An experimental and multiscale numerical study of mechanical and tribological behavior of cemented tungsten carbides and hard rocks in percussive drilling

Abstract

Presented research is about deep rotary-percussive drilling (RPD) in hard rock formations. RPD is recognized as an effective method for tunnel excavation and well boring in hard igneous rocks. Rock fragmentation and chipping are achieved through repetitive impacts of hard indenters onto and the rock. At the head of the drill bit, a number of inserts are placed, which are the main operating components of a drill bit that directly interact with rock formation and perform its fragmentation. Gradual wear of drill bit hardmetal inserts leads to the loss of functional shape which decreases drilling efficiency. One measure to enhance efficiency of RPD is to extend the lifetime of the drill bit inserts. Presented research is focused on the interaction of WC hardmetal insert and hard rock formation and on the wear of the former. The study is divided into two parts: investigation of the rock behavior and of the behavior of WC hardmetal insert. A link between the individual studies is given. The aim of the current research is to develop numerical tools and a methodology for studying the behavior of WC hardmetals under various drilling conditions. The tools and methodology would provide engineers a quantitative description of the drilling process, allow to estimate the drill bit’s lifetime and search for the most optimal solutions in particular drilling tasks. By doing so, the delay times and additional expenses associated with up-lifting of the drill bit for repair or replacement is reduced. The most commonly used material for drill bit inserts in RPD is the WC hardmetal composite, consisting of ceramic tungsten carbide grains (WC) and ductile metallic binder. Studies of the degradation mechanisms and relationship between the types of fracture of the composite are widely reported in the literature. Nevertheless, it still presents a challenge to establish the relative influence of different microstructural features on wear behavior in a particular drilling application. Current research present results on using computational homogenization techniques to obtain representative macroscopic behavior of WC hardmetal composite. The macroscopic behavior is obtained from the knowledge of the constitutive laws and spatial distribution of the components. Reaction forces acting on the hardmetal insert obtained for a particular rock and drilling conditions are one of the essential characteristics of the interaction process. These forces are essentially the boundary conditions for investigation of the hardmetal behavior during the interaction with rock. Rock material under impact loading experiences various states, leading to failure and crater formation. A good prediction of the stress and strain fields under the impactor is crucial for estimation of both, the rock crushing and shipping efficiency and the reaction forces acting on the hardmetal insert. Current research presents a development of rock material model to be used in numerical simulations of an impact. Material model is calibrated for Kuru Grey granite using data from performed laboratory tests. Multiscale tribological analyses have been performed on a damaged surface of drill-bit hardmetal insert and crushed rock surfaces. The study revealed that the rock-tool interaction leading to wear of the drill bit insert is at a scale of about 100μm. Measurements showed that the morphology of the rock surface after impact matched the damage pattern on the surface of drill bit insert. This result provides us with the characteristic length scale of interaction. Information from macroscopic rock response and from hardmetal microscopic behavior can be transferred to the meso-scale in order to account the more realistic contact areas and forces.