Rock drilling tools are widely used in mining, building construction and transportation facilities construction, and other fields. In the process of use, rock drill bits endure complex impact and torsion forces, the working conditions are harsh, and the service life is short. Since rock drill bits are made of carbide with high hardness and good wear resistance welded to the steel, and the broken and fallen-off carbides are the main forms of rock drill bit failure, the welding quality directly affects the service life of rock drilling tools. In addition to external factors, the residual stress in the drill bit and the lack of welding and low brazing strength are the internal reasons for the fracture and shedding of the cemented carbide. Therefore, the manufacturer can effectively prolong the service life of rock drill bit by formulating improvement measures based on the understanding of the quality problems of rock drill bit.

Structure and material of rock drill bit

Rock drill bit is mainly composed of a drill bit steel body and cemented carbide and is brazed with a brazing filler metal. Cemented carbide is WC-Co8, namely YG8C cemented carbide. The common material for the steel body of rock drill bit is 40Cr, which is extruded at low temperatures.

Quality problems of rock drill bit

The fracture and fall-off of cemented carbides are the main form of drill bit failure, which is related to welding residual stress in the drill bit, lack of welding, low brazing strength, and other reasons. There are sometimes stresses in the weld or cemented carbide after welding, and the existence of these stresses has a great impact on the quality after welding. When there are occasional defects at the sharp corners of the cemented carbide or in the inner layer near the surface, it will also cause local thermal stress concentration, and the thermal stress at this time will be magnified by 2 to 3 times. In use, it will often cause the carbides to break, so it should be paid attention to.

1. Low brazing strength

The brazing strength of rock drill bit is an important criterion to measure the quality of welding, and the factors affecting it are mainly the weld gap. During the welding process, if the gap between the weld seam is too small, carbides and steel body in some places will even be in close contact, so that the solder cannot spread to the entire welding surface after melting, reducing the strength of the joint; when the gap is too large, the support of the base body to the weld after welding is weakened, as well as the large gap in the solder is columnar casting organization, coarse grain, loose organization, strength, and toughness is poor, resulting in a decrease in the strength of the joint.

2. The cause of residual stress

2.1 Residual stress caused by difference in thermal expansion coefficient

The thermal expansion coefficient of cemented carbide and steel bodies is very different. In the brazing temperature range, the coefficient of thermal expansion of carbide is about half that of the steel body. The difference in shrinkage during post-weld cooling will generate enormous stresses in the weld and carbide and steel body on both sides of the weld. When this welding stress is serious, it can crack the cemented carbide and the weld, causing rock drill bit to fall off and break when it is working.

2.2 Residual stress caused by uneven temperature

During the brazing process, because of the phenomenon of thermal expansion and cold contraction, the temperature difference between the various parts of the solid material will cause thermal stress inside the material, the size of this thermal stress is related to the material’s elastic modulus thermal expansion coefficient material temperature difference. As the temperature difference increases, the thermal stress also increases, because the carbide has good mechanical properties, the temperature difference is large, and it is not enough to cause cracks in the carbide. However, during welding, the carbide will be integrated with the drill bit through the solder, and the thermal stress of the shell is also added to the carbide blade. In this case, the thermal stress of carbide blade is twice that of the unwelded state. In addition, if the local heating of drill bit is too fast or too high during welding, or the local cooling is too fast, it is possible to make the carbide have too much thermal stress locally.

Measures to reduce stress

1. Control the heating and cooling speed to avoid excessive temperature differences. It is very important to control the cooling speed after welding during brazing.

2. Take necessary measures when heating and cooling to prevent local overheating or rapid cooling.

3. Strive for uniform heating, shorten the solder liquefaction process as much as possible, avoid volatilization of low melting point, produce weak tissue and slag inclusion in pores, and reduce oxidation of the brazing surface.

4. Appropriately increasing the weld gap can reduce the stress added to the cemented carbide.