The occurrence and control measures of cracks in cemented carbide welding tools

As the development process of new machines continues to deepen, the types and quantities of difficult-to-machine materials are gradually increasing, and the demand for cutting tool products is gradually changing from high-speed steel cutting tools to carbide cutting tools. Carbide-cutting tools are widely used in aviation, aerospace and other fields. Applications are becoming more and more widespread.

As a manufacturing and grinding unit of carbide tools, we have encountered quality problems such as tool dissatisfaction, alloy blades tearing off pieces during use, blades breaking as a whole at the welding point, welding blade cracks, etc. Grinding cracks are the most common. common. Therefore, it is necessary to analyze the causes of grinding cracks in carbide tools and control them, summarize improvement measures to prevent grinding cracks, reduce tool cracks, and improve the quality and service life of carbide-turning tools.

 

1. Factors affecting grinding cracks of cemented carbide tools

 

1) Influence of material properties of cemented carbide tools

 

The characteristics of cemented carbide materials include high hardness, high brittleness, poor plasticity, and low thermal conductivity. The material has high hardness and requires greater grinding pressure during grinding. The material is highly brittle and has a high tendency to produce grinding cracks during grinding. The material’s low thermal conductivity does not allow large grinding heat to be generated.

Therefore, the sharpening of cemented carbide tools requires the grinding wheel to have good self-sharpening properties and a reasonable sharpening process, good cooling, and good heat dissipation.

 

2) The influence of grinding wheel on grinding cracks of cemented carbide tools

 

The most serious problem when grinding carbide tools with grinding wheels is that carbide is prone to cracking or cracking. In addition to the quality of the carbide blade, unreasonable tool structure, internal stress in blade welding, large machine tool vibration, and tool collision during process turnover In addition to other factors, the grinding wheel and grinding process can also cause cracks during the grinding process.

The hardness of the carbide used to make cutting tools is high, and the abrasive grains are easily passivated during the sharpening process. If the self-sharpening ability of the grinding wheel is not good, the dull abrasive grains cannot fall off in time, and the grinding wheel loses its cutting ability, which intensifies the relationship between the grinding wheel and the hard metal. The friction between the grinding surfaces of alloy tools increases the grinding heat sharply, and the heat in the grinding area cannot be transferred in time, causing the grinding surface of the cemented carbide to generate an instantaneous high temperature of more than 1000°C, and the temperature rises quickly, causing thermal deformation. In addition, the flexural strength of cemented carbide is low and has almost no plasticity at room temperature, so it cannot remove part of the thermal stress through its own plastic deformation. Under the action of grinding heat, the local stress value generated on the ground surface of the cemented carbide exceeds the strength limit of the cemented carbide, causing cracks or cracks.

Silicon carbide grinding wheels grinding cemented carbide have low grinding efficiency, high grinding force, poor self-sharpening, high local temperature on the surface of the grinding contact area, poor tool edge quality, poor surface roughness, and high scrap rate during grinding. The diamond grinding wheel for grinding cemented carbide has high grinding efficiency, low grinding force, good self-sharpening properties, sharp diamond edge, and is not easy to passivate. The local surface temperature of the grinding contact area is low, generally around 400°C. At present, Wide range of applications.

 

3) Effect of welding stress on grinding cracks of cemented carbide tools

 

Cracks appear on carbide blades, sometimes caused by the welding stress exceeding the strength of the carbide blade during welding. Welding stress is formed during blade welding and heat treatment, which poses the greatest risk to grinding cracks. It is not necessarily revealed during the welding process, and in some cases will be revealed during sharpening. When welding the tool, the height HC of the tool body should be three times greater than the height HT of the blade, that is, HC/HT>3.

HC/HT<3, tensile stress occurs on the surface of the cemented carbide and cracks are prone to occur.

HC/HT=4-5, the surface layer of cemented carbide has no significant stress and is not prone to cracks.

When HC/HT<8, a uniform load is generated on the welding layer

When cemented carbide is rapidly heated and rapidly cooled, the heat is distributed unevenly, sometimes producing significant transient stresses. During welding, due to rapid heating, the outer layer of the cemented carbide is subject to compressive stress and the middle layer is subject to tensile stress. When the allowable heating speed is exceeded, cracks or internal invisible cracks may appear. With rapid cooling, tensile stresses will appear on the outer layer, causing cracks in the alloy.

 

2. Specific control measures for cracks in carbide tools

 

Practice shows that the greater the linear speed and radial feed of the diamond grinding wheel, the higher the hardness of the grinding wheel, the finer the grinding wheel particle size, the lower the thermal conductivity of the tool material, and the more serious the grinding wheel wear, the more the grinding temperature will rise. , the easier it is to produce grinding cracks and grinding burns. To control and avoid grinding cracks and grinding burns, the generation of grinding heat must be reduced and the transmission of grinding heat must be accelerated.

To prevent cracking during the sharpening of carbide tools, in addition to paying enough attention to the inspection of blades, tool structures, welding processes, heat treatment processes, machine tool adjustments and process transfers, the grinding wheel and grinding process must also be adapted to the hard alloy. The grinding characteristics of alloy tools reduce the generation of grinding heat. For example, the self-sharpening ability of the grinding wheel can be improved, so that the passivated abrasive grains can fall off in time, and the grinding performance of the grinding wheel can be improved to reduce grinding heat. Improve the heat dissipation conditions in the grinding area and disperse the grinding heat. Methods can be adopted to reduce the continuous contact and friction time between the grinding wheel and the cemented carbide, so that the heat generated by grinding can have the opportunity to diffuse in time without gathering, thereby improving the grinding conditions.

 

1) Control the welding stress so that the height of the cutter body matches the thickness of the blade

 

When welding the tool, control the ratio of the height HC of the tool body to the thickness HT of the blade to 6-8, that is, 6<HC/HT<8.
In the production process of manufacturing carbide turning tools, some tools are designed with carbide blades with a thickness of 7-10mm. The phenomenon of cracks in the carbide blades during welding of such tools is very serious, and the scrap rate of the blades due to cracks reaches 70%. %above.

The mismatch in the thickness of the cutter body and blade is the main cause of this problem. On the premise of meeting process requirements, we gradually reduced the blade thickness

(1) Use wire cutting equipment to cut the blade, and control the blade thickness to less than 5mm. 2) Heat treat the cut carbide blade to remove cutting stress. Through this method, the welding crack rate of the blade is reduced to less than 10%-15%, and the effect is very good.

(2) Appropriately reduce the grinding speed and reduce the grinding heat

(3) Select a softer diamond grinding wheel to reduce the contact area between the workpiece and the grinding wheel

(4) Reasonably select the grinding wheel particle size according to the grinding requirements, and always keep the grinding wheel grinding under sharp conditions

(5) Accelerate grinding heat transfer

(6) Maintain the rigidity of the processing system

(7) Repair the grinding wheel frequently to keep the grinding wheel with good grinding performance

(8) Avoid rapid cooling and heating of cemented carbide

The main reason for grinding cracks in cemented carbide tools is the improper grinding process and parameters, which leads to excessive local instantaneous temperature in the grinding contact area, causing excessive tensile stress values in the near-surface layer to exceed the fracture stress of the material. By improving and optimizing the grinding process and grinding parameters, the occurrence of grinding cracks and grinding burns can be effectively prevented, the surface quality of the workpiece can be improved, and the strength and service life of the tool can be improved.

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