Influence of the HFIW welded joint in the fatigue resistance of an API 5CT N80 type Q steel tube used in offshore oil and gas exploration

Abstract

Steel pipes used in the oil and gas industry are often subjected to dynamic loading. Therefore, to mitigate fatigue cracks nucleation and growth, these steel pipes should be as flawless as possible. HFIW (High Frequency Induction Welding) process is widely used by industry in the manufacturing of steel tubes. These tubes (like the one used in this research), after being welded, are often subject to heat treatments (to improve their mechanical properties and homogenize their microstructure) and to some grinding to remove excess material from the welded joint. However, even after these processes, a discontinuity will still be present. The aim of this paper is investigate how this weld line may assume the role of a notch (stress riser) reducing the fatigue resistance of casing and tubing quenched/tempered steel pipes. The study of the fatigue resistance has been done through the analysis of results obtained from “Sa × N” curves, linear regressions and estimation of a fatigue stress concentrator Kf′-notch. This Kf′-notch differs from the commonly used Kf, and is presented in the form of an equation Kf′-notch = η.Sa γ. In the fatigue tests, non-standardized specimens, taken directly from an API 5CT N80 type Q steel tube, have been used, part of them aligned with the longitudinal weld line and the others aligned with a position situated 90° from it. To complement the fatigue results, tensile tests have been carried on, as well metallographic analysis and a qualitative analysis of the welded joint geometry. The results obtained indicate that while the tube exhibit good mechanical and metallurgical homogeneity it exhibits lack of circularity (or roundness) in the adjacencies of the welded joint. In addition, from the fatigue results obtained, it is clear that the welded joint act as a stress riser, reducing the fatigue resistance of the steel pipe, with values of Kf′-notch that can be higher than 2.0.