Microstructural and Mechanical Properties of the Intercritically Reheated Coarse Grained Heat Affected Zone (ICCGHAZ) of an API 5L X80 Pipeline Steel

Abstract

The weld thermal cycle, depending on the welding process and steel composition can reduce the toughness of the HAZ when compared with the base metal. In the intercritically reheated coarse grained HAZ (ICCGHAZ) region, microstructural transformations from coarse austenite to bainite or martensite are liable to occur. Reheating into the dual phase field temperature and subsequent cooling can lead to the formation of “microphases” commonly referred to Martensite-Austenite (MA) constituent. Due to the C enrichment of the austenite, this region is regarded as local brittle zones (LBZ) and degradation of HAZ toughness can be attributed to the formation of local brittle zones (LBZ) at the ICCGHAZ. This work will discuss the characteristics of the ICCGHAZ of two API5LX80 steels produced by thermomechanical controlled process (TMCP) without accelerated cooling using a finishing rolling temperature in the dual phase field, where the main hardening mechanisms are grain refining and precipitation. Weld thermal cycle simulation, using a Gleeble 3800®, characterised by the peak temperature (Tp) of 800oC and the cooling time from 800 to 500oC (∆t800–500) were applied in order to obtain an ICCGHAZ equivalent to a 2.5, 3 and 4kJ/mm heat input. Charpy-V tests and metallographic analysis using optical and electron microscopy were carried out to evaluate the simulated zone. The results have shown that 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.