Study On The Residual Stresses And Interface Optimization Of Polycrystalline Diamond Compacts For Oil Drilling

Polycrystalline diamond compacts(PDC)bits have gained wide commercial acceptance in oil and gas drilling due to their high rats of penetration, long life and mechanical simplicity. However, eighty percent of PDC failure, according to statistical data, is due to microchipping, gross fracturing and delamination between diamond-layer and the substrate.The thermal residual stress is the main cause leading to PDC failure.Supported by the Doctoral Fund'Study on rupturing and arresting of crack due to thermal residual stresses of polycrystalline diamond compacts for oil drilling'(No.20070533113),the author studied systematically on the residual stresses and interface optimization of polycrystalline diamond compacts for oil drilling.Theoretical and experimental method, as well as finite element method and in-site drilling test were used in the present investigation.The main research contents and results obtained in the dissertation are as follows:The recombination mechanism and sintering technology of PDC have been studied systematically, the relationship between composite mechanism, synthesis technology and interfacial strength is presented, which provides the theoretical basis for researching the residual stresses and interface composition of PDC.The distribution features of residual stresses in plane-interface PDC was studied by theoretical analysis and finite element method. The formulas for calculating the shear stress in complex material interface and the normal stress in polycrystalline diamond (PCD) layer were presented. The obtained results show that the difference of thermal expansion coefficients between PCD and cemented carbide is the main reason for the initiation of residual stresses in PDC.The relationship between the thickness of PCD layer, residual stresses and deformation of PDC is proposed.The property and composition of interface of PDC complex material have been systematically studied.The theory about interface reaction controlling is described. The residual stresses in PDC with different interface structures were calculated using finite element method. The relationship between residual stresses and interface structure is obtained, based on which a new PDC with an optimized interface is proposed.The residual stresses in flat-interface PDC have been measured in laboratories using improved stress-release method, X-Ray diffraction and Raman Spectroscopy method, respectively. The values of residual stresses and it's distribution on the surface of PCD table were obtained.The tested results were coincident well with that obtained by the numerical method. The shortage existing in traditional experimental method in which only finite points on the PCD table surface can be tested for one specimen has been overcome by the improved method used in the present test. These methods provide an experimental basis for PDC optimum design.The shortcoming of the present method in measuring PDC wear resistance is analyzed. Considering that the weighing of PDC is hard to guarantee the accurate of testing results, the author suggests to measure the length of PDC abrasion facet by microscope and then calculate the abrasion volume instead of weighing it. A formula for calculating the volume was derived. The veracity and reliability of the method have been proved by the testing results.The influence of geometrical shape of PDC on C-ratio testing was also studied. The obtained results show that the test values of PDC wear resistance increase with the increasing of geometric angles.Conventional mechanical properties of PCD, such as Young's modulus and Poisson's ratio,were tested using Instronl342 testing machine.The obtained results show that it's feasible to measure the Young's modulus and Poisson's ratio of PCD by simple compressive loading test.Comparing to ultrasonic transfer velocity method and free vibration frequency method, the method used in present investigation is quite simple, reliable and cost-effective.Istron1342 testing machine and Hopkinson pressure bar(SHPB)were also used to obtain the strain rate varying from 10-6/s to 102/s,and the edgewise compressive strength of PDC in different loading speeds were successfully obtained.The relationship between loading speed and edgewise compressive strength of PDC is presented. The compressive strength under this loading condition for intact and damaged PDC were also obtained. The relationship among damage, loading speed and thickness of PCD layer is presented. This research provides the theoretical and experimental basis for reusing the damaged PDC.A new technique which combines non-planar junction with gradient transition is proposed, with which an inner gradient layer is designed in PDC.It can decrease the greatly problem on the residual stresses in flat-interface PDC.It's excellent service performance was proved by the results of numerical analysis, experimental investigation and in-site drilling test. This new design has been successfully applied for a patent.Rock drilling feature and mechanism of PDC bit in service were studied.A numerical mode for calculating the PDC strength when cutting rock was then presented, which provides the theoretical basis for choosing the working parameters of PDC cutter. On this basis a new PDC cutter was designed and used in the Daqing oil well drilling. The obtained results show the excellent service performance of the new PDC.In short, the research work in this dissertation is based on the frontal of the subject, which is combined closely with engineering practice.The theoretical analysis and methods of mathematical,mechanical,numerical simulation and advanced experimental means,were used in studying on the residual stresses and interface optimization of PDC.It founded a theoretical and technical basis for studying thermal residual stress fracturing and preventing fracture.This research is helpful for reducing effectively the residual stresses, optimizing the composition of the interface and improving the eligible rate in manufacturing.It is of important theoretical significance and engineering application prospect.