Soldagem do aço API 5L X70: controle da dureza na ZGG pela técnica de passe de revenimento

Abstract

Welding processes are widely used in the Oil and Gas industry. However, such procedures promote thermal cycling that often leaves residual stresses and microstructural changes that cause increased hardness in the weld regions - especially in the coarse-grained zone (CGHAZ) of the heat-affected zone (HAZ) of carbon steel and low-alloy steel components. For this reason, post-weld heat treatments (PWHT) are used for the purpose of tempering and controlling micro-constituents that are deleterious to the toughness of steel. However, this procedure requires considerable time and financial costs. In this sense, the Temper Bead Welding (TBW) presents itself as an alternative technique that aims at the metallurgical effect on the hardness of the HAZ under previously analyzed conditions, mainly in the control of the phase transformation in the solid state and its consequent effect on the hardness of the regions welding of these assets, without the need for PHWT, allowing considerable savings. In the present work, API 5L X70 steel, which combines satisfactory weldability and high mechanical strength to be used in pipelines in the oil industry, was selected to study the effect of TBW in terms of reducing hardness in the CGHAZ to levels accepted by codes and standards. For that, the overlapping of the beads at 25, 50 and 75% was considered as a variable, applying to three types of samples: with two layers of different thermal inputs, others with a pair of beads superimposed on each other; plus single passes for comparison purposes. In this way, optical and electronic microscopic analyzes and Vickers microhardness tests were performed to evaluate microstructure and hardness in the CGHAZ and validate the TBW technique. The single pass sample with the lowest heat input (E1 ≅ 0.79 kJ/mm) obtained the highest hardness values, with a high fraction of the ferrite phase with the second phase aligned FS(A). The experimental results, however, show that the 50% overlap obtained optimal values in terms of hardness reduction, representing a drop of 43.4% in relation to the sample with greater hardness and of 48.1% in relation to the maximum hardness allowed by the API 6A standard, and it was found that the tempering passes with higher heat inputs (E2 ≅ 1.58 kJ/mm) caused satisfactory tempering in the CGHAZ of the previously deposited E1 input beads, a fact responsible for the resulting low levels of hardness , presenting results accepted by codes/standards.