Rolling schedules for TMCP processing can be optimized according to a specific alloy design and therefore can produce subtle changes in the final microstructure of a steel of the same class and grade. During pipeline construction and depending on the welding process and steel composition, the weld thermal cycle can reduce the toughness of the heat affected zone (HAZ) when compared with the base metal.
It has been observed that while the improvement of the HAZ toughness of microalloyed steel is due to the occurrence of a microstructure refinement near the fusion line, the degradation of the HAZ toughness can be attributed to local brittle zones (LBZ) which is dependent of the prior austenite grain size, bainite packet size and second phase distribution such as carbides and martensite-austenite (MA) constituents.
This paper will discuss the characteristics of the HAZ of an API 5L X80 produced by thermomechanical controlled process (TMCP) without accelerated cooling and finishing rolling temperature in the dual phase field, where the main hardening mechanisms are grain refining and precipitation.
The Gleeble 3800 simulator, was used to apply the thermal cycle characterised by the peak temperature Tp1 at 1350°C and Tp2 at 1200°C, 1000 °C, 800 °C and 600 °C and average cooling time of 11.3 s from 800 to 500 °C (delta t 800–500), in order to obtain a coarse-grained heat affected zone (CGHAZ), and reheated intercritically (ICCGHAZ) and supercritically (SCCGHAZ), equivalent to a 1.2 kJ/mm heat input. The obtained regions were compared to the same regions of a real multipass welded joint. Charpy-V tests and metallographic analysis using optical and electron microscopy were carried out to evaluate the simulated zones. The ICCGHAZ presented a necklace microstructure at the prior austenite grain boundaries associated with the low impact energy and the presence of the MA microconstituent.