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.
Key words: Hard spot, hard layer, TMCP, Line pipe, sour service