Investigation of residual stresses within a friction welded steel pipeline by the contour and X-ray diffraction methods

Residual stresses (RS) in pipes welded by a fully automated friction welding process, called FRIEX, are investigated using the contour method (CM) and the X-ray diffraction method (XRD). Samples were produced using a self-developed equipment, which is based on the interaction between a consumable intermediate rotary ring and two stationary pipe sections, by varying the loading rate applied during the welding of API 5L B pipes with an API X65 ring. Mechanical and metallurgical properties were locally analyzed through metallography and microflat tensile tests. RS measured by XRD and CM showed comparable values; tensile zones were found in the pipe HAZ and compressive stresses were found in its adjacent areas. Tensile RS of approximately half the local yield strength of the components were also found, mostly on the internal surface, and their values decrease with increasing welding loads. Microflat tensile tests showed an increase in yield strength of pipe TMAZ and HAZ, with a clear influence of loading rates. (author)