Avaliação da confiabilidade da recomposição de espessura por weld overlay em tubos da fornalha de caldeiras de recuperação química em indústrias de papel e celulose

Abstract

The chemical recovery boiler is one of the most important equipment in the pulp industry, and it is the one that usually limits the unit’s operating time. In these boilers, black liquor is used as fuel for burning, unlike other conventional fuels. The burning of this type of fuel produces a melting material, known as Smelt, which will be processed again for reuse of the compounds and return to the system, thus ensuring the chemical recovery cycle. This equipment has a high risk of explosion by changing the physical state of the water in case of contact with the Smelt, in leaks caused by damage to the pipes of the furnace. Welding processes are often used in the repair of these pipes. However, this method of repair can cause changes in the mechanical and metallurgical properties of the base materials. These changes, in practice, cannot always be verified immediately, due to the lack of specific methodologies, which are reliable and economically viable versus the need for operational return of the component. This work was carried out in order to gather the basic information necessary for reliability for operational continuity with the repaired tubes, and what are the risks that these repairs can bring to the operation of the chemical recovery boiler in case of poorly executed. For this, samples of new tubes and tubes that operated in chemical recovery boiler were evaluated. Points with wear and subsequent recomposition of the overlay were inserted in order to simulate the procedures performed during the equipment maintenance stops. The evaluation was made through the microstructural analysis, the residual thickness and the mechanical property of hardness in the region repaired with weld to recompose the coating. As a result, it was observed that the repairs performed above the limits of minimum residual thicknesses, met the expected quality requirements, and can be used safely, provided that they are performed and accompanied by experienced and qualified professionals for such activity.