Microstructure characteristics and mechanical properties of deposited metals with different types of bainite

Abstract

In this work, three types of bainite microstructure have been gained in the deposited metals by adjusting the Ni content (1 wt %, 2.5 wt %, and 4 wt %). The microstructure feature of different types of bainite has been characterized by optical microscope, scanning electron microscope, transmission electron microscope, and electron backscattering diffraction. Microstructure changed from granular bainite with different morphology to lath bainite with the increase of Ni content. The number of elongated Martensite-Austenite constituents in granular bainite increased with the increasing Ni content from 1 wt % to 2.5 wt % due to the increase in hardenability. The complex orientation in the deposited metal with 4 wt % Ni content attributed to the numerous autocatalytic nucleation occurrence during the bainite transformation. After crystallographic characterization, the bainite variant selection tendency decreased with the bainite type change from granular bainite to lath bainite with autocatalytic nucleation, and the density of high-angle grain boundaries increases obviously. The deposited metal with numerous autocatalytic nucleation has an outstanding match of high strength and excellent low-temperature toughness due to the effective grain-refinement strengthening. The large-sized elongated Martensite-Austenite constituents in the deposited metals with 1 wt % and 2.5 wt % Ni contents deteriorate low-temperature toughness in two aspects, promoting crack nucleation and reducing the crack propagation resistance. Interlaced bainite is the most optimized among different bainite types. This work guides the microstructure design in high-strength deposited metals.