Aços Maraging classe 350 e 250 como metal de solda nas juntas dissimilares usadas na indústria do petróleo

Abstract

The welding of dissimilar joints is very common in systems used in oil exploration and production in deep waters. Commonly involves welding low carbon steel pipes with other pipes, valves, pumps, all made of alloy steel. It has been used in these welding Inconel electrodes, however, is made a buttering process with carbon steel electrode in low alloy steel side. An alternative technique to reduce the residual stresses and thereby eliminate the post-welding heat treatment is the use of a welding filler metal that during the cooling process undergo a martensitic transformation to a relatively low temperature, so that the resulting deformation processing offset the contraction that occurs as a result of the cooling process. The aim of this study was to evaluate the application of Maraging steels 250 and 350 in typical dissimilates joints of the oil industry. The tests were performed on joints of ASTM steels A516 grade 70, ASTM A36, AISI 4130 and AISI 8630M welded with and without buttering with steel filler materials maraging 250 and 350, nickel alloy Inconel 625 (AWS ERNiCrMo-3) and low carbon steel (AWS ER80S-D2 electrode) without heat treatment after welding (PWHT). For the joints were evaluated residual stress profiles, hardness, microstructure, tensile strength and impact energy absorbed. The welded joint with maraging 250 showed higher intensity of compressive residual stresses compared to the joint buttery with low carbon steel electrode (ER80S-D2 AWS). The use of maraging 350 as buttering material positively influenced the base metal interface (AISI 8630M) / welding metal when compared to similar configuration, buttery with the electrode ER80S-D2 AWS (carbon steel), with lower levels of residual stresses. It was not noticed the effect of the joint with the deposit of 0.6 mm buttering maraging 350 on the residual stresses. In the the joints 250 and 350 maraging, the bonding zone, ZL, between the base metal and the weld metal, presented a composition gradient creating regions with different mechanical properties called partially dilute zone (ZPD). The higher the impact toughness of the dissimilar joint studied was in the interface region between the base metal and weld metal of the welded joint of 250 maraging presenting 46,1J compared to 21,6J the joint interface buttering maraging 350 steel. The result microhardness showed that the results were superior to the standards recommended by NACE MR0175 DNV RO and B401, however the materials were provided without PWHT. The sample with buttery maraging 350 presented microhardness values consistent with the expected value for the material. Tensile tests performed on specimens taken from the joints with maraging 250 and 350 met the acceptance criteria described by the ASME IV. The SEM of the fractured surface at the Charpy impact test images revelarm cleavage indications at the interface between the base metal and the weld metal in the welded joint of 350 maraging.