In the machining of mechanical parts, the workpiece will be affected by cutting force and cutting heat, which will change the physical and mechanical properties of the surface layer metal. In the process of grinding, plastic deformation and cutting heat will be more serious than cutting edge. In order to ensure the mechanical properties of machined surfaces of parts, we need to know what factors affect the mechanical properties of machined surfaces of parts.
1. Cold work hardening and evaluation parameters
(1) Cold work hardening of metals
In the process of machining, due to the role of cutting force, plastic deformation is very easy to occur, which makes the lattice distorted, distorted, and even broken. These will improve the hardness and strength of the surface metal, which is called cold work hardening.
(2) Main factors affecting cold work hardening
·With the increase of the blunt radius of the cutting edge, the extrusion effect on the surface metal will be enhanced, which will aggravate the plastic deformation and lead to the strengthening of the cold hardness.
·The wear of the rear tool face increases, and the friction fixture between the rear tool face and the machined surface increases the plastic deformation, leading to the hardening.
·The influence of blunt radius of cutting edge on work hardening cutting speed increases, the interaction time between tool and workpiece shortens, so that the extension depth of plastic deformation decreases, and the depth of hardened layer decreases. When the cutting speed is increased, the action time of the cutting heat on the workpiece surface is shortened, resulting in the hardening.
2. Change of metallographic structure of surface layer material
(1) Grinding burn: when the temperature of the grinding workpiece surface reaches above the phase change temperature, the surface metal will undergo metallographic changes, which will reduce the strength and hardness of the surface metal, resulting in residual stress and slight cracks. Tempering burns, quenching burns and annealing burns may occur during grinding of quenched steel.
·Tempering burn: the temperature in the grinding area does not exceed the transformation temperature of the quenched steel, but has exceeded the transformation temperature of the rough body. The tempering martensite structure of the surface metal of the workpiece will be transformed into the tempering structure with low hardness.
·Quenching burn: If the temperature of the cutting area exceeds the phase change temperature, coupled with the role of the coolant quenching, the surface metal will undergo secondary quenching, and its hardness will be higher than that of the tempered martensite, while the lower layer of it will cool slowly, and a tempering fabric with a hardness lower than that of the original tempered martensite will appear.
·Annealing burn: If the temperature of the cutting area exceeds the phase change temperature, and there is no coolant entering the grinding area, the surface metal will produce a return structure, causing a sharp drop in surface hardness.
(2) Residual stress of surface layer: under the action of cutting force, the machined surface will be affected by tensile stress, resulting in elongation plastic deformation. When the surface area tends to increase, the inner layer will be in elastic deformation. When the cutting force is removed, the inner metal tends to recover, but it cannot return to its original shape due to the limitation of the plastic deformation of the surface layer. Therefore, there will be residual compressive stress on the surface layer and tensile stress on the inner layer.