PSI - Issue 17

Jan Kec et al. / Procedia Structural Integrity 17 (2019) 230–237 Jan Kec / Structural Integrity Procedia 00 (2019) 000 – 000

231

2

Nomenclature OD

outside diameter of pipeline

WT USE HIC SCC SSC DBTT LSAW SEM EDXS LM

wall thickness upper shelf energy

ductile-brittle transition temperature

hydrogen induced cracking stress corrosion cracking sulphide stress cracking

longitudinal submerged arc welded

light microscope

scanning electron microscope energy-dispersive X-ray spectrometer

1. Introduction Due to the ever-increasing volume of transported gas all around the world, systematic development and research of steels for transition gas pipelines are going on. For instance, about 50 billion cubic meters per year is transported across the Czech Republic. To make the transport of such enormous amount of gas possible, it is necessary to maintain in operation the gas pipelines of large diameters under high-pressure conditions. It is quite obvious that steel alloys for gas pipelines shall meet exigent requirements. The development of these steels aimed especially to ensuring excellent weldability, mechanical properties, toughness and crack arrestability. Furthermore, all mentioned mechanical properties shall be stable within a large temperature range, e.g. from -50 °C to +50 °C (Petrov et al. 2015). These apparently contradictory properties can be obtained by microalloying, controlled rolling and accelerating cooling from the rolling temperature. The microstructure resulting from this procedure cannot be obtained only by heat treatment and contains very fine ferritic matrix with bands of pearlite and/or bainite. After the controlled rolling process, the ferritic matrix is strongly strengthened owing to deformation (dislocation) and precipitation of carbides and carbonitrides of microalloying elements. However, more than 30 years of research were necessary to obtain the required microstructure containing well selected phases. The manufacture of the first hot rolled steels X60 microalloyed with V and later with Nb started about 1970 (Hillenbrand et al. 2001). A common problem of all hot rolled steels is the anisotropy of mechanical properties brought about by band microstructure, uneven distribution of inclusions and crystallographic texture. The band microstructure affects also unfavorably cold working, resistance to hydrogen induced cracking (HIC) (Koh et al. 2004), stress corrosion cracking (SCC) (Zadow et al. 2015) and sulphide stress cracking (SSC) (Sojka et al. 2005). In case of hot rolled steels, the banded microstructure arises from the segregation of Mn and during the solidification of the steel (Feng et al. 2018). It is well-known that the carbon activity in austenite is reduced by Mn and that is why the formation of ferritic bands and undesirable granular bainite occurs in the zones with higher Mn concentration. The bands of granular bainite, called also abnormal segregation bands, have more than twice higher hardness in comparison with ferrite and deteriorate the plasticity and toughness of the steel (Feng et al. 2018, Nagode et al. 2017, Feng et al. 2015). It was found in X70 gas pipeline steel that the banded microstructure causes crack arrester and crack-divider delamination, which substantially changes the values of absorbed energy during Charpy V notch impact test (Haskel et al. 2014). 2. Experiment

2.1. Procedure

Segment of longitudinal submerged arc welded (LSAW) pipe with OD 813 mm and WT 10,6 mm was cut out for analyses of mechanical properties and microstructure. The pipeline had origin from the first generation of microalloy steels designated X60 according to API 51 (corresponding to L451N according to ČSN EN 10027 -1 Standard). Chemical analysis of the base metal (BM) and weld metal (WM) was carried out via optical emission spectrometer