Research On Functionally Graded Polycrystalline Diamond-Cemented Tungsten Carbide Composite Button Insert

Heavy duty rock bits, such as tri-cone rock bit, down-hole-hammer rock bit and large diameter roller rock bit are extensively employed for drilling wells in subterranean formation for oil, gas, water, geothermal steam and for other geological or engineering purposes. Traditionally, cemented tungsten carbide button inserts are employed for cutting elements. As such bits are used in very hard, tough and abrasive formations, the total useful life of the bits will be unsatisfactorily short due to the relatively lower abrasive resistance of cemented tungsten carbide. As a result, low drilling efficiency and high drilling cost become the inevitable. Polycrystalline diamond compacts (PDCs) are widely used as drill bit cutters in rock drilling and as tools in machining non-ferrous materials due to their very high abrasive resistance. A typical PDC comprises a thin layer of sintered polycrystalline diamond (PCD) bonded to a tungsten carbide-cobalt substrate. A well recognized failure mechanism of PDC is delamination at the interface between diamond and cemented carbide. High stresses at the diamond/carbide interface, due to thermal expansion coefficient and elastic modulus mismatch, are the primary cause of in-service failure under impact loading conditions. As a result, PDC now can only be used in the bits which drilling rock by shear and cutting mechanism and can not be used for percussive drilling. The research work in this dissertation is based on a scientific project entitled diamond-cemented tungsten carbide composite button insert and bit (serial number 9505402-4 ) supported by Ministry of Land and Resource, China. On the basis of tutor's right guidance and the predecessor's research work, the author conducts an investigation into the research on functionally graded polycrystalline diamond-cemented tungsten carbide composite button insert and the following work has been completed in this dissertation. The rock fragmentation mechanism of percussive button bit and the main failure type and wear mechanism of the traditional cemented tungsten carbide insert are analyzed. Considering the inconsistency of hardness and strength of cemented tungsten carbide material, new rock drilling material superior to cemented tungsten carbide should be developed. The development history, synthesis route, key technology, fabrication method and comprehensive properties of PDC material are reviewed. They give a reference to the development of polycrystalline diamond-cemented tungsten carbide composite button insert The residual thermal stresses in PCD-cemented tungsten carbide bi-material are analyzed. The expressions of shear stress at the interface and the normal stress in the PCD layer are derived. The crucial factor affecting the residual stress is the difference of the coefficient of thermal expansion (CTE) between PCD and cemented tungsten carbide. Only when the maximal value of the difference is less than 1.5×10-6K-1, the cracking in the PCD layer and the delamination between PCD layer and cemented tungsten carbide substrate could not appear. However, it is difficult for PCD-cemented tungsten carbide bi-material to achieve that maximal value. The concept and design method of functionally graded material (FGM) is applied for the design of PCD-cemented carbide button insert. The model for designing functionally graded PCD-cemented carbide button insert with graded composition distribution and microstructure is put forward. The constituents and their maximum and minimum volume fraction in the graded layer are determined. The appropriatemethod to estimate thermo-physical and mechanical properties of graded layers is selected. The residual thermal stresses within the FGM button insert induced by cooling the insert from the sintering temperature to room temperature are analyzed by using the finite element method (FEM). The results indicate that with the increase of the number of graded layers, the residual compressive stress in the PCD layer, the shear stress and Von Mises stress at the interface are all increased, while the residual tensile stress in the substrate is decreased. On the basis of comprehensivelly considering of the FEM analysis results and the aim of the button insert designing, the appropriate numerber of graded layers 3~5 is determined, which settled the foundation for the FGM button insert design and fabrication. By analyzing the various fabrication method of PDC and the percussive drilling demand for the properties of button insert, the hybrid sintering method with adding small amount of binder/catalyst is selected for the method to fabricating the FGM button insert. The optimum diamond powder size distribution is calculated. Cobalt (Co), tungsten carbide (WC) and boron (B) are selected for the binder/catalyst of PCD layer. The decontamination processes of diamond powder and cemented tungsten carbide substrate are determined. The tungsten carbide substrate with step structure is designed to effectively support the PCD