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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]

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]