Cemented Carbide Blades Wear Characteristics In Drilling Limestone Ⅳ

3. Results and discussion

3.1. Effect of water jet pressure on the wear rates of the cemented carbide blades

Fig. 4 shows the wear rates of cemented carbide blades under different water jet pressure (the used diameter of the water jet nozzle is 1.2 mm). It is shown that the wear rate is quite high (nearly 10%) without the help of water jet, but the wear rates decrease sharply when the water jet joins in. The wear rates decrease when the jet pressure increases. Nevertheless, the wear rate decrease slowly when the jet pressure is over 10 MPa.

The wear rates are affected by the mechanical stress and temperature of the blades, and the water jet is helpful to reduce the mechanical stress and temperature. The more impact stress the water

jet gives to the rock, the less mechanical stress is on the blades. The maximum impact stress of the water jet on the rock (Pi) is given by the Eq. Pi=v*ρ1*c1*ρ2*c2/(ρ1*c1+ρ2c2)

where v is the jet velocity of the liquid and ρ1, ρ2 and c1, c2 are the densities and the shock velocities in the liquid and the solid. According to the equation, the impact stress will increase with the jet velocity. The relationship between jet pressure and velocity could be estimated by the expression, where P is jet pressure. So the impact stress will increase with the jet pressure: bigger jet pressure means lower mechanical stress on the blade.

Higher jet pressure could also increase the thermal exchange efficiency to reduce the working temperature. Heat transfer takes place when the water jet flows through the surface of blade, with a cooling effect. This cooling process can approximately be regarded as the process of convective heat transfer outside flat plate. The surface heat transfer coefficient of the blade will be, where h is the heat transfer coefficient of the surface, λ is the coefficient of thermal conductivity of water, l is the flow length on the blade, ξ is the kinematic viscosity of water, μ is the dynamic viscosity of water, Cp is the specific heat capacity of water. The power of heat exchanging is given by Eq. where Φ is the power of heat exchanging, TH is the temperature of blade, TL is the temperature of water, S is the area of flow on the blade.

In a word, the cooling effect is proportional to the square root of jet speed v, and according to Eq. (3) , it is also proportional to the forth root of jet pressure P.

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