Study On Microcosmic Mechanism Of Adiabatic Shear Behavior In Orthogonal Cutting Of High Strength Steel

In recent years, the technology for high speed machining is applied widely in manufacture companies. The formation of sawtooth chips in high speed machining can influence on machining precision, surface roughness and tool life. The occurrence and development of adiabatic shear bands of materials may play a key role during sawtooth chip formation. To investigate the microcosmic mechanism of adiabatic shear during sawtooth chip formation have important meaning to more know chip formation during high speed machining. In this paper, by microstructure observation and microcosmic theoretic analysis, the microcosmic mechanism of adiabatic shear behavior during orthogonal cutting of 30CrNi3MoV high strength alloy steel is investigated, the main investigation contents are listed as following:1. Experimental investigation of evolvement process for chip morphology during orthogonal cutting of high strength alloy steel. As the cutting speed increases, the ribbon chips transform into the sawtooth chips when a critical cutting speed is reached. The reason for the sawtooth chips is that the adiabatic shear localization occurs in the primary deformation zones when thermal softening exceeds strain and strain rate hardening. For the ribbon and sawtooth chips, the measurement and analysis results of the cutting forces and parameters of chip deformation indicate that the formation mechanism of sawtooth chips is distinctly differ from ribbon chips.2. Calculation of deformation and temperature in the adiabatic shear bands within the sawtooth chips. Based on analyzing various stages of adiabatic shear during sawtooth chip formation, a calculational method of the deformation and temperature in adiabatic shear bands is proposed.3. Based on the theoretic model of one-dimensional shear deformation under high rate loading, a relation of width and spacing for adiabatic shear band to cutting speed is proposed. As the adiabatic shear bands form and develop, a series of changes in chip morphology occur which is mainly influenced by the cutting speed and the tempering hardness of workpiece material. The deformation bands change into the transformation bands in microstructure as the cutting speed increases. The microhardness deformation bands results from work hardening, but the microhardness in the transformation bands is influenced by phase transition hardening.4. The results of composition analysis near the adiabatic shear bands by electron probe indicate that the short distance diffusion of carbon and the precipitation of carbide may occur within the adiabatic shear bands. The TEM film samples used to observe the microstructure of the adiabatic shear bands in the sawtooth chips are attained by a new sample preparation method. The observation and analysis of microstructure of transformed band by TEM indicate that the equiaxed grains in the band center are considered to be a recrystallization structure, a microstructure evolvement model in the adiabatic shear bands of high strength alloy steel is proposed,. The rotational dynamic recrystallization model based on mechanical mechanism is analyzed, the results indicate that the rotational dynamic recrystallization mechanism may rationally explain the recrystallization process occurred in the adiabatic shear bands within the sawtooth chips.5. Based on the observations of the surface of adiabatic shear fracture and the analysis of microcosmic fracture mechanism, a microcosmic model of adiabatic shear ductile fracture controlled by nucleation, growth and coalescence of microvoids during sawtooth chip formation is proposed.