Assessment of the In-Service Degradation of Pipeline Steel by Destructive and Nondestructive Methods
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ASSESSMENT OF THE IN-SERVICE DEGRADATION OF PIPELINE STEEL BY DESTRUCTIVE AND NONDESTRUCTIVE METHODS Yu. V. Mil’man,1 H. М. Nykyforchyn,2 K. E. Hrinkevych,1 O. T. Tsyrul’nyk,2, 3 І. V. Tkachenko,1 V. A. Voloshyn,2 and L. V. Mordel’1
UDC 620.17:669.017
We analyze the influence of the long-term operation of АРІ Х52 pipeline steel on the degradation of its mechanical properties. It is established that, after operation for 30 years, its strength and plasticity characteristics decrease substantially, and the fatigue toughness decreases particularly abruptly, which is connected with the development of the damageability. We use indentation methods for comparing the states of the operated and nonoperated material. It is shown that the indices of spread of the results of measurements (the coefficient of variance) of the mechanical properties (hardness, wear, etc.) are more sensitive to the damage of the material in the initial state and the damage after operation than their averaged values. Some correlations between the Vickers hardness, cyclic hardness, strength, and plasticity of the steel are established. Keywords: in-service degradation, pipeline steel, diagnostics of the state of metal, indentation methods.
In Ukraine, most main pipelines have exhausted their design life or are approaching its end, and, therefore, the justification of their safe operation with regard for the possible degradation of properties of the metal is a particularly urgent problem. The assessment of the degree of degradation of steels after long operation is a difficult and complex task. The number of works that deal with these problems is insufficient, though, recently, investigation on this subjected have intensified [1–7]. Along with the assessment of changes in the mechanical characteristics, structural transformations in steels are studied [2, 5, 6], and measurements of acoustic [4] and electrochemical [8] characteristics after long operation in a corrosive medium are performed. At the same time, in view of the possibility of substantial degradation of the initial properties, the monitoring of the current mechanical properties of steels [4, 8, 9] and, hence, the development of nondestructive methods of their diagnostics, which must be based on, first of all, the assessment of characteristics sensitive to transformations in the metal on the structural and the substructural levels, have also become urgent [5, 6]. Here, methods of assessing the wear resistance, hardness, and friction force are promising. It is also important to take into account the spread of results of measurements of these parameters, whose indices were found to be more sensitive to the state of the method (the LM-hardness method [10]) than the averaged values in the investigation of the influence of long operation on the development of the dispersed damage in steels. The aim of the present work is to analyze the degradation of the steel of a main gas pipeline by indentation methods with the determination of wear and friction characteristics under conditions of cyclic loadi
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