Avaliação de trinca a quente em ligas cuproníquel e monel aplicando o ensaio trans-varestraint

Abstract

Due to the challenges associated with deepwater oil extraction, technological advancements in this sector have been aimed at pushing boundaries. The development of new materials and manufacturing processes, with an emphasis on welding techniques used for joining pipelines and claddings, demands the use of advanced technologies, which contributes to the increased cost of oil extraction. Among the most commonly used materials in refinery cooling units, water intake systems, and fire-fighting systems are cupronickel and Monel alloys, which are essential due to their high corrosion resistance. In these applications, welding of piping made from these alloys is a common practice. However, the occurrence of hot cracking during welding is a critical issue, considering the high costs associated with repairs and replacements. The objective of this study is to evaluate the metallurgical phenomena related to solidification during the formation of hot cracks in CuNi and NiCu alloys, using the Trans-Varestraint (TVT) weldability test. Three factors were analyzed in relation to hot cracking formation: material, welding energy, and imposed strains. The alloys studied were UNS C70600 and AWS A5.7 ERCuNi, commercially known as CuNi 90/10 and CuNi 70/30, respectively, as well as the AWS A5.14 ERNiCu-7 alloy, corresponding to Monel 60. The tests were conducted with five levels of strain and two welding energy inputs: 1.09 and 0.74 kJ/mm. The results from the weldability tests were compared with thermodynamic simulations based on solidification curves using the Scheil model. Paradoxically, it was observed that the CuNi 90/10 alloy—despite not forming low-melting phases and having the narrowest brittle temperature range (BTR)—showed greater susceptibility to cracking. This contradiction was explained by the occurrence of different cracking mechanisms. In the CuNi 90/10 alloy, cracks were associated with both ductility loss and solidification, whereas in CuNi 70/30, cracks were exclusively related to solidification. All analyzed alloys exhibited grain boundary cracks with preferential crystallographic orientation. It was concluded that the CuNi 90/10 alloy is more susceptible to cracking due to the combined effect of solidification and ductility loss mechanisms, while the CuNi 70/30 alloy exhibited only solidification-induced cracks.