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What are the factors that affect the mechanical properties of the processed surface of parts?
In the machining of mechanical parts, the workpiece will be affected by cutting force and cutting heat, so that the surface metal will have a change in physical and mechanical properties. In the process of grinding processing, plastic deformation and cutting heat will be more serious than cutting edge. In order to ensure the mechanical properties of the processing surface of parts, we need to understand what are the factors that affect the mechanical properties of the processing surface of parts.
1. Cold work hardening and evaluation parameters
(1) Cold work hardening of metals
In the process of machining, due to the action of cutting force, it is easy to have plastic deformation, so that the lattice is distorted, distorted, and even broken, which will increase the hardness and strength of the surface metal, which is called cold work hardening.
(2) The main factors affecting cold work hardening
The blunt radius of the cutting edge increases, and the extrusion effect on the surface metal will be enhanced, which will aggravate the plastic deformation and lead to cold hard reinforcement.
The wear of the rake face of the tool increases, and the friction fixture between the rake face and the surface to be machined increases the plastic deformation, resulting in cold hard strengthening.
The influence of the blunt radius of the cutting edge on work hardening increases the cutting speed, shortens the action time between the tool and the workpiece, reduces the expansion depth of plastic deformation, and reduces the depth of the cold hard layer. After increasing the cutting speed, the time of action of cutting heat on the workpiece surface is shortened, resulting in cold hard reinforcement.
2. Changes in metallographic structure of surface layer materials
(1) Grinding burns: When the temperature of the surface of the grinding workpiece reaches above the phase change temperature, the metallographic structure of the surface metal will change, so that the strength and hardness of the surface metal will be reduced, residual stress will occur, and slight cracks will appear. During the grinding of hardened steel, tempered burns, quenched burns, and annealed burns may occur.
Tempered burn: The temperature of the grinding zone does not exceed the phase change temperature of the quenched steel, but has exceeded the transition temperature of the wool body, and the tempered martensitic structure of the surface metal of the workpiece will be transformed into a tempered structure with low hardness.
Quenching burn: If the temperature of the cutting zone exceeds the phase change temperature, coupled with the action of quenching the coolant, the surface metal undergoes secondary quenching, and its hardness will be higher than that of tempered martensite, and in its lower layer, cooling is slower, and a tempered structure with a lower hardness than the original tempered martensite appears.
Annealing burns: If the temperature of the cutting zone exceeds the phase change temperature and no coolant enters the grinding area, the surface metal will produce a return tissue, causing the surface hardness to drop sharply.
(2) Surface layer residual stress: Under the action of cutting force, the processed surface will be affected by tensile stress, resulting in elongation plastic deformation. The surface area tends to increase, and the inner layer will be in a state of elastic deformation. When the cutting force is removed, the inner metal tends to recover, but it cannot be restored to its original shape due to the limitation of the surface layer that has produced plastic deformation. Therefore, there will be residual compressive stresses on the surface layer, and the inner layer will be balanced by tensile stresses.
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