Residual Stress Assessment and its Effect on SCC of Pipelines Steel in Acidic Soil Environment

Abstract

ABSTRACTEffect of residual stresses of multiple welding repairs on API 5L X52 pipeline steel on stress corrosion cracking (SCC) in a simulated acidic soil solution was studied. Four conditions of repairs of the girth weld were evaluated. The residual stresses were measured through X-ray diffraction (XRD) on the internal side of the pipe in longitudinal and circumferential direction. The circumferential and longitudinal residual stresses values are compressive on the inner surface of the welding joints. The highest residual stresses were measured in the hoop direction reaching values of about 98% of the yielding strength (360 MPa). It was observed that its magnitude increases as move away from weld center line. The effect of residuals stresses in the SCC susceptibility of X52 pipeline steel was evaluated through slow strain rate tests (SSRT) in a simulated acidic soil solution. Relation between mechanical properties obtained from SSRT and residual stresses on the SCC susceptibility was analyzed. Results of SCC index taking account the ratios obtained from the mechanical properties of the welding joints evaluate, showed good SCC resistance in acidic soil solution at low pH. Scanning electron microscopy (SEM) observations showed that the region with high residual stresses prior to generate cracks in the steel due to the combination of soil solution and the strain exerted, should favor pitting formation and not cracking.