layer. The pyrophyllite pressure medium, steel ring, graphite heater and refractory metal enclosure are correctly selected. The appropriate high pressure reaction cell for sintering FGM button insert and the manufacturing flow chart are designed. Based on the analysis of ultra high pressure-high temperature sintering condition of PDC material, the rational sintering parameters are determined and the sintering experiments are conducted using a cubic press. In order to ensure the formation of very abrasive resistant polycrystalline diamond, the sintering must be conducted in the area of carbon phase diagraph where diamond is thermodynamically stable. Generally, the pressure ranges from 5.5 GPa to 7.5 GPa, the temperature ranges from 1450℃to 1650℃and the keeping time ofpressure and temperature ranges from 10 to 20 minutes. The mechanical properties of the FGM button insert are tested and the microstructure is analyzed. The abrasive resistance of the FGM button insert is about 1100 times as large as conventional WC-15Co hardmetal. The drop hammer impact test to single button insert shows that the maximum impact energy it can bear is 110 Joule. In addition, the damage of the FGM button insert during impact testing is relatively minor and the representative damage style is microchipping rather than delamination, which implies that there is a strong bond between PCD layer and cemented carbide substrate. The thermal stability of FGM button insert is analyzed by the DTA and TGA, and the result indicates that the thermally stable temperature of FGM button insert is about 700℃. The optical micrograph of the longitudinal section of FGM button insert shows that the graded layers have been realized in the insert. The SEM photograph of the PCD layer indicates that there exists diamond-diamond bond. XRD pattern of PCD layer shows that there exists diamond phase, Co phase, WC phase and W2C phase. No graphite phase has been found. That has proved the successful sintering of the insert. The down-hole-hammer bit with diameter of 70mm is designed. The geometry parameters and the steel grade of bit blank are selected. The size, number and arrange style of button insert is determined. Rock drilling tests of the down-hole-hammer bit are performed. New FGM button inserts are placed on the gauge of the bit, that is, the row of inserts that drills adjacent to the wall of the hole. The rock being drilled is very hard and abrasive and has a drillability of about Ⅶ~Ⅷ. Such bit drills satisfactorily a depth of 1.05 m until one of the inserts drops out. The result of the drilling tests indicates that the FGM button insert has good impact resistance and can meet the demand for the percussive drilling. But it is a pity that the total depth drilled is not enough to compare the useful life between the diamond enhanced insert bit and conventional bit.The innovations of the dissertation are as following. Firstly, the residual thermal stresses in PCD-cemented tungsten carbide bi-material are analyzed. The expressions of shear stress at the interface and the normal stress in the PCD layer are derived. Only when the maximal value of the difference of thermal expansion coefficient between PCD and cemented tungsten carbide is less than 1.5×10-6K-1, the cracking in the PCD layer and the delamination between PCD layer and cemented tungsten carbide substrate could not appear. Secondly, the functionally graded PCD-cemented tungsten carbide button insert is designed. The model for designing FGM PCD-cemented carbide button insert with graded composition distribution and microstructure is put forward. The constituents and their maximum and minimum volume fraction in the graded layer are determined. The appropriate method to estimate thermo-physical and mechanical properties of graded layers is selected. The residual thermal stresses within the FGM button insert induced by cooling the insert from the sintering temperature to room temperature are analyzed by using the finite element method (FEM). The results indicate that with the increase of the number of graded layers, the residual compressive stress in the PCD layer, the shear stress and Von Mises stress at the interface are all increased, while the residual tensile stress in the substrate is decreased. On the basis of comprehensivelly considering of the FEM analysis results and the aim of the button insert designing, the appropriate numerber of graded layers 3~5 is determined, which settled the foundation for the FGM button insert design and fabrication. Thirdly, the tungsten carbide substrate with step structure is designed to effectively support the PCD layer and improve the impact strength of the button insert. Fourthly, the functionally graded PCD-cemented tungsten carbide button inserts with good performance are successfully fabricated. The inserts are mounted on a Φ70mm down-hole-hammer bit and the drilling test on very hard and abrasive rock with the bit is performed. The test result indicates that the FGM button insert has good impact and abrasive resistance and can meet the demand for the percussive drilling.