| Hot dry rocks(HDR)are becoming the hot spot of current geothermal energy development due to its stable output,and low impact on the environment.At present,the common approach of developing HDR in the world is to use the enhanced geothermal system(EGS)to exchange heat in the target formation,so as to realize geothermal power generation.Drilling production wells and reinjection wells are important steps in developing EGS.Most of the HDR geothermal resources exist in gneiss or granite,whose drillability is poor and their uniaxial compressive strength can reach more than 200 MPa.At present,rotary drilling with roller-cone bit or PDC bit is used in drilling hard rocks with poor drillability.However,there are problems which can not be solved when using the above conventional drilling methods,such as slow drilling speed when drilling in HDR or other hard rock formations.Thus,during the development of EGS,drilling costs often accounted for more than 80% of the total cost,which severely restricted the development and utilization of HDR.For this purpose,Prof.Jianming Peng put forward High-Energy Down-the-hole(DTH)Fluidic Hammer drilling technology.Compared with the down-the-hole(DTH)air hammer,the mud drilling fluid used in the drilling process of the High-Energy Down-the-hole(DTH)Fluidic Hammer has the function of borehole wall protection.In addition,the drilling process will not produce dust,and a suitable mud pump can satisfy the working conditions.The High-Energy Down-the-hole(DTH)Fluidic Hammer has the characteristics that its performance does not change with the hole depth,and can maintain a high drilling speed for a long time in the HDR formation.Moreover,the High-Energy Down-the-hole(DTH)Fluidic Hammer has a simple structure and fewer moving parts.It does not require vulnerable parts such as springs to participate in the movement process.The High-Energy Down-the-hole(DTH)Fluidic Hammer has a rated working pressure of 15 MPa,and the speed of the impact system can reach 7m/s,which greatly improves the impact energy compared with conventional fluidic DTH hammer.Its driving medium has wide adaptability,including clear water and mud,which greatly reduces drilling costs while ensuring drilling efficiency,and is expected to improve the drilling efficiency in hard rock formations.Because of the high impact frequency and high end velocity of the High-Energy Down-the-hole(DTH)Fluidic Hammer,the conventional 35 Cr Mo piston rod has insufficient abrasive wear resistance,which limits the service life of the DTH hammer.This paper proposes to use WC-Co cemented carbide and related materials to manufacture the piston rod.Due to the hard and brittle characteristics of WC-Co cemented carbide,the WC-Co cemented carbide piston rod suffered fatigue fracture during the working process.This article discussed the following issues,including that the influence of assembly stress of the piston rod and the mass-block on the stress distribution of the piston rod after impact,the stress amplitude change trend after geometry optimization of the impact system,the wave cutting effect of the new type of tapered gaskets with different materials,the wear resistance and mechanical properties of thermal sprayed WC-Co cemented carbide under the working condition of the HighEnergy Down-the-hole(DTH)Fluidic Hammer.The research content and main conclusions of this article are listed as follows:(1)The reason of the fatigue fracture of the cemented carbide piston rod is the stress concentration at the connecting part of the piston rod after impact.The assembly stress generated during the assembly of the piston rod has little effect on the stress distribution of the piston rod after impact.Numerical simulation indicated that the assembly stress of the conical joint is in the state of compression stress on the piston rod surface.To some extent,the assembly stress of the conical joint played a protective role in the cemented carbide piston rod.(2)The length of taper fitting section of the piston rod,the taper of the piston rod and the change of the external chamfer of the mass block have certain influence on the subsequent stress distribution on the joint of the piston rod.The length of the taper fitting section has the most significant influence on the stress concentration.After optimization,the chamfering of the mass block is set to 60°,the taper is set to 1:20,and the matching length of the piston rod is set to 35 mm.Compared with the unoptimized structure,the maximum principal stress amplitude is reduced by 19.4% at the joint of the piston rod.(3)The taper mating joint made of tapered gaskets of different materials has a significant wave cutting effect on elastic waves.When the impact velocity ranges from3m/s to 5m/s,the three-layer gaskets combination joint can reduce the transmitted stress wave amplitude by 60%-62%,and the five-layer gasket combination joint can reduce the transmitted stress wave amplitude by 71%-77%.The cutting effect mainly depends on the difference in wave impedance between adjacent gaskets and the number of interface between gaskets.The contact percentage between adjacent gaskets also has a significant effect on the cutting effect of the new type of joint.In addition,the taper of the joint can also affect the cutting effect of the joint.(4)The use of HVOF to manufacture WC-Co cemented carbide sprayed piston rod is one of the effective methods to balance the wear resistance and toughness of piston rods.It has been proved that,the service life of the WC-17 co sprayed piston rod in the High-Energy Down-the-hole(DTH)Fluidic Hammer is obviously better than that of35 Cr Mo piston rod.The results show that the wear mechanism of sprayed WC Co cemented carbide coating has little change compared with other working conditions in previous papers,but its wear resistance is obviously lower than that under other working conditions.The wear resistance of the piston rod is reduced by the cracks caused by the working process of the High-Energy Down-the-hole(DTH)Fluidic Hammer.In addition,the coating porosity caused by spraying conditions also has a certain influence on the wear resistance of the coating.Therefore,on the basis of the existing spraying process,improving the spraying parameters and reducing the radial vibration of the impact system after impact the bit are effective methods to improve the service life of the thermal sprayed piston rod.(5)By simulating the working condition of the High-Energy Down-the-hole(DTH)Fluidic Hammer,the effect of heating and water-cooling cycle on the mechanical properties of the WC-Co thermal sprayed coating was studied.The results illustrated that the mechanical properties of the coating changed significantly under the condition of heating and water-cooling cycle.The microhardness and residual stress related to the wear resistance of the coating will be significantly reduced.The adhesive strength of the coating related to the coating integrity would also decrease significantly at high temperature.The main reason for the high temperature of the coating surface is radial vibration after the impact.After the design of the guide-cooling structure on the mass block,it is expected to reduce the radial vibration and reduce the surface temperature of the piston rod.The main innovations of this article are listed as follows:(1)The influence of assembly stress on the impact stress distribution in the piston rod was studied.On this basis,the influence trend of the change of the main geometric dimensions of the piston rod and mass-block on the stress concentration at the connection position was explored,and the structure of the piston rod and mass-block was optimized.(2)Based on the stress wave theory,the difference of wave impedance is proposed to weaken the elastic stress wave.According to the geometric structure of impact system,the new type of taper mating joint made of tapered gaskets of different materials was proposed.The geometric structure factors that affect the cutting effect of this new type of conical mating joint are studied.(3)The performance change of HVOF WC-Co thermal sprayed piston rod under the working condition of the High-Energy Down-the-hole(DTH)Fluidic Hammer was studied.The reasons for the change of wear resistance and mechanical properties of HVOF sprayed cemented carbide piston rod under the working condition of HighEnergy Down-the-hole(DTH)Fluidic Hammer were analyzed in detail.In order to extend the service life of the HVOF sprayed cemented carbide piston rod in the HighEnergy Down-the-hole(DTH)Fluidic Hammer,the guide-cooling structure in the impact system was put forward. |
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