Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal Failure of electric resistance welded API pipes – Effect of centre line segregation Arnab Karani ⁎ , Soumyajit Koley, Mahadev Shome Research and Development, Tata Steel, Jamshedpur, Jharkhand 831 001, India ARTICLE INFO Keywords: ERW Phosphorus Embrittlement Centre line segregation API pipe ABSTRACT Electric Resistance Welding (ERW) is a widely used manufacturing method for API grade steel pipes. Failure due to cracking near apparently good welds is a major concern. Failure analysis studies were carried out on ERW pipes which cracked during ring flattening test, a standard method for evaluating the weld quality of pipes. The weld joint apparently perceived to be weak is found to be sound and intact. Optical and electron microscopy studies of the failed location show the presence of extensive centre line segregation in the steel. Specially colour etched specimens revealed the presence of segregation band containing high concentration of phos- phorus merging almost at the weld joint. This embrittled band covering a large portion of the thickness of the pipe has been attributed as the cause for failure during the flattening test. 1. Introduction Pipelines continue to be the most cost effective and safest mode of liquid transfer over large distances [1].Steels complying to the American Petroleum Industry (API) specification are largely used for making pipes [2].With increasing use of higher strength steels for petroleum transmission pipes it is expected that lower wall thickness should be able to withstand higher operating pressure [3]. Welded pipes are manufactured by cold forming of the hot rolled plates followed by seam welding of the edges. Submerged Arc Welding (SAW) and High Frequency Induction Welding (HFIW) also termed as Electric Resistance Welding (ERW) are the two most commonly practised welding methods [4].In HFIW process, the skelp is progressively cold formed into a tubular shape by means of a series of forming rolls. A high frequency current of 400 kHz is induced at the open seam by means of an induction coil located ahead of the welding point [5]. Due to skin and proximity effect, the current gets localized at the edges and melts them. The squeeze rolls at the welding point squeezes the abutting edges together and the molten metal containing the oxides are squeezed out towards the inner diameter (ID) and outer diameter (OD) of the pipe and the edges are forged together to form a joint. HFIW is an autogenous process and the welded region has a granular microstructure, unlike other welds which have cast structure [6]. The quality of the weld depends on (i) ERW (HFIW) welding process parameters and (ii) the stock material. The common weld defects are hot crack, hook crack, cold weld and penetrators [7]; While the first two are material defects, the others are process defects. While the role of inclusions in causing hook crack is fairly understood [7,8], the role of centre line segregation (CLS) in the skelp on weld defects is less explored [9].The segregation happens during slab casting at the time of steel manufacturing. During continuous casting of steel, due to rejection of solutes into surrounding liquid, a gradual build-up of solutes takes place [10]. During solidification, the partitioning of solute elements between solid and liquid phase is governed by equilibrium partition ratio, k. which is defined as: https://doi.org/10.1016/j.engfailanal.2018.10.018 Received 23 May 2018; Received in revised form 1 October 2018; Accepted 22 October 2018 ⁎ Corresponding author. E-mail address: arnab.karani@gmail.com (A. Karani). Engineering Failure Analysis 96 (2019) 289–297 Available online 23 October 2018 1350-6307/ © 2018 Elsevier Ltd. All rights reserved. T