Avaliação da tenacidade à fratura e propagação de trinca por fadiga do aço ASTM A285C na parede de pressão de um digestor contínuo de polpação kraft

Abstract

The continuous digester of the Kraft pulping process is a pressure vessel that plays a fundamental role in producing cellulose fibers. The delignification process takes place inside it, i.e., the removal of lignin from wood chips in an alkaline aqueous solution composed mainly of NaOH and Na2S, denominated white liquor, under high temperatures and pressures. Thus, its pressurized structure is subjected to conditions of oscillating stresses and corrosive processes, which can result in damage mechanisms such as crack nucleation and propagation. Consequently, knowledge about damage behavior as a function of the construction material and usage conditions characteristics is of great relevance in the maintenance and design of pressure vessels, in particular, continuous digesters. In this work, samples of ASTM A285 grade C carbon steel, a common specification for pressure vessel manufacturing, were analyzed in terms of fracture toughness properties by the J-Integral parameter and resistance to fatigue crack propagation as a function of parallel and perpendicular orientation relative to the rolling direction. Additionally, the degradation caused by surface corrosion on these properties was also evaluated. For this purpose, fracture toughness, fatigue crack propagation and white liquor immersion corrosion tests were performed on C(T) specimens with crack planes parallel and perpendicular to the rolling direction, according to ASTM E1820, ASTM E647 and ASTM G31 standards, respectively. Results showed anisotropy of the JIC parameter; C(T) specimens with notch-oriented transversal to the plate rolling direction had an average 1.71 higher values when compared to parallel-oriented specimens, in both as-received and after immersion corrosion test conditions. A degradation in the JIC parameter due to corrosion was also verified; residual fracture toughness of corroded specimens decreased by an average of 0.73 in relation to as-received condition specimens in both orientations. On the other hand, the property of resistance to fatigue crack propagation did not present anisotropy between orientations. However, it also presented degradation due to corrosion, mainly in the region of low and intermediate K values in the range of 15 to 40 MPa√m. In addition, EBSD crystallographic microtexture analyses were performed and showed that the anisotropy of fracture toughness was influenced by the characteristics of the grain orientations. On the other hand, the resistance to fatigue crack propagation was less influenced by the microtexture with distinct crystallographic orientation and dislocation boundaries maps between crack orientations and conditions.