Influence of the technology for manufacturing sucker rod pump cylinders on the structure and acoustoelastic characteristics

Problem. After the ion nitriding operation, unacceptable geometric deformations of the inner diameter (circular and oval distortions) occur in sucker rod pump (SRP) cylinders, presumably associated with an uneven distribution of residual stresses. Existing methods do not provide non-destructive testing of these stresses throughout the entire volume of the finished product. Aim. To develop a methodology for the ultrasonic assessment of the non-uniformity of residual stresses arising after nitriding in actual SRP cylinders and to establish its relationship with geometric deformation. Methods. The microstructure near the inner surface of the cylinders was investigated after four key stages: delivery (as-received condition), hightemperature tempering, grinding, and ion nitriding, including in deformation zones. For the non-destructive evaluation of residual stresses, the acoustoelasticity method was applied using a SEMA structuroscope and a DIO-1000 PA flaw detector. The acoustoelastic coefficients were determined during compression tests on specimens. Results. It is shown that after nitriding, a heterogeneous nitride zone with coarse inclusions forms in the areas of geometric deformation. The maximum nonuniformity of residual stresses is observed after the as-received and grinding stages. Tempering and nitriding reduce both the absolute stress values and their scatter. A correlation was established between the degree of stress non-uniformity and the wall thickness variation in the deformed cross-section. Conclusions. The discovered correlation between the ultrasonic testing parameters and geometric defects confirms the practical applicability of the acoustoelasticity method for the rapid identification of defective SRP cylinders at the post-nitriding inspection stage. The results make it possible to adjust the technological process to minimize deformations