Cemented carbide is a kind of special material and most of brazing technology is used in actual production.

First, the cause of cemented carbide welding crack

There are many factors that lead to crack on cemented carbide brazing workpiece, such as groove design, brazing technology, heating process and cooling conditions.

Some hard alloys with high hardness and low strength, such as YT60, YT30, YG2 and YG3X, are easy to produce welding cracks.Especially when the brazing area of cemented carbide of these grades is relatively large, more attention should be paid.

(2) The closed or semi-closed groove shape design is an important reason for increasing the brazing stress and causing cracks.Free opening groove should be adopted as far as possible to reduce brazing area and brazing stress under the condition of meeting the requirements of brazing strength.

(3) Welding heating speed too fast or after welding cooling speed too fast will cause the heat distribution is not uniform, resulting in greater stress caused by crack.Visible cracks and internal cracks may occur when the outer compression stress and intermediate tensile stress exceed the allowable heating rate.During the rapid cooling after brazing, tensile stress will appear on the outer layer, which will lead to cracks in the alloy.Avoid putting the workpiece on wet ground or in a wet lime tank, which will crack the hard alloy due to severe cold.

(4) The cemented carbide itself has defects, which could not be found in the pre-welding inspection and lead to cracks after brazing.For large area or special shape of hard alloy, the needle welding must be strictly inspected piece by piece.In the sintering process, the defects of cemented carbide, such as small cracks, Angle collapse, loose, etc., may enlarge and form large cracks after heating and brazing.

(5) After brazing, improper grinding of the edge will also lead to cracks, such as the unsuitable material, hardness and particle size of the grinding wheel, water cooling during grinding, too large grinding margin, improper grinding process will also easily lead to cracks.

Secondly, measures to reduce brazing crack of cemented carbide

(1) Adding compensation gasket in the stitch is one of the effective measures to reduce welding stress.There are many ways to add compensating gaskets to the brazing joint, such as wire mesh, punch filler, nickel-iron alloy gasket and electroplating pure iron on hard alloy.As the melting point of these compensators is above 200℃, the gasket does not melt and is clamped in the middle of the brazing joint.When the brazing joint is cooled, each layer of the brazing joint between the hard alloy and the base metal has sufficient plastic deformation, so that each part of the brazing joint can contract freely and the welding stress is reduced.However, the addition of filler tape will lead to a significant decrease in the strength of the brazing joint.

Among them, the strength of brazing joints with additional wire mesh or punch gasket is reduced by about 60%. Although the Ni-FE alloy compensation gasket composed of 50%Ni and 50%Fe can better eliminate stress and do not reduce the strength of brazing joints, it is not suitable to be used in large quantities in production because of the excessive Ni content.In production, low carbon steel sheet or nickel-plated iron sheet with thickness of 0.4~0.5mm can be used as compensation gasket to achieve good results.

The double - layer cemented carbide brazing method is an effective measure to prevent crack.This method does not require special materials for easy popularization and use.It can eliminate the brazing crack of high hardness hard alloy such as YT30, YT60, YG2 and YG3X.It is to weld high strength YG8 hard alloy as gasket with the matrix, and then weld the hard alloy with low strength and high hardness on it.Its advantage lies in that the brazing stress is concentrated on the high-strength hard alloy as the gasket, while the hard alloy which is prone to crack on the top is close to the line expansion coefficient due to being welded together with YG8, and the stress is small after brazing, so no cracks will occur.Because there are two layers of cemented carbide welded together, the compressive strength of the whole cemented carbide is improved, and the service life of the tool is increased.

(3) Although the use of copper as compensation gasket can effectively reduce the brazing stress and prevent the generation of cracks, but must use the melting point below 850℃ solder, such as L-AG-49 silver solder, otherwise in the brazing is easy to make the copper sheet melting and lose its role.When using red copper as gasket, it is not suitable for use under impact or heavy load and high temperature due to its soft nature.

(4) When brazing long and narrow shaped hard alloy workpiece, in order to reduce the brazing stress and prevent the occurrence of cracks, the double layer of hard alloy brazing can be used, the next layer is made up of small pieces of hard alloy and becomes the prefabricated "crack" form.This method is particularly effective for crack elimination and can be used on large carbide tools and special carbide dies.

Third, the reason of carbide brazing unsoldering

(1) The needle welding surface of cemented carbide has not been sandblasted or polished before welding, and the oxide layer on the brazing surface reduces the wetting effect of the filler metal and weakens the bonding strength of the brazing joint.

(2) improper selection of brazing agent will occur, such as the use of borax as a brazing agent, due to borax containing more water and can not effectively deoxidize, the needle material can not be well wetting the brazing surface, and the occurrence of welding phenomenon

(3) The correct brazing temperature should be above the solder melting point 30~50℃ is the most appropriate, the temperature is too high or too low may occur off welding.Too high heating temperature will cause oxidation in the joints, and the use of zinc filler metal will make the joints blue or white.When the brazing temperature is too low, a relatively thick brazing joint will be formed. The brazing joint is full of pores and slag. The above two situations will reduce the strength of the brazing joint and make it easy to dissolder when the edge is ground or used.

(4) In the process of brazing not timely slag discharge or slag discharge is not sufficient, so that a large number of injection slag residue in the joint, reduce the strength of the joint, resulting in the welding.