PSI - Issue 30

L.V. Moskvitina et al. / Procedia Structural Integrity 30 (2020) 100–104 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The analysis of fatigue degradation of metal of wheel rims of heavy dump-trucks has allowed to fix a unique case of development of rotary processes in the open and closed system and at opening of mainstream crack at a temperature -40 0 С under the influence of high intensity alternating pressure. The purpose of this work is to establish a connection between micro – and macroscopic signs of plastic deformation and damage of metal of the wheel rim of tipper under alternating loadings. The identification of features of temperature fatigue destructions by macroscopic signs will allow to take measures in time on warning destruction of the workings parts of tippers. 2. Material and Methods The wheel rim of the 150-ton dump-body truck М -200 consists of two parts connected by welding - cylindrical and conical. They are made of steel with a ferrite – pearlite structure of as a following chemical compound: C - 0,11 %; Mn - 0,38 %; Si - 0,06 %; Sr, Ni - 0,30 %. Hardness by Brinnel is 136 kg/mm 2 . Mechanical properties of metal of the rim at test temperature +20 0 C were the yield strength σ 0.2 =141 MPa, ultimate strength σ 0,2 = 350 MPa, uniform strain δ= 35,62% , relative strain ψ =60,66 %. In the connection of cylindrical and cone parts of the wheel rim an opening is drilled for a valve of the rubber chamber, designated as a valve opening. To study the deformation structures, thin sections were cut along the fracture profile, along the zones of initial breakages and other areas of the destruction selected on macroscopic signs. Electric-microscopic images of microstructures and breakages are obtained by a method of two-level celluloid – carbon replicas using the translucent electronic microscope (TEM) -200. The prepared sections subjected to special etching to detect plastic deformations at different levels of magnification. 3. Results and Discussion 3.1. Rotational processes in the open system The macroscopic examination of the wheel rim has revealed that the nidus of breakdown was a mouth of the valve opening. The valve opening is a concentrator of tension. The distance from the opening edge to the fusion line is 5 mm. At automatic welding the metal where the opening is located could be heated to the temperature up to 100 200 0 С that corresponds to the ageing zone (Grabin 1982). On the circle of the mouth parts small cavities and radial cracks initiated by them are found out. The cavities were formed for a long time and are divided into different stages of the development. For studying the initial damages a plastic zone under the superficial cavity (Fig. 1 а ) was considered. The electronic-microscopic image of the area shows heterogeneous distribution of deformations between separate grains. Strongly deformed grains on the edge of the plastic zone are covered by thin lines of intragranular shifts. Frontier grains are inclined aside less deformed grains and are divided by shift strips into separate disoriented fragments. In the center of the image there is a large ferrite grain, its surface is splitted into fragments, contours of true boundaries are blurred. The edges of the grain are divided into fragments that are tilted relative to each other. The boundaries of slightly deformed grains are convex (Fig. 1b). The convexity of grain boundaries is caused by grain-border migration (Fig. 1 с ). The migration of boundaries is inherent to early stages of the plastic deformation (Gordeeva et al 1978). It specifies to the large gradient of deformation between the central and border-line grains of the plastic area. Koneva et al (1990) and Panin (1990) obtained that such heterogeneity of deformations leads to realization of large effects of extrusion-intrusion in the border-line active grains. The movement of grains is conducted as a whole by their shifts on boundaries and by turns of crystal macrovolume. According to the characteristic signs, it is possible to assert that turns of grains and their macrovolumes take place in metal of the valve opening mouth. The intragranular surface of destruction in the cavities is noted in Fig. 1d. Due to the corroded surfaces of intragranular facets, the presence of corrosive material along secondary cracks as well as the strips along grain boundaries, it is possible to suppose the corrosive-fatigue mechanism of destruction.