Longevity of tungsten-cobalt alloys under load in a corrosive environment

Abstract

We studied the influence of the chemical composition of the WC-Co cermet system on the operating characteristics of hard-alloy tips of drill bits manufactured from it. We established that the cyclic contact strength of tips in air at room temperature and the threshold range of the stress intensity factor in the tip material increase with the cobalt content. Using the results of tests of the materials for cyclic crack resistance, as well as of cavitation-erosion and electrochemical tests in 3% NaCI aqueous solutions, we ascertained that the optimum corrosion and mechanical characteristics correspond to the composition containing 8 mass% Co. We noticed a significant contribution of the anodic dissolution of the Co-phase and its hydrogen embrittlement in the process of corrosive-mechanical damage of hard alloys. We discovered a satisfactory correlation between the parameters of cyclic contact strength, threshold range of the stress intensity factor, and steady-state rate of cavitation-erosion failure of WC--Co hard alloys. This fact demonstrates the possibility to use these parameters as reliable and interchangeable criteria of the operational reliability of hard-alloy products.Cermet tungsten-cobalt alloys of WC-Co system are widely used for the manufacture of tools which are used in the mining, oil, and gas industries and also for metal machining. During operation, products made of cermet tungsten--cobalt alloys are subject to the influence of various kinds of mechanical loads often in the presence of a chemically active environment, which results in their damage, wear, and failure, e.g., failure in consequence of contact fatigue, hydroabrasive erosion, or cavitation-erosion failure. To intensity the processes of machining of articles and products and of rock mining, one successfully applies technological solutions with various degrees of corrosivity. Therefore, investigations of the characteristics of resistance of cermet tungsten-cobalt alloys to corrosive-mechanical failure depending on the combined action of the environment and mechanical factors are becoming very urgent. From this viewpoint, in particular, the behavior of cermet tungsten-cobalt alloys under the influence of cavitation is interesting. In this case, intensive cyclic loading in a domain commensurable with the structural components of the material occurs. Investigations of this problem are well known [1 ] but they did not deal with the effect of the amount of the cobalt phase in materials with identical parameters of structural elements on the behavior of cermet tungsten--cobalt alloys. Moreover, a sufficiently wide spectrum of loading conditions has not been considered.The present work investigates the influence of a corrosive environment on the characteristics of resistance of WC-Co cermet compositions with different ratios between the metallic and ceramic phases to mechanical damage and failure under different kinds of loading and searches for a criterion which would characterize the kinetics of damage of the structure of material under certai...