Causes of hard alloy brazing cracks

There are many factors that cause cracks in hard alloy brazed workpieces, such as groove design, brazing process, heating process and sharpening, etc.

1. Some hard alloys 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, more attention should be paid.

2, the closed or semi-closed trough shape is an important reason for increasing the brazing stress and causing cracks. Should meet the requirements of the use of weld strength, the brazing area should be reduced as much as possible to reduce the brazing stress.

3, too fast welding heating speed or too fast cooling speed after welding will cause uneven heat distribution, resulting in instantaneous stress and cracks. During rapid heating, the outer layer of the cemented carbide is subjected to compressive stress and intermediate tensile stress. When the heating rate exceeds the allowable heating rate, visible cracks and internal invisible cracks may occur. When rapidly cooling after brazing, tensile stress will appear on the outer layer, causing cracks in the alloy. Avoid placing the workpiece on the wet ground or in a wet lime tank, which will cause cracks in the cemented carbide due to quenching.

4, the cemented carbide itself has defects, which were not found in the pre-welding inspection, resulting in cracks after brazing. For large-area or special-shaped hard alloys, strict inspections must be carried out block by block before brazing. The defects of cemented carbide in the sintering process, such as small cracks, corner chipping, porosity, etc., may expand to form large cracks after heating and brazing.

5. Improper sharpening will also cause cracks after brazing, such as inappropriate selection of the material, hardness and particle size of the grinding wheel, water cooling during grinding, excessive grinding allowance, improper grinding technology, etc., may also cause cracks. .

Measures to reduce hard alloy brazing cracks

1, adding a compensation gasket in the weld is one of the effective measures to reduce the stress of the weld. There are many ways to add compensation gaskets in the weld, such as using wire mesh, punching fillers, nickel-iron alloy gaskets, and electroplating pure iron on hard alloys. Since the melting point of these compensations is higher than the melting point of the solder by more than 200°C, the gasket does not melt during brazing and is sandwiched between the welds. When the weld is cooled, the layers of the weld between the cemented carbide and the base metal are fully plastically deformed, so that each part of the weld can shrink relatively freely, reducing the brazing stress. However, the addition of compensation gaskets will result in a substantial decrease in the strength of the weld. Among them, the weld strength of the wire mesh or punching gasket is reduced by 60%. Although the nickel-iron alloy compensation gasket composed of 50% nickel and 50% iron can eliminate stress and not reduce the weld strength, it is not suitable to be used in large quantities in production due to the excessive nickel content. In production, using low-carbon steel sheet or nickel-plated iron sheet with a thickness of 0.4-0.5mm as compensation gaskets can achieve good results.

2. Although the red copper sheet is used as a compensating gasket can effectively reduce the brazing stress and prevent cracks, it is necessary to use a solder with a melting point below 850°C, such as L-Ag-49 silver solder, otherwise it will be easy to braze. Melt the copper sheet and lose its effect. The copper itself is relatively soft and is not suitable for use under impact or heavy load and high temperature. Zhengzhou Institute of Machinery has systematically studied the characteristics of cemented carbide brazing, and introduced the sandwich composite solder CT861 for cemented carbide brazing. The CT861 sandwich silver solder sheet is made of silver solder from the top and bottom, and the middle layer is made of pure copper. Three-layer structure composition melting temperature range: 640-695 ℃, the application of sandwich solder can effectively prevent cemented carbide welding cracks.

3. When brazing long and narrow carbide workpieces, in order to reduce the brazing stress and prevent cracks, double-layer carbide brazing can be used, and the lower layer is made of small pieces of carbide, becoming Prefabricated "crack" form. This method is particularly effective in eliminating cracks, and can be used on large cemented carbide tools and special cemented carbide molds.

Causes of desoldering in cemented carbide brazing

1, the brazing surface of the hard alloy has not been sanded or polished before welding. The oxide layer on the brazing surface reduces the wetting effect of the solder and weakens the bonding strength of the weld.

2. Desoldering can 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, and the solder cannot be well wetted by brazing. Surface, 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 appear blue or white. When the brazing temperature is too low, a thicker weld will be formed, and the weld will be full of pores and slag inclusions. The above two conditions will reduce the strength of the weld, and it is prone to desoldering when sharpened or used.

4, in the brazing process, the slag is not discharged in time or insufficient, so that a large amount of flux slag remains in the weld, which reduces the strength of the weld and causes desoldering.