We investigate the kinetics of damage of the material of hard-alloy (tungsten carbide-cobalt) teeth of drill bits under cyclic compressive load. Three stages of damage of the structure up to the critical fracture of a tooth are found. The dependences of the relative duration of each stage on the amplitude of the applied loading and on the alloy composition are established.For estimating the serviceability of tools fitted with hard-alloy components, methods of testing are in use that reproduce, as precisely as possible, the conditions of their functioning [1]. The characteristics of wear resistance of material during dynamic contact with the abrasive are regarded as the main criterion of serviceability [2][3][4][5][6][7]. However, although they are informative as a whole, they do not give information on the tendency of an individual hardalloy tool to accumulate damages under specified conditions. The methods of fatigue tests of the teeth of drill bits under cyclic compressive load proposed in the Physicomechanical Institute of the Ukrainian Academy of Sciences makes it possible to register the evolution of damage of the material of the tool which puts this hard-alloy tool out of order.The aim of this investigation is to study the evolution of damage of the material of a hard-alloy tooth under cyclic compressive load up to its complete failure in detail.Teeth 12.4 mm in diameter with a spherical working part 14 mm in height made of tungsten-cobalt compositions VK6KS, VK8KS, and VK12KS with increased plasticity and resistance to failure were taken for tests [9] (here, "KS" denotes coarse-grained alloys where the main part of grains of the WC phase is characterized by a size equal to 3.7-4.5 ~tm). The method [8] includes, as the type of loading, cyclic compression of two teeth with contact over their working surfaces. A sinusoidal change of voltage with a frequency of 16 Hz and the cycle asymmetry R = 10 to 20 was used. The damage of the material was monitored after a certain number of cycles, increasing the intervals between observations and decreasing the amplitude of loading.We established that, at the first stage of tests (I), the material undergoes intensive plastic deformation in the region of the contact of the teeth (Fig. la-c). Then (stage II) a network of microcracks forms, and microvolumes of the material crumble out in the region of the contact of the teeth (Fig. ld-h). It was also found that, for a material with lower cobalt content, the area of the deformed surface is smaller (Fig. la-c), but the relative duration of stage I is greater than that for the material with greater cobalt content (Fig. 2). At the stage II, in a tooth with small cobalt content (mass part is at most 8%), we observe that the region of crumbling out of microvolurnes of the material is smaller as compared with the region of deformation (see Fig. ld, e, f). The localization of the site of initiation of the main macrocrack as the final stage (III) of the material damage is more pronounced (Fig. li-n). This is responsible for the lowe...

We present experimental estimates of the resistance of 30KhGSNA steel after heat treatment in various modes to corrosive-erosive wear in inhibited water. We established the existence of threshold amplitudes under cavitation, below which wear of the steel is not observed. They vary more significantly in the case of inhibiting the medium than owing to a change in the steel structure from perlite to martensite. We compared the results obtained with the literature data concerning the structural sensitivity of the steel under study to corrosion fatigue in the range of high-and low-amplitude loading. We conclude that corrosive-erosive fracture is controlled by the process of initiation and growth of short corrosion cracks.Corrosive-erosive fracture during cavitation is considered, as a rule, from the position of cyclic deformation of local regions in the surface layer of a material [1][2][3]. For this reason, investigators often try to find correlation relationships between the characteristics of fatigue fracture and corrosive-erosive fracture. One can use them, on the one hand, for predicting the resistance of a material to corrosive-erosive fracture according to data of cyclic tests and, on the other hand, for its structural optimization aimed to achieve a strong counteraction to both loadings often corresponding to actual conditions of operation. Such relationships are usually based on the" characteristics of the resistance to cyclic fracture in air, neglecting the role of the corrosion factor. However, the process of corrosiveerosive fracture is purely mechanical [4,5], and the role of the corrosion factor can be substantial and even predominant [5]. This fact, in turn, indicates the expediency to use just the characteristics of corrosive-fatigue fracture upon the construction of correlation relationships with the parameters of the resistance to corrosive-erosive fracture.The goal of this work is to study the structural sensitivity of economically alloyed 30KhGSNA steel to corrosive-erosive fracture, to compare it with the sensitivity to corrosive-fatigue fracture, and, on this basis, to analyze the role of the corrosion factor in the processes of corrosive-erosive fracture. Experimental ProceduresWe tested steel after hardening in olive oil from a temperature of 940~ with subsequent two-hourly tempering at temperatures of 150~ 350~ and 550~ and also after annealing at 960~As a result, microstructures of martensite, troostite, sorbite, and fine-grained perlite with ultimate strengths of the material of 1930, 1590, 1010, and 950 MPa, respectively, were formed.We tested cylindrical specimens 12 mm in diameter and 8 mm in length in tap water on a UZDN-1 magnetostrictor with a frequency of 22 kHz and amplitudes of magnetostrictor oscillations A in a range 4-55 pro. In certain cases, we added the corrosion inhibitor piperidine with a concentration of 10 g/liter to water. We periodically recorded a loss in weight of the specimen W (with an accuracy of _+ 0.00005 g), which made it possible to construct the time dependence...

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