Efeito do aporte térmico na microestrutura de cordões de solda de arame tubular E81T1-Ni1C depositados em 3 variantes de aço API X65

Abstract

The objective of the present work is the microstructural characterization of weld beads deposited with E81T1-Ni1C tubular wire on three variants of X65 API steels. These are alternative materials used in the assembly of pipelines for the transportation of oil and gas, where the qualification of the welding procedures is a prerequisite for guaranteeing the mechanical properties of the component in service. In this context, the detailed characterization of the microconstituents that make up the three sub-regions of the weld is an essential tool, since it contributes with important elements for the understanding of the correlation between process variables and the final properties of the fabricated assembly. For this research, FCAW deposition of bead-on-weld beads under controlled welding conditions was performed, which allowed thermal input of 7.3, 11.3 and 22.1 KJ / cm which correspond to Δt 8/5 in the order of 3, 6-7 and 20 s, respectively. The microstructural characterization was done by optical microscopy and scanning electron microscopy with Field Emission Gun (FEG), where the high contrast quality in the images allows the identification and more detailed classification of the microconstituents present. The results for the welding deposits with medium heat input and Δt8 / 5 = 6 to 7 s, showed that the fused zone (ZF) of the three alloys is predominantly formed by the microcitre acicular ferrite (AF), with average hardness of the order of 240 HV0,3, aspects that according to the literature are requirements of excellent tenacity. When welded with low thermal input, the ZF showed AF and medium hardness of 260 HV0.3 and Δt8 / 5 = 3.5 s. The high thermal input provided a hardness of less than 200 HV0.3 and formation of AF in the ZF, and greater Δt8 / 5 of approximately 20s. The area affected by the heat of coarse grains (ZAC-GG) showed for the 3 steels microstructure basically formed by second phase ferrite (FS) and small fraction of primary ferrite. According to the microhardness profile along the sub-regions of the weld, the maximum peaks located in the ZAC-GG are lower than 340 HV for the cord made with medium heat input, which is indicative of absence of martensite and good toughness for this critical region Of the affected area. A hardness profile made in the ZAC-GG, with values higher than 350 HV0,3, indicates indications for the formation of martensite in the three alloys of API X65 welded steel with low thermal input.