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Vacuum Heat Treatment Process Prevention of Cemented Carbide Tool Cracks

Welding cracks are common welding defects of alloy cutting tools. The reasons are mainly caused by the following aspects:

1.1 The effect of cemented carbide hardness on crack formation

Generally, the higher the strength of the cemented carbide, the less likely to have cracks during welding. However, the main characteristics of cemented carbide, hardness and wear resistance, are often inversely proportional to their strength, which determines when we require high hardness and high resistance. When abrasive cemented carbide is used, the possibility of crack defects due to low strength is faced. Therefore, alloy tools with higher hardness are required to pay more attention to the problem of cracks in machining.

1.2 Influence of insert, cutter body and kerf shape on crack formation

If the welding surface is not flat and the shape of the blade and the kerf does not match, the solder distribution will be uneven, which will cause a large stress concentration during the welding process, resulting in cracks or fractures. If the groove is a closed surface or a semi-closed surface groove, due to the excessive welding area and the difference in heat shrinkage, a large stress will be generated at the welding layer, resulting in cracks or chipping. Therefore, in the case of meeting the requirements of welding strength, the welding surface and welding area should be reduced as much as possible.

1.3 Influence of heating process on crack formation

Welding stress is the direct cause of welding cracks, which is mainly caused by the large thermal shrinkage of the carbide insert and the rigid body shank. Because the coefficient of linear expansion of cemented carbide (blade) and steel (blade) is very different, which is 1/3~1/2 lower than that of steel, when heated rapidly, a large internal stress will be generated inside the tool, which promotes heat If the stress is too large, it will appear as tensile stress inside the alloy. When the tensile stress is large enough, cracking will occur at the welding layer of the cemented carbide insert. The poor thermal conductivity of cemented carbide is also one of the important reasons for welding stress, which puts forward high requirements for the brazing process. Therefore, the control of vacuum brazing temperature and the selection of solder are very important.

Generally speaking, it is ideal to control the temperature between 30 and 50 degrees higher than the melting point of the solder. Therefore, it is necessary to choose a solder whose melting point is 40 to 60 degrees lower than the melting point of the steel (blade). When heating, the tool holder should be preheated first, and the tool should be preheated slowly from the bottom to the top, and the welding should be carried out evenly by heating, which can reduce the internal stress caused by the difference in the expansion coefficient and reduce the possibility of thermal stress causing the blade edge to crack.

1.4 Influence of cooling process on crack formation

Due to the difference in heat shrinkage, the cooling process after brazing of cemented carbide tools is also an important cause of cracks in cemented carbide. After vacuum brazing, cooling, too fast cooling, rapid cooling, etc. will cause cracks in the blade. In addition, poor solder dehydration can also cause cracks to appear. Therefore, when brazing, choose a solder with good dehydration. After welding, do not rapidly cool with water or air, but do heat preservation treatment. You can choose to put it into dry asbestos powder, sand, limestone and other media for slow cooling. When slow cooling to 300 When the temperature is reached, it is placed in a holding furnace at a constant temperature for 6 hours, and then cooled with the furnace.

2.Analysis of the Causes of Cemented Carbide Tool Grinding Cracks

The physical properties of cemented carbide determine its characteristics of crack generation. Due to its high hardness, high brittleness, poor thermal conductivity and poor vibration resistance, cracks are more likely to occur during sharpening. The main types are thermal cracking, vibration cracking, chipping, Bursting, mainly in the form of network cracks. The defect of the crack seriously affects the cutting performance of the tool, and the strength is reduced by more than half.

2.1 Sharpening and hot cracking

Due to the high hardness of cemented carbide inserts, it is easy to passivate the grinding wheel grit during the sharpening process. Additional thermal stress will be formed due to local high temperature, resulting in thermal cracks. This cracking is due to the combination of the properties of the cemented carbide and the selection of the grinding wheel. Therefore, it is necessary to start with eliminating the factors of additional thermal stress to achieve the purpose of preventing sharpening and hot cracking.

2.2 Sharpening and vibration cracking

In the sharpening process, factors such as poor rigidity of the tool bar, beating of the machine tool spindle, excessive sharpening allowance, unstable clamping and other factors will cause the sharpening process to vibrate continuously, thus causing the blade to break. Therefore, it is necessary to start from eliminating various factors that cause vibration to achieve the purpose of preventing sharpening and vibration cracking.

2.3 Sharpening and cracking

When sharpening, the axial and radial runout of the grinding wheel is too large, the rotation direction of the grinding wheel is different from the grinding direction of the cutting edge, and the sharpening sequence of the rake angle and the relief angle is improper, which will cause the cutting edge to crack, and the cutting edge of the tool will crack. On the sharp arc, there may be slight chipping on the main and auxiliary cutting edges, front and rear surfaces. Therefore, controlling the beating of the grinding wheel and formulating a reasonable process chain are the keys to controlling the sharpening and cracking.

3.Crack prevention

According to the cause of the crack, find out the preventive measures respectively, which can be summarized in the following aspects:

3.1 Prevention of brazing cracks

Before vacuum brazing, process a flat, suitable welding surface that matches the blade, and if necessary, do sandblasting to reduce stress concentration; the solder should choose high-strength solder, such as L104, etc.; Dehydrated borax can be used for welding, or 70% can be used A mixed solvent of dehydrated borax and 30% boric acid; when welding, preheat the tool holder first, and master the heating sequence and temperature; after welding, the carbide tool should be tempered in a low-temperature vacuum and cooled with the furnace.

3.2 Prevention of sharpening cracks

Choose a silicon carbide grinding wheel with a grain size of 60-80 and medium hardness, and grind the surface of the grinding wheel in time. You can choose intermittent grinding to achieve a good heat dissipation effect; first use an alumina grinding wheel for rough grinding, and then use a silicon carbide grinding wheel for fine grinding and sharpening. When grinding, the beating of the grinding wheel should be controlled, the rotation direction should be opposite to the grinding direction of the population, and the order of the previous tool face – chip breaker – auxiliary chamfer – tool tip arc – wiper edge should be observed. Do not use excessive force. Violent, both hands should be supported to prevent chattering. The blade should be 1~2mm higher than the center line of the grinding wheel, and the contact area with the grinding wheel should be minimized. Rapid cooling after sharpening is prohibited. enough coolant.

4.Summary

The factors that affect the cracking of cemented carbide cutting tools are complex and diverse. It is necessary to analyze the specific reasons, starting from the process of vacuum brazing and sharpening, and actively prevent and improve the performance of cutting tools, so as to achieve the purpose of saving costs, reducing consumption and improving work efficiency.