Surface work hardening and its influencing factors
Surface work hardening is a common phenomenon during machining. It refers to the plastic deformation of the surface metal of a workpiece under the influence of external forces such as cutting force and friction, resulting in increased hardness and strength of the surface metal, and decreased plasticity and toughness. The formation of a work-hardened layer has a dual impact on the performance of the part: on the one hand, appropriate work hardening can improve the wear resistance and fatigue strength of the part surface, extending its service life; on the other hand, excessive work hardening can increase the brittleness of the surface metal, making it prone to cracking, and at the same time bring difficulties to subsequent processing, such as increased tool wear and reduced processing efficiency. Therefore, understanding the formation mechanism and influencing factors of surface work hardening is of great significance to controlling machining quality.
The formation of work hardening is closely related to the plastic deformation of the metal. During the cutting process, the rake and flank faces of the tool come into contact with the workpiece surface, generating intense extrusion and friction, causing the surface metal grains to slip, break, and fiberize. As the degree of plastic deformation increases, the grains are elongated and aligned along the deformation direction, forming a fibrous structure. At the same time, the dislocation density within the metal increases, and the dislocations become entangled and obstructed, resulting in increased slip resistance, thereby increasing the hardness and strength of the surface layer. For example, when turning steel parts, the surface metal is squeezed by the tool, and the thickness of the plastic deformation layer can reach several microns to tens of microns, and the hardness can be increased by 20%-50% compared to the base metal. In addition, cutting heat also affects work hardening. If the cutting heat causes the surface metal temperature to rise above the recrystallization temperature, dynamic recovery or recrystallization will occur, partially offsetting the work hardening effect. If the temperature does not reach the recrystallization temperature, the hardening will be exacerbated.
Cutting force is one of the key factors affecting the work hardening of the surface layer. The magnitude of the cutting force directly determines the degree of plastic deformation. The greater the cutting force, the more severe the plastic deformation of the metal surface, and the higher the depth and hardness of the work hardening layer. The magnitude of the cutting force is closely related to the cutting parameters. When the back cutting depth and feed rate increase, the cutting force increases significantly, which will aggravate the work hardening. For example, when the back cutting depth increases from 0.5mm to 2mm, the depth of the work hardening layer may more than double. In addition, the tool angle will also affect the cutting force. When the main rake angle decreases and the secondary rake angle increases, the radial cutting force increases, which increases the extrusion effect on the surface metal and improves the degree of work hardening. Increasing the rake angle can reduce the cutting force, thereby reducing work hardening.
The wear state of the tool has a significant impact on the work hardening of the surface layer. The cutting edge of a new or sharpened tool is sharp, resulting in less friction with the workpiece surface, less plastic deformation, and a lower degree of work hardening. As the tool wears, the cutting edge becomes rounded, the contact area between the flank and the workpiece surface increases, and friction increases significantly, causing the surface metal to be subjected to more intense extrusion and friction, significantly increasing the depth and hardness of the work hardening layer. For example, when the wear of the tool flank increases from 0.1mm to 0.3mm, the hardness of the work hardening layer may increase by 10%-20%. In addition, the wear resistance of the tool material will also affect work hardening. Tools with poor wear resistance are prone to wear, which in turn exacerbates the work hardening phenomenon.
The mechanical properties of the workpiece material are intrinsic factors that affect work hardening. The better the plasticity of the material, the easier it is to undergo plastic deformation during the cutting process, and the more obvious the work hardening phenomenon. For example, low-carbon steel has higher plasticity, and the degree of hardening of the surface layer after processing is more significant than that of high-carbon steel; while brittle materials such as cast iron have relatively poor plasticity, and the work hardening phenomenon is relatively mild. The hardness and strength of the material will also affect the work hardening effect. Under the same cutting conditions, materials with lower hardness will have greater plastic deformation and higher hardening degree. In addition, the grain size of the material also affects work hardening. Fine-grained materials have more uniform plastic deformation and a more uniform distribution of the work hardening layer; while the deformation of coarse-grained materials is concentrated at the grain boundaries, which is prone to local hardening.
The effect of cutting speed on surface work hardening is complex and exhibits a nonlinear relationship. At low cutting speeds, the cutting force is high and the cutting time is long, giving the surface metal ample time to undergo plastic deformation and a high degree of work hardening. As the cutting speed increases, the cutting force decreases while the cutting heat increases. If the temperature rises to a certain level, the surface metal will undergo dynamic recovery, partially eliminating the work hardening, and the degree of hardening will decrease. When the cutting speed is too high, although the cutting time is short, the friction between the tool and the workpiece intensifies, generating instantaneous heat that causes the surface metal to undergo significant plastic deformation even at high temperatures. Furthermore, the cooling rate is rapid, further enhancing the hardening effect. For example, when machining 45 steel, the degree of work hardening is highest at cutting speeds of 30-50 m/min. Once the speed exceeds 80 m/min, the degree of hardening gradually increases. Therefore, selecting the appropriate cutting speed is an important means of controlling work hardening.
