Specific features of the Analysis of the Initial Stage of Corrosion-Erosion Destruction of High-Strength Steel
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SPECIFIC FEATURES OF THE ANALYSIS OF THE INITIAL STAGE OF CORROSION-EROSION DESTRUCTION OF HIGH-STRENGTH STEEL V. А. Voloshyn1,2 and R. Ya. Kosarevych1
UDC 620.193.16: 519.23; UDC 621.391
We study the initial stage of corrosion-erosion destruction of thermally treated ShKh15 steel in alkaline environments and establish the character of the surface damage in the course of the tests and the laws of its degradation during the incubation period of cavitation. We also develop an automated method for the numerical analysis of the surface cracks, which enables one to give an objective description the degree of surface damage. Keywords: cavitation, corrosion-erosion destruction, image processing.
Cavitation is responsible for the cyclic impact loading of the surface layer of the metal and its frequency and intensity are determined by the hydrodynamic conditions and relaxation properties of the liquid and metal [1]. Therefore, the mechanism of corrosion-erosion destruction (CED) under the conditions of cavitation is usually regarded as the process of crack initiation and growth up to the crumbling of some volumes of the metal [1–3]. The researchers often consider only the cavitation destruction of metals in the form of craters without paying attention to objects of smaller sizes. The formation of small craters 10 µm in diameter for the first several seconds was described in [4]. In this case, the metal surface becomes wavy. Large craters (∼ 150 µm) are formed on the surface as the time of holding increases further. It was also emphasized that craters and blisters are randomly distributed over the surface and this is not connected with the grain boundaries or slip lines. As a specific feature of CED according to the mechanism of crack initiation, we can mention the absence of mass losses of the material in the initial stage [5]. At first sight, this means that the problem of CED is not important at all. However, one should keep in mind that the cavitation action of the environment is often accompanied by some other kinds of mechanical loads, which may lead to the formation of the main crack (fatigue, corrosion fatigue, corrosion cracking, etc.). Thus, the microcracks caused by CED may lead to the acceleration of this destruction, practically without the stage of crack initiation by the mechanisms corresponding to certain kinds of mechanical loads [6]. This means that the metallographic analysis of the surfaces subjected to cavitation becomes especially important from the viewpoint of formation of microcracks. The aim of the present work is to develop a method for processing the metallographic images of the surface of low-alloy high-carbon ShKh15 steel with martensitic structure under the action of cavitation, which is susceptible to CED just by the mechanism of crack initiation. Materials and Experimental Procedures The heat treatment of ShKh15 steel includes oil quenching from a temperature of 980°С and tempering at 200°С, which ensured its high brittleness and low resistance to surface crack formation. As a cavitation env
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