Analyze the causes of cracks in welding tools and improve hardware tool technology

1 The mechanism and type of welding knife crack formation

1) The effect of heating on the formation of cracks in cemented carbide

The thermal expansion coefficient of cemented carbide blade and steel (toolholder) is quite different, and the thermal conductivity of the alloy is also worse than that of the tool body material. If it is heated quickly during welding, a large internal stress will be generated, which will promote the blade in the welding layer. Excessive thermal stress caused the blade to crack.

Therefore, the soldering temperature is controlled at approximately 30～50℃ higher than the melting point of the solder. The melting point of the selected solder should be lower than the melting point of the tool shaft by 60°C. During welding, the flame should be uniformly heated from bottom to top and slowly preheated for welding. Therefore, the welding surface of the knife groove and the blade are required to be consistent. Local overheating will cause the blade itself or the temperature difference between the blade and the tool shaft to be large (large and thick blades are more serious), and thermal stress will cause the blade edge to crack. Therefore, it is required to preheat the tool bar when preheating. If the blade and the tool bar are heated together, the flame should be moved back and forth and left and right to heat, so as to avoid local overheating and cracks caused by heat concentration.

2) The influence of sipe shape on crack formation

The shape of the knife groove is inconsistent with the welding surface of the tool holder or the difference is large, forming a closed or semi-closed groove shape, which is easy to cause too much welding surface and too large welding layer. Due to the inconsistent shrinkage rate after thermal expansion, it is also easy to Excessive stress is caused at the welding part of the blade, causing cracks. To meet the requirements of the weld strength required for use, reduce the area of u200bu200bthe brazing surface as much as possible.

3) The effect of cooling on the formation of cracks in cemented carbide

Cooling or rapid cooling during or after welding and poor flux dehydration will cause the blade to burst and crack through. Therefore, the solder is required to have good dehydration properties. After welding, it must not be quickly cooled in water, but slowly cooled in lime, asbestos powder, sand, etc. It is best to keep the temperature at 300℃ for more than 6 hours and then cool down with the furnace after cooling slowly.

4) The effect of defects on the bottom of the sipe on the formation of cracks

The contact surface between the blade and the knife groove is not flat. If there are black skin pits and local unequal causes, the welding cannot form a flat joint, resulting in uneven solder distribution, which not only affects the strength of the weld but also causes stress concentration, resulting in The blade is broken, therefore, the contact surface of the blade should be ground, and the welding surface of the blade slot should be cleaned.

In the process of matching between the milling insert groove and the insert, it is required that the insert extends beyond the support part of the arbor not more than 0.5mm. If the support part of the insert protruding from the arbor is too large or the support part of the arbor is weak, the tool will be During the welding process, the tensile force is subjected to the fracture phenomenon.

5) The effect of blade secondary heating on crack formation

After the blade is brazed, the red copper solder does not completely fill the gap, and there are some false welding. Some tools fall off the blade during the process of being out of the furnace, so they need to be heated again. This way, sticky The binding agent Co is severely burned, and the WC grains grow up, which may directly cause the blade to crack.

2 Characteristics of cracks caused by welding stress

Cracks appear on the cemented carbide blades, in some cases due to excessive welding stress, which exceeds the strength of the cemented carbide blades. When welding the cutter, the height hc of the cutter body should be 3 times greater than the height of the blade ht. For example, hc/ht, after welding, it is easy to cause the alloy blade to break; if hc/ht<3, the surface of the cemented carbide will generate tensile stress and cracks are also prone to; when hc/htu003d4～5, the surface of the cemented carbide is not Significant stress, so it is not easy to produce cracks, even if there is a crack, it is not obvious; when hc/ht<8, a uniform load is generated on the welding layer. The bending of the alloy blade produces tensile stress along the thickness direction of the alloy blade, and the strength of the welding layer exceeds the force distribution of the alloy itself, which is more complicated, because it is not joined on one surface, but on two, three or four surfaces. .

In addition, when the cemented carbide is rapidly heated and rapidly cooled, due to uneven heat distribution, significant instantaneous stress may be generated. During rapid heating, the outer layer of the cemented carbide is under compressive stress, and the middle is under tensile stress. When the allowable heating rate is exceeded, cracks or invisible internal cracks may appear. When cemented carbide is welded, rapid cooling is also very dangerous. In this case, tensile stress will appear on the outer layer, causing cracks in the alloy.