Measurement of residual stress due to deformation caused by slight hydroforming deformation process in double-walled pipes through external pressure

In this study, the measurement of residual stress due to deformation caused by slight hydroforming deformation process in double-walled pipes through external pressure was studied. Residual stresses were measured by nanoindentation of ST52 steel/GGG70 iron double-walled pipes. The results showed that the highest compressive residual stress was created on the inner surface of the GGG70 with effective parameters of pressure of 146 tons, the temperature of 73 °C, and percentage of Al₂O₃ nanoparticles powder in fluid of 5.6 %. Repeated nanoindentation measurements on the reference (stress-free) sample showed a variation of approximately 10 %, indicating acceptable repeatability of the method. The stresses are almost uniform and the same along the length of the part, and only very slight changes are observed in the part's initial part. The stress in the outer wall is obtained at the outer surface of ∼ 502.6 MPa and the inner surface of 506.6 MPa. Considering that the yield stress of the outer wall is considered to be ∼ 500 MPa, the entire outer wall is in a plastic state but has undergone a good amount of plastic deformation. Residual stresses are initially tensile and then reach their maximum value and with increasing depth, they are converted into compressive residual stresses and then reach an equilibrium state in the base metal. With increasing depth, there is not much change in the maximum tensile residual stress.