Rise in the efficiency of the use of cemented carbides as a matrix of diamond-containing studs of rock destruction tool

Abstract

To improve strength and plasticity of a matrix of diamond-containing studs for rock destruction tools and thereby to increase their efficiency, we offer to replace the VK6 and VK10 carbides currently used as a matrix with the VK6S and VK10S alloys based on a high-temperature tungsten carbide. These alloys feature an increased work of plastic deformation and ultimate strain. For this reason, they ensure, by the Morrow equation, a higher service life of the carbide matrix and add to the diamond grit retention in it. The efficiency of diamond-containing studs based on VK6S and VK10S alloys is further increased if they are strengthened by heat treatment.

Introduction

Recent years have seen a wide application of drilling bits set with diamond-impregnated WC–Co studs for mining minerals, particularly for drilling for oil and gas. Diamond concentration does not exceed 38 vol%. The principle of the work of diamond-impregnated tools lies in ensuring its self-sharpening, i.e., proportional wear of diamond grits and a bond (a matrix) for the continual exposure of fresh cutting grits. Thus, the performance characteristics of the tools depend on the proper choice of the matrix properties and diamond grit strength. The matrix should not only be strong to retain diamond grits but also highly resistant to the abrasive action of rock to provide a given value of the diamond grit exposure.

Based on the foregoing, diamond-containing studs for rock destruction tools are made of VK6 and VK10 cemented carbides used as a matrix. Apart from tungsten carbide, the above cemented carbides contain 6 and 10 wt% cobalt, respectively. In some cases, when drilling horizontal wells, mechanical strength (durability) of these cemented carbides appears to be inadequate and diamond-containing studs suffer from premature failure when drilling horizontal wells. Mechanical characteristics of the stud matrix can be improved by increasing the binding phase content of the cemented carbide. This, however, will cause the wear resistance of studs to decrease substantially. For this reason, we propose to replace the traditional VK6 and VK10 cemented carbides which are used as a matrix in diamond-containing rock destruction studs with VK6S and VK10S grades based on high-temperature tungsten carbide. In the manufacture of VK6 and VK10 cemented carbides, WC is used which is produced by the reduction of oxides at temperatures up to 900°C followed by carbidization at temperatures up to 1600°C, while in the case of the manufacture of VK6S and VK10S cemented carbides, WC is produced by the reduction of oxides at 1200°C and subsequent carbidization at 2200°C (high-temperature carbide).

It is known [1], [2], [3] that cemented carbides based on a high-temperature tungsten carbide possess higher deformation characteristics and fatigue life under dynamic loading as compared with traditional cemented carbides. Specifically, the fatigue life of cold-heading dies made up of alloys based on a high-temperature tungsten carbide is 3–5 times that of dies made of alloys based on a low-temperature carbide.

A factor which is responsible for a higher durability of cemented carbides based on high-temperature tungsten carbide is a shorter length of WC–WC boundaries in these cemented carbides [4]. A reduction in contiguity and connectivity of carbide grains, the volume content of the binding phase (V_VCo) and the mean size of WC grains ($\text{d}{}_{\mathrm{<mtext>WC</mtext>}}$) being equal contributes to the improvement of strength characteristics and plasticity of cemented carbides [5] which are responsible for a pronounced increase in the carbide product fatigue life under cyclic loading. It was shown in [2], [5] that contiguity and connectivity of carbide grains in sintered carbides can be changed by heat treatment. For this reason, we have studied the possibility of improving mechanical characteristics of a cemented carbide matrix of diamond-containing studs by using heat-treated alloys based on high-temperature tungsten carbide. As diamond-containing studs for a number of rock destruction tools are secured in a steel body by welding, diamond-free specimens were heat-treated in two different modes. The first mode: heating up to a given temperature followed by oil quenching. This procedure is usually used to modify properties of cemented carbides [6]. The second mode: heating up to a given temperature, holding for some minutes at this temperature and air cooling. This procedure simulates the conditions of welding diamond-containing studs in a tool body.