Experimental and numerical analysis of residual stresses induced by the manufacturing process of longitudinal welded tubes

In the present work, the process chain in tube production by roll forming of the steel 34MnB5 was examined in more detail. The process chain consisting of the steps (i) roll forming, (ii) HFI welding, and then (iii) straightening and final calibration was mapped using the finite element method. In addition to the pipe geometry, the residual stress distribution was considered as an essential target value for the assessment of the performance of the tubes under service conditions. The objective of the project is to describe the manufacturing process and thus the processing induced residual stress distributions as realistically as possible. For a more detailed characterization of the process and above all for the validation of the numerical simulations, experimental residual stress analyses were carried out for the final state of the tubes using complementary analysis methods. The contour method was used to determine the 2D-distribution of residual stresses across the transverse and longitudinal sections of the manufactured pipes. These measurements were supplemented by local residual stress analyses using the incremental hole drilling method and X-ray residual stress analyses. The work thus pursues both a methodical approach to the analysis of the internal stresses induced by the process using complementary methods and also a manufacturing approach to the analysis and evaluation of the production chain. In particular for the axial components of the residual stresses, it can be shown that experimentally determined residual stresses correlate well with the numerically calculated values. The contour method is excellently suited to monitor the uneven distribution of the internal stresses over the pipe cross-section. The results of the simulation show that the overall residual stress distribution that is determined using the contour method mainly results from the plastic strains introduced by the roll forming process. X-ray and hole drilling analyses are shown to be more suitable for measuring locally present residual stresses at defined positions on the outer tube surface. In this way it can be shown that the simulation approach described provides an accurate model of the process.