Reposição de espessura por soldagem em tubos de caldeira de recuperação e seu efeito nas propriedades mecânicas, metalúrgicas e de corrosão

Abstract

A recovery boiler is part of the pulp production process in which the burning of black liquor produces a molten material known as smelt. There is a high risk of explosion in this kind of equipment because of the phase change of water when exposed to the smelt due to the existence of holes caused by external corrosion in the water-tubes. The execution of weld buildup in corroded portions of such tubes tends to cause mechanical and metallurgical changes in their materials and may affect their integrity. This study aimed to evaluate the influence of weld buildup in the structure, hardness and corrosion resistance of recovery boiler water-tubes. The samples consisted of tubes with weld buildup that resumed operation in the boiler, new tubes and laboratory-welded tubes. They were prepared by metallography and evaluated by means of micro and macro photography, followed by hardness measurements, chemical analyses of the materials and a laboratory corrosion test to simulate the corrosion phenomenon by black liquor inside the boiler. The results led to the conclusion that the weld buildup process changes the microstructure of the tubes throughout their thickness. The laboratory-welded samples presented similar microstructure in the fusion zone (FZ) and in the heat-affected zone (HAZ), indicating that operation in the boiler did not have significant influence on the structure of the weld regions. However, there were considerable differences between them and the weld performed in the field. The area filled with weld has showed an increase in hardness of the repaired region and it has a lower tendency to be attacked by corrosion than the base metal.