Influence of Residual Stress on the HIC Resistance of High Frequency Induction Welded Pipes With Regard to Process-Specific Influencing Factors

Abstract

Over the last decades an increase in the exploration and exploitation of impure oil and gas resources in remote environments under aggravated conditions has become necessary. This led to a growing demand for pipes with resistance to sour service conditions. Salzgitter Mannesmann Line Pipe has enhanced its product range of High-Frequency-Induction (HFI) welded pipes in recent years accordingly. In the process of HFI welding of pipes, forming roles bend steel coil into a pipe which is then welded together without any filler metal. This cold forming results in residual stress, depending on the diameter and wall thickness of the pipe. The current state of technology is based on the perception that this residual stress has an adverse effect on the resistance of line pipes to HIC, because it amplifies — or if it is sufficiently high — even triggers the onset of HIC. Aim of this paper is to study the influence of residual stress on the resistance in HFI welded pipes to HIC with regard to process-specific influencing factors. Four material strengths are selected for the tests. The first three material strengths (API 5L Grade from Grade B up to X65) are intentionally produced from non sour service material in order to obtain sufficient HIC damage. The highest material strength examined is a sour service material alternative to ascertain whether under optimal material conditions HIC indications can result solely from high residual stress. Plate and pipe segments are examined by means of the cross-sectioning method for longitudinal and circumferential residual stress at the process steps that influence the residual stress. A series of experiments under simulated residual stress to determine the HIC resistance of these pipe materials in NACE TM0284 test solution A is carried out using the four-point-bend test according to ASTM G 39, usually applied in sulphide stress cracking tests of line pipe steels. A characteristic HIC value, the crack area ratio CAR, is determined as a function of C and Mn content and residual stresses. To verify the results, FEM was used to model a test bar with the same geometry and to re-calculate the above-mentioned case. The results of these experiments combined with the supporting theoretical considerations and modelling prove that in the case of HFI welded line pipes, the residual stress induced by the process has no negative impact on the resistance of HFI welded pipes to HIC.