Analysis of Factors Affecting Physical And Mechanical Properties of Surface in Machining

Views: 14     Author: Site Editor     Publish Time: 2024-10-25      Origin: Site

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Analysis of Factors Affecting Physical And Mechanical Properties of Surface in Machining

With the rapid development of China's industrial technology, in industrial production activities, the level of mechanization has been significantly improved, and today's mechanized production has been widely used in the processing industry, and the application of various mechanical equipment has greatly reduced the workload of processing personnel and improved work efficiency. The quality of mechanical processing is closely related to various machines and equipment, and the parts are assembled after the processing of the city machine, if the surface mechanical properties of the parts fail, it is easy to cause the performance failure of the entire machine. In the machining process, the parts are susceptible to the influence of the surrounding environment, especially the role of cutting force and cutting heat, so that the physical and mechanical properties of the surface metal change, and then affect the quality of the machining products. There are three main changes in the physical and mechanical properties of machined products, which are cold hardening of the surface layer, changes in the metallographic structure and residual stress.


1 Cold hardening of the surface layer


The cold hardening of the surface layer refers to the subtle changes in the hardness of the surface layer during machining, that is, the plastic deformation phenomenon that occurs during processing, which is mainly the result of the comprehensive effect of the cold hardening of the surface layer caused by the cutting force and the metallographic structure changes caused by the cutting heat. The plastic deformation of the machined product will cause the crystal of the product to change, and elongate the grain, resulting in serious distortion of the crystal lattice, grain fragmentation, and eventually the hardening of the machined product.


1.1 Characteristics of cold hardening


Due to the influence of the two factors of force and heat in the machining process, the deformation phenomenon appears, and the elongation of the processed product will be reduced after this, but the deformation resistance will be improved, that is, the yield point will be improved. The cold hardening of the surface layer makes the machined product in an unstable state, especially in a high energy state, when the conditions are suitable, the chilled structure of the machined product will automatically change to a more stable state, in technical terms, it is weakened. In other words, after the surface physical machinery is affected by mechanical processing, the chilling phenomenon generated in the molding deformation can be restored, and the nature of the surface metal after recovery mainly depends on the effect of strengthening and weakening. The degree of mechanical processing products surface layer is greatly affected by external conditions, including molding deformation, deformation speed and deformation temperature, the greater the force, the greater the effect of molding deformation, the degree of hardening is more serious. However, if the speed of deformation is too fast, the force is relatively small, the conditions for plastic deformation are not sufficient, and the degree of hardening is relatively weak.


1.2 Factors affecting cold hardening


The first is the material of the mechanical processing products, the material of the product itself has a greater impact on the processing, if the hardness of the material itself is low, then the deformation phenomenon occurred after the cutting force and the cutting heat effect is serious. The second is the amount of cutting, cold hardening of the surface is mainly caused by the cutting operation, when the cutting speed increases, the contact time between the mechanical equipment and the product is relatively short, the degree of molding deformation is relatively light, the degree of hardening and hardness will be reduced. When the cutting amount increases, the phenomenon of molding deformation and hardening will be aggravated, which should be noted that the cutting amount can not be too small, otherwise the degree of hardening will also increase because of the increase in the number of extrusion. Finally, the tool, the damage caused by the tool in the machining process is also large, which is also a major factor in the depth of the chilled layer and the increase in hardness.


2. Metallographic structure changes of the surface


In the process of machining, the temperature of the machining area has been increased, especially the surface temperature of the processing, because the cutting process needs to consume a lot of energy, which is converted into heat in the process, which adds a lot of heat for the machining, and the temperature rises. The metallographic structure of the product surface needs to change under certain conditions, and when the temperature to be processed reaches that critical point, the metallographic structure will change. Under normal circumstances, the heat generated by the general cutting process will be taken away by the cutting operation, which will not cause large changes in the temperature of the processing interval, and the impact on the surface of the product is not large. However, for some machining methods that consume a lot of cutting power, changes in the metallographic structure are common. Some enterprises, in order to improve the efficiency of processing production, the grinding processing method is used, the cutting force required by this processing method is much larger, the cutting speed is also fast, and the heat generated in the processing process is greatly increased, and then transmitted to the processed products, causing changes in the metallographic organization.


There are two main factors affecting the change of metallographic structure. The first is the grinding amount, which also includes the grinding depth, the longitudinal feed amount and the workpiece speed. When the grinding depth increases, the surface temperature of the machinized product will increase, and the higher the rise, causing the metallographic structure change; When the longitudinal feed increases? r, the surface temperature of the product will be reduced to reduce the damage to the workpiece; When the workpiece speed increases, the temperature of the grinding zone rises, causing a small damage to the grinding depth, but it will cause the surface of the product to become rough. The second is the cooling method, grinding processing method will inevitably cause the workpiece burn, if can not take effective cooling method, will also affect the surface of the workpiece.


3 Residual stress on the surface


There are residual stresses on the surface of the workpiece after machining, which are completely opposite to the effect of the residual compressive stress. The residual compressive stress can play a certain protective role and increase the wear resistance of the workpiece surface, while the residual stress will cause damage to the workpiece. The main manifestation of residual stress is the crack on the surface of the workpiece, because the fatigue strength of the workpiece material can not withstand too much tensile stress, and then accelerate the damage of the workpiece.


The residual stress of the machined surface is the result of the joint action of three aspects, therefore, the influencing factors of the residual stress are mainly cold molding deformation, thermoplastic deformation and metallographic structure, according to the above description, it is not difficult to find the connection between the three. Of course, in addition to the influence of the main factors of this result, the generation of residual stress is also related to the processing technology.


4 Measures to solve the problem


4.1 Basic Management


"Reasonable allocation, safe and practical, both maintenance and repair, professional management" is the principle that should always be implemented in the use and management of machinery and equipment. The management of machinery and equipment should be vigorously strengthened to ensure that when the machinery and equipment are processed, the relevant person in charge of the machinery and equipment on the site carries out the whole process and all-round monitoring and control, and the division of labor of on-site processing operations is clear. Use the machinery strictly according to the plan. Check the mechanical equipment regularly and deal with the problems in time. For the stopped or used machinery and equipment in the absence of approval, it can not be disassembled, but should actively innovate in the basic management methods and technology, widely use modern management methods and technical means, and strive to improve the mechanical equipment integrity rate, utilization rate and economic benefits.


4.2 Security Management


Safety management is an important part of all machining projects, so it is necessary to strengthen the leadership and education of relevant personnel in the safe production of equipment, establish sound production rules and regulations, regularly inspect the machinery and equipment, find problems to solve in time, and accumulate experience and summarize. For machining operators, professional technical training and safe operation procedures must be carried out, and after the examination to obtain an operating certificate before operation. And strictly abide by the operating procedures, warranty procedures, mechanical equipment safety protection devices must be complete and reliable.


4.3 Operator Management


There are many processing methods of mechanical processing, and in order to meet the needs, the types of processing are becoming more and more diverse, resulting in the greater mobility of processing personnel, the metabolism of processing personnel is getting faster and faster, and the requirements for the operators of mechanical equipment are relatively improved. In order to solve this problem, corresponding management measures should be established in the management process of machining to ensure the stability and enthusiasm of the staff to the maximum extent.


5 Conclusion


There are many factors that affect the physical mechanical properties of the machined surface, and it is required to be careful in the machining process and try to avoid the occurrence of problems.


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