| Although bit cost accounts for a small proportion of total drilling cost, bit quality has significant effects on drilling efficiency and borehole quality. Therefore, bit plays a very much important role in drilling, especially in deep drilling and in hard, compact rock drilling. Since its invention, the manufacturing technique of electroplated diamond bit has been improved greatly. As one of the manufacturing methods, electroplating method occupies an important position in the production of impregnated diamond bit. Up to now, research work on electroplated diamond bit has mainly centered on nickel based matrix, emphasizing on the improvement of plating bath, the optimization of plating parameters and the imposition of physical field (magnetic field, ultrasonic field, etc) to obtain matrix material with different behaviors to meet the demands of drilling in different formations. However, limited by the variability of the behavior of nickel based matrix material, electroplated nickel-based diamond bit has comparatively less applications for its lower drilling efficiency. It is very necessary to develop a kind of electroplated diamond bit with a new matrix material, in order to widen the application range of electroplated diamond bit.Due to its high electrodepositing speed, desirable hardness and wear resistance, as well as its low cost and less pollution, iron deposit is used for numerous applications, such as restoration of worn mechanical part, electroforming mold and iron foil. In recent years, its application has expanded to magnetic deposit, nanometer deposit and functional composite deposit. However, some significant problems still exists with present iron plating, such as low stability of iron plating bath, high internal stress of deposit, difficult preparation of particulate reinforced composite deposit. In consideration of the wide application of electroplated diamond bit and our country’s abundant iron resource, this dissertation aims at the shortages of common iron plating at the aspects of bath composition and plating technique, investigates the bath composition for electroplated iron based diamond bit, electrodeposition mechanism of iron and manufacturing technique of iron based bit, studies the relationship between the hardness, wear resistance, internal stress of deposit and bath composition, plating parameters, to understand the adaptability of iron based diamond bit to hard, compact formation. Through this work, the improvement of comprehensive effects of drilling with electroplated diamond bit in proper formations and the reduction of bit production cost are expected.Considering the inevitable fact that in plating solution Fe2+will be oxidized to Fe3+through contact with air and the practice of operating procedures of electroplated diamond bit, this dissertation chooses this route to increase the stability of iron plating solution:adding complex agent to selectively complex Fe3+oxidizing from Fe2+to decrease its concentration in plating solution, preventing its negative effect on iron deposit. By titration tests, observation of solution’s color and experimental electrodepostion, F-was thought to be a proper stabilizing agent to iron plating solution. On the basis of analysis of the reason that diamond grains is hard to co-deposit, orthogonal tests were planned and carried out to solve the problem of co-deposition. The optimal co-deposition condition is bath pH3.0, cathodic current density1.0A/dm2, sodium dodecyl sulfate (SDS)0.15g/L, ammonium chloride5.0g/L. The smooth realization of diamond co-deposition removed a key obstacle to the manufacture of electroplated diamond bit. Cathodic current efficiency of iron electrodeposition was tested with copper Colombo counter. Throwing power of iron bath was measured by Haring-Blum method. The experiment results showed that: the addition of NH4HF2could increase current efficiency and throwing power; with increase in current density, cathodic current efficiency increased, while throwing power decreased to some extent. With increase in bath pH value, there was obvious increase in current efficiency; the addition of MnCl2has significant effect on throwing power, however, the difference between the throwing power of iron plating solutions containing different concentration of MnCl2was small; within experimental range, the addition of sodium dodecyl sulphate has basically no effect on current efficiency and throwing power.The cathodic polarization curves of iron electrodeposition were measured by linear potential sweep voltammetry. Nucleation process of iron electrocyrstallization was studied by potential step chronoamperometry and some kinetic parameters of electrocrystallization were calculated. The results of linear potential sweep voltammetry showed that:NH4HF2had the effect of depolarization, while MnCl2and SDS enlarged polarization; with the increase in bath pH, the deposition potential shifted positively. The results of potential step chronoamperometry and related calculation showed that:in plating solutions containing different concentration of NH4HF2, MnCl2or SDS, the electrocrystallization of iron on glassy carbon electrode all underwent nucleation process and followed three-dimension instantaneous nucleation mechanism; when the potential step increased negatively, the current peak of iron electrodeposition increased and the time corresponding to current peak became shorter gradually. With increase in the concentration of NH4HF2, the vertical growth rate of nuclei rose while the diffusion coefficient of iron ions decreased obviously (up to17%), compared to that of basic bath. The addition of magnesium chloride also enhanced the vertical growth rate of nuclei visibly and increased the diffusion coefficient of iron ions by up to74%. The addition of SDS lowered the vertical growth rate of nuclei, especially at low potential; at