these phases is accelerated evidently by the substructure. Laves phase particles are observed formed at dislocations and distributed inside grains with sub-micron scale. Precipitates of chi and sigma phase are found at grain boundaries and along shear bands, as well as around the TiN particles. Recovery during 800 °C aging is impeded by the precipitation of Laves phase at dislocations and chi phase at shear bands. Tensile ductility and micro hardness are significantly affected by the precipitation of intermetallics. The breakage pattern after tensile test transformed from ductile to brittle fracture when the aging time increases from 10 min to 4 h. Graphical abstract Unlabelled Image Highlights • Hot-rolling deformation accelerated the formation of intermetallics: chi, Laves and sigma phases. • Formation of intermetallic phases impeded the recovery of deformed ferritin grains. • Precipitation of brittle phases led to brittle fracture mode during tensile test. [ABSTRACT FROM AUTHOR]