Formation of Residual Stresses in the Cylinders of Deep-Rod Pumps after Manufacturing Operations

Abstract

The influence of technological operations on the formation of residual stresses during the manufacture of cylinders of deep-rod pumps made of steel grade 38Х2МUА has been studied. The following operations are considered: delivery status, tempering, mechanical processing, ion nitriding. The measured parameters were the transverse velocities of ultrasonic waves, used to calculate changes in residual stresses, which were determined using the phenomenon of acoustoelasticity. The magnitude of the average values of residual stresses was assessed by the destructive method according to the manufacturer's method by cutting ring samples cut from pipes after the corresponding technological operations. The heterogeneity of residual stresses along the perimeter of the pipe relative to the average value after all technological operations of processing the product was noted. According to the research results, the maximum deviations of residual stresses were obtained after the nitriding operation. Witness samples were made from the cylinders, with the help of which the hardness of the nitrided layer was checked and metallographic analysis of the microstructure of the nitrided layer was carried out. The studied microstructure consists of pearlite and ferrite at the first 3 stages of operations; after ion nitriding, the microstructure consists of sorbitol-like nitrogenous pearlite with inclusions of aluminum nitrides at a depth of up to 0.3 mm from the inner surface. The microstructure of the core is ferrite-pearlite, normalized, highly tempered. The conducted research revealed the following. The heterogeneity of residual stresses along the perimeter of the pipe relative to the average value after all technological operations of processing the product, reaching 10 MPa, was noted; the maximum deviations of residual stresses were obtained after the nitriding operation and exceed 60 MPa; the microstructure in the delivered state after tempering and machining consists of pearlite and ferrite; the microstructure after ion nitriding consists of sorbitol-like nitrogenous pearlite with inclusions of aluminum nitrides. Hardness meets the requirements of design documentation.