What is the cause of the welded knife crack?

Causes and types of welded knives

  •  Effect of heating on the formation crack of cemented carbide

The thermal expansion coefficient of the cemented carbide blade and steel (knife rod) is large, and the thermal conductivity of the alloy is also poor than the tool material, and if rapid heating in the welding, it will produce large internal stress, so that the blade is thermally stressed at the weld layer. Excessively causes the blade to crack.

Therefore, the welding temperature is controlled at about 30 to 50 ° C which is greater than solder. The selected solder should be lower than the tool rod melting 60 ° C, and the flame should be welded upward when welding, and therefore, the slot is required to coincide with the blade welding surface.

Local overheating will make the blade itself or the blade and the knife rod have a large temperature difference (large and thick blades more serious), the thermal stress will make the blade cut. Therefore, the knife rod is preheated when the blade is preheated. If the blade is heated with the knife rod, it should be heated to return to the flow of flames around and off, so that the heat is concentrated to cause local overheating to produce cracks.

  • Influence of the shape of the slot on crack formation

The shape of the slot is inconsistent with the knife bar welding surface or a large difference, forming a closed or semi-closed groove shape, which is easy to cause excessive welding surface and excessive solder layer, and is also easy to weld in the blade after the shrinkage after the thermal expansion is inconsistent. The stress is too large, forming a crack. Under the required weld strength requirements, the area of ​​the brazing surface is reduced as much as possible.

  • Effect of cooling on cracks in cemented carbide

Cooling or rapid cooling after welding or welding and poor floating dehydration can cause the blade to burst and crack. Therefore, the solder is required to have good dehydration. After the welding is absolutely unable to be rapidly cooled in the water, it is necessary to place slowly in lime, asbestos powder, sand, etc. It is best to cool around 300 ° C for more than 6 hours and above.

  • Influence of defective crack formation in the bottom surface of the sipe

The contact surface of the blade and the sipe is not flat, such as a black leather pit, local inequality cause, causing the welding to form a plane bond, resulting in uneven solder distribution, but not only affect weld strength and causing stress concentration, causing the blade to break, Therefore, the blade should be ground to the contact surface, and the welding surface of the blade sipe should be cleaned.

During the milling cutting tool, the blade is required to reach out of the knife bar support portion is not more than 0.5mm, and if the blade projects out of the knife bar support portion, the knife bar support is weak, the knife will cause the knife in the welding process. Belt pull and generate fracture.

  • Influence of the secondary heating of the blade on crack formation

After brazing, the copper solder is not completely filled with gaps, and the individual is solder. Some knives are fell on the knife bar, so it takes secondary heating, so that the binder CO Serious burning, WC grains grow up, it is possible to directly lead to blade cracks.

Solder stress causes crack characteristics

  • Cracks appear on the carbide blade, in some case due to too high welding stress, exceeding the strength of the hard alloy blade. At welding, the height Hc of the blade should be greater than the blade height HT3 times. Such as HC / HT, after welding, it is easy to cause alloy blade breaks; if HC/HT <3, the surface of the cemented carbide produces a stress, it is prone to cracks;
  • When HC/HT = 4 ~ 5, there is no significant stress of the surface of the carbide, so it is not easy to produce cracks, even if there is crack, it is not obvious; in HC/HT <8, a homogenizer is produced on the welding layer. The alloy blade is bent along the thickness direction of the alloy blade, and the strength of the weld layer exceeds the force distribution of the alloy itself is more complicated because it is not engaged on one surface, and combined on two, three or four faces .
  • In addition, when the carbide is rapidly heated and rapidly cooling, there may be significant instantaneous stress due to the uneven heat distribution. At the time of rapid heating, the outer layer of the carbide is pressed, and the middle is protruded. When the allowed heating speed is allowed, the invisible cracks or internal invisible cracks may occur. When the cemented carbide welding, the rapid cooling is also very dangerous. In this case, there will be a stress on the outer layer, and the alloy has cracks.