The manufacturing and field experience of steel plates used to manufacture line pipes produced by Thermo-mechanically Controlled Processing (TMCP) are well defined in industry standards and literature. Compared to the Quenched & Tempered heat treatment process, TMCP plates are designed with a leaner chemical composition combining micro-alloying elements, precipitation, recrystallization and phase transformation during rolling and accelerated cooling. Technical challenges and process improvements moved older generation TMCP pipes from coarse microstructures and presence of non-metallic inclusions and/or mid-thickness segregation, to finer, homogenized microstructures and improved properties typically present in modern TMCP pipes.
Despite such an improvement, local hard zones (LHZ) have recently been experienced in the Oil & Gas industry on large diameter line pipes manufactured from TMCP plates. These LHZ must be distinguished from regular hard spot formation mechanisms known for years and highlighted in industry standards such as API† Spec. 5L / ISO‡ 3183.
The current paper deals with a thorough analysis of these newly recognized LHZ with Scanning Electron Microscope (SEM) and Electron Back Scattering Diffraction (EBSD) investigations through the wall thickness of pipes. Internal diameter (ID) surface, intermediate zone and bulk metal microstructures showed an increase of a strong misorientation while approaching the ID. Thus, LHZ is characterized by the presence of lath and especially lower bainite type microstructures associated to high local hardness above the NACE MR 0175 / ISO 15156 limits for sour service applications.
The manufacturing and field experience of high strength low alloy (HSLA) steel plates produced by Thermo-Mechanical Controlled Process (TMCP) are well defined in industry standards e.g. American Petroleum Institute (API) and literature. The TMCP method consists of a well-prescripted rolling pass schedule followed by accelerated cooling that leads to a fine-grain microstructure with the desired mechanical properties of the produced plates. From a metallurgical point of view, TMCP utilizes the complex interrelationship between (micro) alloying elements, precipitation, recrystallization and phase transformation during rolling and accelerated cooling to systematically adjust the microstructure and crystallographic texture. Due to the economical benefits of TMCP manufacturing over quench and tempered processes, development of the processes continued over the preceding decades. By further research and development, the steel manufacturers were able to attain a good combination of required strength and toughness properties by optimizing technologies such as micro-alloying and TMCP.