Microstructural Characterization of CGHAZ of HSLA Steel Welded by a Hybrid of Semi-Automatic Waveform Controlled GMAW and Mechanized FCAW-G Welding Process

Abstract The semi-automatic waveform-controlled gas-metal arc welding (GMAW) and mechanized flux-cored arc welding (FCAW) processes are commonly used in mainline tie-in welding. Like mainline girth weld construction by the mechanized GMAW process, to reduce the construction cost and minimize unnecessary weld repairs, the fitness-for-service (FFS) approach using Engineering Critical Assessment (ECA)-based weld flaw acceptance criteria were explored for mainline girth weld tie-in welding. The welding, mechanical testing and ECA details were presented in paper IPC2022-86737 for high strength low alloy (HSLA) Grade 483 steels using a hybrid of semi-automatic waveform-controlled GMAW and mechanized gas-shielded FCAW (FCAW-G) processes. This paper is a continuation of that previous work. It is focused on further studies of the specimens from the welding procedure qualification test welds where low fracture toughness values were associated with crack tip opening displacement (CTOD) pop-ins. In this paper, the CTOD pop-in fracture regions were characterized using optical metallography (OM), macro- and micro-scale hardness testing, scanning electron microscopy (SEM) and transmission/scanning transmission electron microscopy (TEM/STEM). From the fractography and detailed post-test metallographic analysis, cleavage crack initiation was observed in the coarse-grained HAZ (CGHAZ). The CGHAZ microstructural zones were large enough to be effectively sampled for hardness testing; however, the suspected brittle phases within the CGHAZ were too small to be assessed with available hardness testing equipment. From metallography examinations using SEM and TEM/STEM, irregular Fe3C particles were identified through carbon extraction replicas and focused ion beam (FIB) samples of the prior austenite grains (PAG) and prior austenite grain boundaries (PAGB) of the CGHAZ. In the FIB samples, twinned martensite (M) and retained austenite (RA) were also identified within the PAG and along the PAGB of the CGHAZ, confirming the presence of MA. The formation of these brittle phases in the HAZ of the HSLA steel is explored and their effect on the low fracture toughness of the HAZ is discussed. This investigation indicates that the inter-critically reheated CGHAZ (ICR-CGHAZ) region is considered to be responsible for the low fracture toughness. An important finding is presented related to the use of heat-tinting and its adverse effect on detailed post-test characterization of CGHAZ/ICR-CGHAZ regions in CTOD specimens. It is also important to mention that it is difficult to precisely identify the region that triggered cleavage fracture or a pop-in without extensive fractography. For pop-ins in the ICR-CGHAZ region due to suspected M-A secondary microphases, it is much more challenging to find the conclusive contributing factor or direct cause because of the effect of multiple thermal cycles of welding, phase transformation during and after welding, the inhomogeneous nature of the weld and HAZ, and the amount and morphology of the local microphases triggering the initiation of a fracture event.