Abstract

Carbon steel line pipes for sour service are required to exhibit hardness levels less than 250 HV10. The hardness of the inner pipe surface is especially critical, since sulphide stress cracking (SSC) may initiate at hardened surfaces. In recent years, high profile failures in sour environments were attributed to the presence of identified hard zones on TMCP pipe inner surfaces, and located far from welds. To help define the SSC limits of such material it is critical to know how these areas form.

Based on these considerations, the European Pipeline Research Group has launched at the end of 2017, a research project aiming to reproduce these hard zones via lab scale TMCP, aiming to identify the root cause.

This work deals with the first phase of this project, namely a state of the art review covering potential hard zone formation mechanisms in TMCP pipes. From the analyses performed, several mechanisms were found to be potentially relevant for hard zone formation during TMCP plate manufacture. First, possible carbon contamination from the casting phase. Second, the use of intense accelerated cooling equipment. Third, the effect of oxide scale in enhancing cooling power with standard accelerated cooling.

Introduction

Carbon steel line pipe manufactured from plates via thermo-mechanically controlled rolling (TMCP=Thermo-Mechanically Controlled Processing) has been successfully used in sour service applications for many years.1 Resistance to SSC is traditionally obtained by keeping hardness below 250HV10 and obtaining a fine and homogeneous microstructure. Additionally, resistance to SSC and HIC is achieved by the use of inclusion shape control, lean steel chemistries, and process control during casting.1 Moreover, pipes are also qualified for specific sour or a range of sour environments via SSC and HIC fitness for service tests as specified in ISO15156-2.2 Nevertheless, recent, high profile SSC failures of TMCP pipes have occurred.3 These were attributed to crack initiation at hard zones located on the inner surface of the pipes in base metal i.e. far from welds. These hard zones were located at a depth of less than 1 mm from the pipe inner surface, making their detection difficult by standard QA/QC.

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