Weldabilty evaluation of corrosion-resistant alloy weld metals for high-temperature service in oil and gas industry using trans-varestraint test

Abstract

The flare in the oil and gas industry is responsible for burning harmful gases, ensuring safety. Flares are commonly made of austenitic stainless steels (ASSs), which are highly recommended for application at high temperatures and in reducing/oxidising atmospheres. Despite this, several failure cases of ASS components utilised in oil and gas production and refining have been reported in the literature, mostly related to metallurgical phenomena occurring due to harsh operation conditions. The processes of fabrication and repair per welding may also cause failures once the welding leads to nonequilibrium solidification, microsegregation of alloy elements, and potential rejection of impurities from the solid toward the interdendritic liquid, which are critical factors for the occurrence of solidification cracking. Therefore, this work aimed to assess the weldability through Trans-Varestraint Test (TVT) of consumable electrodes which can be employed in the fabrication and repair of flare components. The welds were made with the shielded metal arc welding (SMAW) using three filler metals: the ASS electrodes AWS E310-15 and AWS E347-17 and the nickel-based alloy electrode AWS ENiCrFe-2. The ASSs AISI 310S and AISI 304H were applied as the base metals. The TVTs was performed with strain levels ranging from 0.5% to 8%. The weldability was evaluated based on the susceptibility of the weld metal to solidification cracking, which was determined through the critical strain, saturated strain, maximum crack length, maximum crack distance, total crack length, and critical strain speed. Additionally, samples of the fusion zones resulting from each welded combination were investigated by light optical microscopy and scanning electron microscopy, with a focus on correlating the microstructural characteristics to the resistance to solidification cracking. Electron backscatter diffraction (EBSD) analysis was used to determine the effects of stress applied during TVT on the microstructures of the weld metals. Considering all the results analysed, the ranking from least to most susceptible to solidification cracking was as follows: (AISI 310S + AWS E347-17) > (AISI 304H + AWS E347-17) > (AISI 304H + AWS E310-15) > (AISI 310S + AWS E310-15) > (AISI 310S + AWS ENiCrFe-2) > (AISI 304H + AWS ENiCrFe- 2). The weld metals most resistant to solidification cracking resulted from the tests with the AWS E347-17 electrode, for which solidification occurred in the primary ferrite/secondary austenite mode. These fusion zones consisted of a combination of austenite and δ-ferrite, and the presence of δ-ferrite is known to be beneficial for preventing solidification cracking. The welds with the AWS E310-15 electrode solidified in the fully austenitic mode, which is favourable for solidification cracking. The fusion zones of tests using the AWS ENiCrFe-2 filler metal were composed of the γ-Ni fcc phase, and evidence of the precipitation of Nb(C,N) was found in the interdendritic volumes. The formation of secondary phases may have some influence on the cracking propensity, explaining the lower resistance to solidification cracking of these weld metals. The EBSD results showed higher misorientation levels surrounding the cracks and on subgrain boundaries. This may be attributed to the severe deformation level at which the microstructure was subjected to cracking. The regions where dislocations accumulate eventually became subgrain boundaries.