Avaliação do desempenho de juntas soldadas do aço inoxidável 304H em altas temperaturas na presença de SO2, SO3 e O2

Abstract

This work investigates the cracks formed in welded joints with covered electrode E308L used for joining 304H stainless steel sheets when subjected to long periods at high temperatures (600 ° C), in an environment containing SO2, SO3 and O2. This application is common in sulfuric acid producing units and cracks that occur in welded joints cause loss of production due to unit stops for correction. Beside that can occur environmental damage and risk of accidents due to the leakage of toxic gases. Initially, a crack was analyzed in a weld that remained in operation in the environment mentioned above in a superheater of a sulfuric acid producing unit for approximately 19 years ( 15,000 hours). The result indicated that it is a stress corrosion process involving sulfide and oxidation. To develop a solution capable to reducing the appearance of such cracks, ways of testing the welded joints were selected by varying the welding consumable and using different thermal spray coatings. For the manufacture of these specimens, AISI 304H was used. As addition material, electrodes of E308L and E316L were chosen. Additionally, aluminum and a nickel superalloy were selected, in this case, Colmonoy 6, to be applied by thermal spraying on the welded joints. Spraying processes were selected in a way that could later be used in the field to protect welded joints, resulting in compact layers that can reduce the diffusion of sulfur. The specimens were designed to work by the Double-beam principle (ASTM G39 (2016), allowing the allocation of samples in sufficient quantity to carry out field tests, even though a small space was available. The welding parameters used, as well as the geometry of the chamfers in the plates, were qualified and guaranteed through tensile and bending tests, and the results met the respective standards. The field test was carried out at a temperature of almost 600 ° C for 122 days, totaling almost 3,000 h of exposure to the corrosive atmosphere. After the removal of the specimens and its analysis using optical microscopy and scanning electron, it can be concluded that the joints produced with the E308L electrode developed cracks, while those produced with the E316L electrode did not develop these cracks. Showing the best performance of these joints when produced with this material. Weld beads microstructural analyzes in the region of the introduced notches showed that the crack, when formed, evolves preferentially through the fragile microconstituents that are formed during the operation of the superheater. It was observed that the E316L showed a slightly slower formation of the intermetallic constituents, especially the sigma phase. In addition, this material is more mechanically resistant at high temperatures. Some evidence of the presence of a corrosive process was found that added to the formation of sulfides found in specimens after 19 years operation ( 166 kh) gives evidence of the influence of a corrosive process on the crack formation. The use of aluminum coatings and the thermally sprayed nickel superalloy has not proved to be efficient because it cannot guarantee the necessary adhesion to the base metal. That is, it did not guarantee the insulation of the joint in the corrosive atmosphere.