Failure Mechanism And Performance Strengthening Of Cemented Carbide Rotor For Mud Pulser

Mud pulser is the core part of measuring while drilling instrument,which is mainly composed of stator,rotor and cover.Rotor is generally made of cemented carbide material,which will suffer erosion wear under the action of high speed of mud containing solid particles,thus reducing the quality of downhole signal transmission.There are many kinds of cemented carbides.In order to select suitable cemented carbides to make rotors,three cemented carbides with different Co contents(WC-5%wt Co?WC-6%wt Co?WC-10%wt Co)were tested for basic mechanical properties,erosion wear and electrochemical corrosion to compare their performance differences.The results showed that the density and vickers hardness of cemented carbides decrease with the increase of Co content,while the bending strength and fracture toughness increase with the increase of Co content.Among the three cemented carbides,the density and vickers hardness of WC-5%wt Co are the largest,which are 14.95g/cm³ and 1899.36N/mm²,respectively.The maximum bending strength and fracture toughness of WC-10%wt Co are2573.58MPa and 18.309MPa·m^1/2,respectively.The erosion rates of the three cemented carbides increased with the increase of erosion angle.When the erosion angle was 90°,the maximum average erosion rates were 2.39%,2.42%and 3.22%,respectively.The erosion rates of the three cemented carbides increase first and then decrease with the increase of abrasive particle size.When the abrasive particle size was 100-200?m,the maximum average erosion rates were 1.84%,1.98%and 2.48%,respectively.The results of electrochemical corrosion test show that the three cemented carbides have the worst corrosion resistance in the solution of p H=3,and the best corrosion resistance in the solution of p H=11.In the solution of the same p H,WC-5wt%Co has the strongest corrosion resistance.The erosion wear mechanism of cemented carbide was that Co was removed due to plastic deformation caused by furrow and cutting,and WC particles were washed off by abrasive due to the loss of Co bonding.The electrochemical corrosion mechanism of cemented carbide was Co oxidation dissolution,Co(OH)₂ was formed by coordination between Co²⁺and hydroxyl group in H₂O to cover the surface of cemented carbide and inhibit further corrosion of the cemented carbide.However,Co(OH)₂ was not produced in the acidic solution.The basic mechanical properties,erosion wear properties and electrochemical corrosion properties of three cemented carbides with different Co contents were comprehensively compared,and WC-5%wt Co was selected as the cemented carbide material for rotor.Polycrystalline diamond composite(PDC)has high hardness and wear resistance.In order to evaluate the feasibility of PDC as rotor reinforcement material,the basic mechanical properties,erosion wear and electrochemical corrosion of PDC with four different diamond particle sizes(PDC4,PDC10,PDC2&30,PDC20?30)were tested.The results show that the vickers hardness of PDC decreases with the increase of diamond particle size,and the fracture toughness increases with the increase of diamond particle size.PDC4 has the highest vickers hardness of 8254.96N/mm².The fracture toughness of PDC20?30 was 8.197MPa·m^1/2.The influence degree of three factors on erosion rate of PDC20-30 was erosion angle more than abrasive particle size more more than abrasive displacement.When the abrasive particle size was 100-200?m and the erosion angle was90 and the abrasive displacement was 240 g/min,the erosion rate of PDC20?30 was the largest.The erosion rate of PDC20?30 increases first and then decreases with the increase of abrasive particle size.When the abrasive particle size was 100-200?m,the erosion rate of PDC20?30 was 1.158%.The erosion rate of PDC20?30 increases with the increase of erosion angle.When the erosion angle was 90°,the maximum erosion rate was 1.538%.The erosion rate of PDC increases with the increase of diamond particle size.PDC4 has the lowest erosion rate of 1.24%.PDC with different diamond particle size has the worst corrosion resistance in acidic solution and the best corrosion resistance in alkaline solution.The erosion wear mechanism of PDC was Co plastic deformation caused by furrow,and also accompanied by micro-cutting and micro-fracture at the edge of diamond particles.The repeated collision of particles caused transverse and radial cracks to form on the surface and sub-surface of PDC,and the cracks expand along grain boundaries and interlace with each other,leading to flake spalling of PDC polycrystalline layer.The basic mechanical properties,erosion wear properties and electrochemical corrosion properties of PDC with four different diamond particle sizes were comprehensively compared,and PDC4 was selected as the reinforcing material for rotor.In order to determine the appropriate position for PDC reinforcement on cemented carbide rotor,the hydrodynamic numerical simulation analysis was carried out on the rotor.The results show that the maximum compressive stress on the rotor surface was mainly concentrated in the middle of the rotor blade.With the increase of axial clearance between stator rotors,the compressive stress on the rotor surface decreases gradually.When the axial clearance was 1mm,the maximum average compressive stress on the rotor surface was 6.5×10⁶Pa,When the axial clearance was 2.5mm and the average compressive stress was 4.5×10⁶Pa.the fluid velocity of the rotor surface was mainly concentrated at the bottom of the rotor passage,and the erosion wear was the most serious in this area.According to the fluid dynamics analysis results of the rotor,PDC reinforcement was determined at the bottom of the rotor passage and its vicinity.In order to reasonably select the fixed connection mode of PDC reinforcing material on cemented carbide rotor,the welding and bonding experiments of cemented carbide and PDC samples were carried out.The results show that the shear strength of the welded sample was 34.5 times that of the bonded sample,indicating that welding was stronger than bonding.There was little difference between the erosion resistance and electrochemical corrosion resistance of the welded joint.According to the above research results,the welding method was selected to conduct PDC reinforcement on the cemented carbide rotor.Cemented carbide rotors and PDC reinforced cemented carbide rotors were designed and fabricated,and the experimental study of liquid-solid two-phase flow erosion wear was carried out.The results show that the weight loss rate of the former is 3.27 times that of the latter,indicating that the PDC reinforced cemented carbide rotor was more resistant to erosion.During the experiment,PDC did not fall off from the cemented carbide rotor body,indicating that the welding process was reliable.