the same time, the diffusion coefficient of iron ions decreased slightly. The results of electrochemistry alternating current impedance fit well with those results of above-mentioned two kinds of experiment methods and provided strong evidences for related arguments.The surface topography of iron deposits was observed with scanning electron microscope and confocal laser scanning microscope. From the observation of iron deposit, it can be seen that overall, the deposit obtained with direct current from common iron plating solution was smooth and the deposit was formed by hemispheric particles; however, cracks resulted from large tensile stress and pits from hydrogen bubble’s staying distributed over the deposits. After adding wetting agent and stress-reducing agent, the surface structure of iron deposit turned from tiny hemispheric particles to comparatively big pyramid particles and no crack or pit was observed. The change of surface topography of iron deposit indicated that the internal stress of deposit was reduced effectively and the brittleness of deposit decreased. Observation of diamond/iron composite deposit showed that the deposit among diamond grains was smooth and adjoined diamond grains closely.The results of metallurgical analysis, hardness test and SEM EDS showed that deposit obtained from common iron plating solution was a single phase pure iron and the black stripes were micro cracks. The result of XRD also verified that the deposit only contained iron; with increase in bath pH, the grain size of iron deposit decreased from13～16μm. Moreover, with SEM EDS line scan method, relatively high content of carbon element was detected in pits. Carbon element probably came from substrate,45#steel, which indicated that these pits were directly on the substrate, that is, from the very beginning of plating, there was no deposition at the place of the pits.The hardness of deposit was measured with microhardness tester. The wear resistance of deposit was measured with MG-2000friction&wear tester. Wear resistance was weighed by the mass loss of sample in wear test. Wear scar was observed with confocal laser scanning microscopy. Within experimental range, the addition of MnCl2had no effect on deposit hardness, but influenced wear resistance markedly. With increase in the concentration of MnCl2, wear loss decreased obviously. The hardness of deposit from solution containing FeCl2and FeSO4was higher than that of deposit from solution containing only FeCl2or FeSO4. Cathodic current density had limited effect on microhardness, causing hardness difference of about50kg/mm2. Increasing current density, the wear loss of deposit decreased. After Heat treatment at150℃, the hardness difference between deposits obtained at different current density diminished, which indicated that the hardness difference of deposit at different current density was closely related to hydrogen permeation. Bath pH affected deposit hardness greatly. Increasing bath pH from2.3to3.2, deposit hardness enhanced by160kg/mm2and the wear resistance of deposit rose clearly, too. Within experimental range, NH4C1had little effect on both microhardness and wear resistance of iron deposit.The internal stress of iron deposit was measured by flexible cathode method. The experiment results showed that the internal stress of iron deposit was tensile and with increase in MnCl2concentration, pH value, or current density, the internal stress of deposit all decreased. The effect of these factors on internal stress can be explained by Panganov’s thermodynamic theory of internal stress, that is, these factors can accelerate the vertical growth rate of nuclei and then reduce the excessive surface energy of nuclei, lowering internal tensile stress in iron deposit.Considering the results of overall experiments on iron plating solution performance, diamond co-deposition and deposit behavior, the main bath composition and plating parameters for manufacturing of electroplated iron-based diamond bit are recommended as follows:bath pH value2.9～3.2; current density for diamond co-depositing0.9-1.OA/dm2, for diamond impregnating1.6～3.OA/dm2; main salt, mixture of FeCl2and FeSO4; concentration of MnCl2, no less than12g/L; concentration of sodium dodecyl sulfate,0.10-0.15g/L; concentration of NH4HF2,10-15g/L.Iron deposit electrodeposited under direct current had the characteristics of high hardness and low wear resistance. Therefore, it is believed that electroplated diamond bit has comparative advantage in drilling hard and compact formation, besides cost advantage. The results of indoor drilling test showed that compared to common iron plating formula, the drilling efficiency of diamond bit manufactured with the formula and technique optimized in this dissertation was higher (2.3m/h); in drilling, the iron based matrix displayed plasticity to some extent, embedded diamond grains well, ensuring higher diamond protrusion; the adhesive strength of deposit to substrate met the demand of drilling. The results of primary field drilling test showed that compared to nickel based diamond bits used in the same field, iron based diamond bit had higher drilling efficiency (increased by34%) and longer life (increased by16%).In a word, through experimental investigation and theoretical analysis, this dissertation determined the solution formula and technique for electroplating iron based diamond bit under direct current, successfully solved some key problems, such as bad stability of plating solution, high internal stress of deposit, difficult co-deposition of diamond, and revealed the electrodeposition mechanism of iron to certain degree. However, it is still necessary to further study the control of internal stress of deposit and the match of deposit behavior, diamond parameters and drilling program. |
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