Factors affecting the mechanical properties of the machined surface

In the cutting process of mechanical parts, the workpiece will be affected by cutting force and cutting heat, so that the metal of the surface layer will change the physical and mechanical properties. In the process of grinding, plastic deformation and cutting heat will be more serious than blade cutting. In order to ensure the mechanical properties of the machined surfaces of parts, we need to understand what factors affect the mechanical properties of the machined surfaces of parts.

1. Cold work hardening and evaluation parameters

(1) Cold work hardening of metals

During the machining process, due to the cutting force, it is easy to appear plastic deformation, distortion, distortion, and even fracture of the crystal lattice, which will increase the hardness and strength of the surface metal Cold work hardening.

(2) Main factors affecting cold work hardening

·The blunt radius of the cutting edge increases, and the squeezing effect on the surface metal will increase, exacerbating the plastic deformation, resulting in enhanced cold hardness.

·The wear of the flank of the tool increases, and the frictional fixture between the flank and the surface to be processed increases plastic deformation, resulting in enhanced cold hardness.

·The influence of cutting edge blunt radius on work hardening The cutting speed is increased, the action time of the tool and the workpiece is shortened, the expansion depth of plastic deformation is reduced, and the depth of the cold hard layer is reduced. After increasing the cutting speed, the action time of cutting heat on the surface of the workpiece is shortened, resulting in enhanced cold hardness.

2. Changes in the metallographic structure of the surface layer material

(1) Grinding burn: When the temperature of the grinding workpiece surface reaches above the phase transition temperature, the metallographic structure of the surface metal will change, which will reduce the strength and hardness of the surface metal, and residual stress will occur, and slight cracks will occur. In the process of grinding hardened steel, temper burn, quench burn and annealing burn may occur.

Tempered burn: The temperature in the grinding zone does not exceed the phase transformation temperature of the quenched steel, but it has exceeded the transformation temperature of the wool body. The tempered martensite structure of the surface metal of the workpiece will be transformed into a tempered structure with low hardness.

·Quenching burns: If the temperature in the cutting zone exceeds the phase transition temperature, coupled with the quenching effect of the cooling liquid, the surface metal undergoes secondary quenching, and its hardness will be higher than that of tempered martensite, while in its lower layer, cooling is slower A tempered structure with lower hardness than the original tempered martensite appeared.

· Annealing burn: If the temperature in the cutting zone exceeds the phase transition temperature, and no coolant enters in the grinding zone, the surface metal will produce a return structure, allowing the surface hardness to drop sharply.

(2) Residual stress of the surface layer: under the action of cutting force, the processed surface will be affected by the tensile stress and produce elongational plastic deformation. The surface area tends to increase, at this time the inner layer will be in a state of elastic deformation. When the cutting force is removed, the inner metal tends to recover, but due to the limitation of the plastic layer that has undergone plastic deformation, it cannot be restored to its original shape. Therefore, there will be residual compressive stress on the surface layer, and the inner layer will be balanced with the tensile stress.