Causes of Cemented Carbide Brazing Cracks
There are many factors that cause cracks in carbide brazed workpieces, such as groove design, brazing process, heating process and sharpening.
①Some cemented carbides with high hardness and low strength, such as YT60, YT30, YG2 and YG3X, are prone to brazing cracks. Especially when the brazing area of these grades of cemented carbide is relatively large, it should be paid more attention.
② The closed or semi-closed groove is an important reason for increasing the brazing stress and causing cracks. The brazing area should be reduced as much as possible to reduce brazing stress while meeting the requirements for the use of weld strength.
③ If the welding heating speed is too fast or the cooling speed after welding is too fast, the heat distribution will be uneven, and the instantaneous stress will cause cracks. During rapid heating, the outer layer of cemented carbide is under compressive stress and the middle is under tensile stress. When the allowable heating rate is exceeded, visible cracks and invisible cracks may occur inside. During rapid cooling after brazing, tensile stress develops on the outer layer, causing cracks in the alloy. Avoid placing workpieces on wet floors or in wet lime troughs, which can cause cracks in the cemented carbide due to quenching.
④ The cemented carbide itself has defects, which were not found during pre-welding inspection, resulting in cracks after brazing. For large-area or special-shaped hard alloys, strict inspection must be carried out block by block before brazing. Defects of cemented carbide in the sintering process, such as small cracks, chipping, looseness, etc., may expand to form large cracks after heating and brazing.
⑤ Improper sharpening after brazing will also cause cracks. For example, the material, hardness and particle size of the grinding wheel are not properly selected. Water cooling during grinding, excessive grinding allowance, and improper grinding process can also easily cause cracks.
Measures for Reducing Cemented Carbide Brazing Cracking
① The addition of compensating gaskets in the weld is one of the effective measures to reduce the stress of the weld. There are many ways to add compensating gaskets in welds, such as using barbed wire, punching fillers, nickel-iron alloy gaskets, and electroplating pure iron on cemented carbide. Since the melting point of these compensators is more than 200°C higher than the melting point of the solder, the gasket is not melted during brazing and is sandwiched in the middle of the weld. When the weld is cooled, the layers of the weld between the cemented carbide and the base metal have sufficient plastic deformation, so that each part of the weld can shrink relatively freely, reducing the brazing stress. However, the addition of compensating gaskets will lead to a substantial decrease in the strength of the weld. Among them, the strength of welds using barbed wire or punched gaskets is reduced by 60%. Although the nickel-iron alloy compensation gasket composed of 50% nickel and 50% iron can better eliminate stress and does not reduce the strength of the weld, it is not suitable for mass use in production due to excessive nickel content. In production, low carbon steel sheets or nickel-plated iron sheets with a thickness of 0.4 to 0.5 mm are used as compensation gaskets, which can achieve good results.
③ Although the use of red copper sheet as compensation gasket can effectively reduce the brazing stress and prevent cracks, it is necessary to use solder with a melting point lower than 850 ℃, such as L-Ag-49 silver solder, otherwise it will be easy to cause damage during brazing. The copper sheet melted and lost its function. Copper itself is relatively soft and is not suitable for use under shock or heavy load and high temperature. Zhengzhou Machinery Research Institute has systematically studied the brazing characteristics of cemented carbide, and introduced the sandwich-type composite brazing filler metal CT861 for cemented carbide brazing. The three-layer structure is composed of melting temperature range: 640-695 ℃. The application of sandwich brazing filler metal can effectively prevent cemented carbide welding cracks.
④ When brazing long and narrow carbide workpieces, in order to reduce brazing stress and prevent cracks, double-layer carbide brazing can be used, and the lower layer is made of small pieces of carbide to become prefabricated. “Crack” form. This method is particularly effective in eliminating cracks and can be used on large carbide tools and special carbide molds.
Causes of Desoldering in Cemented Carbide Brazing
① The brazing surface of cemented carbide is not sanded or polished before welding, and the oxide layer on the brazing surface reduces the wetting effect of the brazing filler metal and weakens the bonding strength of the weld.
② Desoldering can also occur due to improper selection and use of the flux. For example, when borax is used as the flux, the raw borax cannot effectively deoxidize due to its high water content. As a result, the solder cannot well wet the surface to be brazed. , and desoldering occurs.
③ The correct brazing temperature should be 30-50℃ above the melting point of the brazing filler metal. Desoldering will occur if the temperature is too high or too low. Excessive heating will cause oxidation in the weld. The use of zinc-containing solder will make the weld bluish or white. When the brazing temperature is too low, a relatively thick weld will be formed, and the inside of the weld is full of pores and slag inclusions. The above two conditions will reduce the strength of the weld, and desoldering is easy to occur when sharpening or using.
④ During the brazing process, the slag discharge is not timely or insufficient, so that a large amount of flux slag remains in the weld, which reduces the strength of the weld and causes desoldering.