Microstructure Evolution of 16Cr3NiWMoVNbE Steel Treated by Carburizing and Laser Transformation Hardening

As a special grade high-quality steel, 16Cr3NiWMoVNbE is the representative of the material of transmission components such as gear and shaft of aeroengine. At present, 16Cr3NiWMoVNbE steel is faced with the problems of large thermal deformation, long process cycle and poor matching of strength and toughness after carburizing strengthening. Laser phase transformation strengthening technology has the advantages of high processing efficiency, small thermal deformation and high surface quality, which can significantly improve the surface performance of the workpiece. However, this technology is often suitable for medium and high carbon steel. Therefore, the work aims to further increase the depth of the strengthened layer, refine the grain size, improve the mechanical property of surface, reduce the thermal deformation of the workpiece and shorten the process circle. The carburizing was combined with laser phase transformation strengthening. The "short-time" carburizing was used to improve the surface carbon content, and then the laser local rapid heating was used to provide an ideal channel for carbon atom diffusion. The microstructure evolution and strengthening-toughening mechanism of 16Cr3NiWMoVNbE steel strengthened by carburizing and laser phase transformation were revealed, providing new ideas and theoretical support for surface strengthening of key transmission parts of aero-engine. The metallographic structure and high-power microstructure were evaluated by optical microscopy (OM), scanning electron microscope(SEM) and transmission electron microscope (TEM). The section hardness and modulus of elasticity of strengthened layer after laser transformation strengthening treatment was tested by microhardness tester and nano indenter to reveal the microstructure evolution and strengthening and toughening mechanism of 16Cr3NiWMoVNbE steel strengthened by carburizing and laser transformation. After the composite strengthening of 16Cr3NiWMoVNbE steel by carburizing and laser transformation, the composite strengthened layer had a good matching relationship between strength and toughness, that was, there were a large number of residual austenite and slight decarburization on the outermost surface, which had good plasticity and toughness, while the sub-surface had a fine martensite + residual austenite + carbide composite phase structure, which had the characteristics of high hardness and high strength. With the increase of laser energy input, the depth of the composite strengthened layer was increased by about one time, the maximum microhardness was 792HV, and the microhardness was increased by about 1.3 times. The elastic modulus and microhardness of the strengthened layer tended to increase at first and then decrease. The microstructure lath of the strengthened layer gradually decreased, and the size of the strengthened layer was coarsened continuously. The retained austenite changed from thin film to block, and the number of retained austenite gradually increased, while the carbide aggregates and spheroidizes and the number decreased. After the composite strengthening of carburizing and laser transformation of 16Cr3NiWMoVNbE steel, the depth of the strengthened layer is further expanded, and the surface hardness is further improved. A composite strengthened layer with excellent plasticity and toughness is obtained. Taking full advantage of the characteristics of rapid heating and rapid cooling of laser transformation strengthening, it provides a new idea and theoretical support for the surface strengthening of key transmission components of aviation engine.