Characterization of phase fractions and misorientations on tempered Bainitic/Martensitic Ni-Cr-Mo low alloy RPV steel with various Ni content

Abstract

The aim of this paper to quantitatively characterize the microstructure of tempered martensitic/bainitic Ni-Cr-Mo steels with different Ni contents. Though it is difficult to characterize the phase factions in the Ni-Cr-Mo low alloy steel due to the microstructural similarity of tempered lower bainite and tempered martensite, the phase fractions could determined with the as-quenched specimens by using three successive measurements of ferrite scope, scanning electron microscopy images and electron back-scattered diffraction. The martensite fraction is increased as the Ni content increases, and it causes the reduction effective grain size. When compare the boundary distributions of both as-quenched and tempered specimens, larger amounts of Σ11, Σ13, and Σ39 CSL boundaries were observed only in as-quenched model alloys. These boundaries were diminished by the tempering process. However, the peak of the Σ3 boundary was observed in both the as-quenched and tempered states. The frequency of Σ3 boundaries in the tempered samples is significantly increased as the Ni content increases, and it is closely related with the martensite fraction.