Caracterização microestrutural e mecânica do aço inoxidável ferrítico AISI 409 submetido à oxidação cíclica e isotérmica

Abstract

Ferritic stainless steel AISI 409 is the leading material in the manufacture of components of the cold part of the vehicle exhaust system, such as the muffler and the exhaust tube, places where condensation of gases from the combustion of the engine occurs. Such service conditions result in microstructural and mechanical damage to AISI 409 steel. The point of this research is to evaluate the microstructural stability and the mechanical comportment of material to suffer isothermal and cyclic oxidation, as a continuation of the research of Silva (2021) e Rodrigues (2014), respectively. Isothermal oxidation occurred in a tubular furnace for 2h, 4h, 8h, 16h, 32h and 50h, between 850°C and 950°C, in an argon atmosphere, containing 1ppm of O2. The samples subjected to cyclic oxidation underwent an oxidation test with immersion in condensate (TOC) at 70 ºC and a pure oxidation test (TOP) at 300 ºC, without immersion in condensate. The oxidized samples were manually sanded with silicon carbide sandpaper of particle sizes 240, 400, 800, 1000, 1200 and polished with 3 and 1μm Alumina, to remove the passive layer formed during the oxidation tests. The microstructure of the metallic substrate was observed using the Optical Microscope. Qualitative phase analysis was performed using X-ray diffraction (XRD). For mechanical characterization, the Vickers microhardness test (HV) was performed, and from the microhardness the corresponding tensile strength limit was determined. According to the micrograph obtained, there was no change in the ferritic microstructure. Through X-ray diffraction, was observed the presence of MnO, 〖"Cr" 〗_"2" "O" _"3" e 〖"Fe" 〗_"2" "O" _"3" (in sample subjected to cyclic oxidation). There is no presence of chromium carbides in the diffractograms. There was a substantial increase in mechanical resistance and microhardness, with the increase in the oxidation temperature from 850ºC to 900ºC and its reduction at 950ºC.