Estimation of residual stresses in bimetallic plunger pump cylinders after thermal deformation

Plunger pumps used in oil production are made of long hollow bimetallic cylinders. These components are thermo-mechanically treated to improve strength and other physical and mechanical properties. These operations result in residual stresses within the parts, which can lead to positive, undesirable and unacceptable changes in the geometry. In the present work, we consider the problem of choosing the optimal machining modes. Estimations of residual stresses in the whole product take too much time, so it was decided to use small rings, which are representative for each particular pipe. In view of complexity or impossibility of applying the existing methods, the authors have designed a novel technique to estimate the level of residual stresses. For this purpose, we formulated and solved this problem within the theory of elasticity. An analytical solution, which makes it possible to find the level of stresses depending on the experimental measurements when cutting the rings, has been obtained. Three different steels were chosen. i.e. 38Cr2MoAl, 15Cr5Mo, 12Cr18Ni10Ti. Based on operating conditions, four optimization criteria for the heat treatment have been produced: the minimum level of residual stresses in the pipe; the minimum difference between stresses in the shell and liner; the minimum change in the pipe radius after the treatment; the highest value of adhesion between the liner and the shell. The obtained results have been analyzed based on the above four criteria. We revealed the optimum and intolerable modes of thermo-mechanical processing, which enabled undesirable changes in products. The required degree of deformation and temperature of the post-deformation heating have been found for each steel under study. As a result, recommendations for industrial enterprises have been drawn up.