Investigation of penetrator defect formation during high frequency induction welding in pipeline steels

Christos Sofras,Nikolaos Voudouris,Marianthi Bouzouni,Spyros Papaefthymiou,S Pantelakis,G Lampeas,K Tserpes

doi:10.1051/matecconf/202134904002

Abstract

The aim of this study is to investigate the formation of oxide defects known as penetrators during high frequency induction welding process of high strength low alloy pipeline steels and to correlate their formation with the steel chemical composition. Penetrators formed during the welding process can be detrimental for the impact properties of the weld seam. For this purpose, three different samples, with different chemical compositions, were intentionally produced with penetrator-type oxides and investigated. In order to characterize the oxide defect and correlate their formation with the chemical composition of the steel, optical microscopy and scanning electron microscopy paired with energy dispersive spectroscopy were employed. In addition, thermodynamic calculations were performed in order to examine whether the chemical composition of pipeline steels is prone to oxide formation. The results showed that oxides with pancake type morphology were found alongside the fusion zone of the samples. They mainly consisted of manganese and silicon. First findings on the the Mn/Si ratio showed that the lower ratio is less susceptible to oxide formation.

Highlights

IntroductionThe world’s ever-increasing energy demands impose new challenges in the field of oil and gas exploration, related both to the medium being transported (i.e. high H2S content, high operating pressure) and to the environment in which the pipeline operates (i.e. low temperatures)
The world’s ever-increasing energy demands impose new challenges in the field of oil and gas exploration, related both to the medium being transported and to the environment in which the pipeline operates
The first one is the High Frequency Resistance Welding, where the required current is applied by a contractor and the second one is the High Frequency Induction Welding (HFIW), where the required current is applied to the steel strip by the passage through an inductive coil

Summary

Introduction

The world’s ever-increasing energy demands impose new challenges in the field of oil and gas exploration, related both to the medium being transported (i.e. high H2S content, high operating pressure) and to the environment in which the pipeline operates (i.e. low temperatures). Modern pipeline steel grades, utilized for this applications, are characterized by a lower carbon (C) content paired with an increased Mn content Microalloying elements, such as niobium (Nb) and molybdenum (Mo), have a significant impact through the formation of carbides, both increasing strength and aid the austenite grain refinement during thermochemical processing. During HFIW, the steel strip is appropriately machined in order to ensure that its edges are ideal for the welding process It passes through a series of rolls, taking the characteristic “O” shape of the pipeline. During the passage of the steel strip through the induction coil, the abutting edges are heated and they are forced together by pressure rolls In this way, the two edges are autogenously welded together. The rejected material is mechanically removed. [2]–[4]

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Conclusion