Problems and Solutions of YG8 Tungsten Carbide Drill Bits

The main failure forms of YG8 tungsten carbide drill bits includes the broken and shedding of tungsten carbide insert sheet. It is for these reasons that the residual stress, lack of welding and low strength of brazing after the welding. Welding strength is the most direct criterion for testing the quality of welding quality and the main influencing factor is the welding gap. During the welding process, if the welding gap is too small, the parts of tungsten carbide insert sheet will contact with the steel closely, so that the molten solder after soldering can not spread to the entire surface to reduce the joint strength; if the welding gap is too large, after welding the supporting role of the matrix is weakened, solder large gap casting columnar structure, grain is relatively coarse, loose tissue, strength and toughness relative decline, a direct result of the strength of the joint is reduced.

The reasons of residual stress appearing include the difference of the coefficient of thermal expansion and uneven temperature distribution. There is a great difference in the coefficient of thermal expansion between tungsten carbide and steel (the linear expansion coefficient of WC-Co is 5-7×10^-9, the linear expansion coefficient of steel is 12.6×10^-9), so it will produce great internal stress by the large difference of shrinkage after cooling, which is also the main causes of tungsten carbide and welding gap cracking. In addition, the brazing process accompanied by thermal expansion and contraction phenomenon and thermal stresses within the material, with the increasing temperature difference, thermal stress is increasing, tungsten carbide insert sheet will appear cracks easily. There are some solutions to reduce the stress as followed:
1. Control heating and cooling rates to avoid excessive temperature difference, especially the cooling rate after welding;
2. Take the necessary measures to prevent partial hot or cold phenomenon;
3. Uniform heating, minimize solder liquefaction process, to avoid the volatile low-melting phase, resulting in weak tissue and pores and also reduce the phenomenon of the brazing surface oxidation;
4. To enlarge the welding gap appropriately to reduce the additional stress on the cemented carbide piece.

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