The growing process of a spinel-like oxide film can be obtained through different atmospheres; in this sense Air, O2, steam, and nitrogen atmospheres were used in different steps during the aging heat treatment at 490ºC of maraging 300 and 350 alloys. The oxidation layer produced under different atmospheres was chemically, microstructurally and mechanically characterized by means of advanced techniques. The results showed (in both alloys) a non-homogeneous mix oxide formed by magnetite, nickel and cobalt ferrites and MoO3 heterogeneously distributed along the oxide thickness. It was also founded TiO2 in the innermost areas and a particularly important quantify of hematite on the external surface. A nickel-rich austenite phase was produced at the interphase due to the combination of nickel stability and the preference of cobalt, molybdenum, and iron diffuses through the film as ions. The highest thickness values were found in the oxides produced in maraging 300 steels which could indicate greater susceptibility to oxidation comparing with grade 350 maraging steel. The oxide films produced in both maraging alloys using air atmosphere presents excellent adherence, and the results show the capability for being used for tribological applications under sliding contact tests.

The corrosion behavior of austenitic and superaustenitic stainless steels was investigated in carbon dioxide-saturated synthetic oil field formation water using cyclic polarization tests. In order to measure the effect of carbon dioxide pressure, the samples were also exposed in a pressurized medium containing carbon dioxide and synthetic air. For this purpose, tests were performed for long exposure time at 80 °C under 8 MPa of a mixture of carbon dioxide and synthetic air both acting togethere. The results indicated that the type of corrosion on the surfaces of the samples after all the tests was pitting corrosion. According to the results, the AL-6XNPLUS™ steel presented the best performance in all experiments. The results also indicated that the conventional austenitic stainless steels are not suitable for the use in carbon dioxide containing environment in aqueous medium.

The liquefied natural gas volumes that currently need to be stored and/or transported require engineering to consistently develop materials that combine high mechanical strength and low manufacturing costs. The present work brings the study of four models of high manganese steels (non-commercials), to demonstrate the feasibility of being used in the petrochemical industry. The tests contemplate the characterization (composition, phase diagram, metallography, EBSD, microhardness), corrosion resistance in NaCl solution (Open Circuit Potential, Linear Polarization) and characterization of corrosion products (EDS, SEM, Raman Spectroscopy). The obtained results showed a relationship between the increases of the manganese content with: formation of twins that help their hardening, as well as the decrease of their corrosion resistance; the steel with less amount of this element had a more noble behavior. Quantities of chromium, silicon and aluminum in these steels could not contain the high dissolution of manganese, which caused low stability of the formed oxides that allows the electrolyte ions to reach the metal surface, accelerating the dissolution process.

RESUMO Aços inoxidáveis super ferríticos são utilizados em equipamentos como trocadores de calor e torres de destilação que operam em condições extremas de temperatura, pressão e em ambientes contendo altas concentrações de cloretos, H2S e CO2, encontradas nas novas áreas de exploração de petróleo e gás na costa brasileira. Uma liga super ferrítica com composição baseada em aços comerciais foi desenvolvida para a realização do presente estudo que visa adaptar a composição dos aços super ferríticos, com o objetivo de aprimorá-los. A liga desenvolvida contem 4,4%p de níquel (Ni) para melhorar a sua tenacidade, 25,7%p de cromo (Cr) para que a estabilidade da fase ferrítica fosse mantida devido a adição de Ni, 4,6%p de Mo para aumentar a resistência à corrosão. O diferencial para o estudo desta liga foi a adição de 0,07%p (700 ppm) de boro, visando o aumento da temperabilidade, tenacidade e estabilidade de contornos de grão. Foram realizados cálculos termodinâmicos através do programa computacional Thermo-Calc para a simulação dos diagramas de fase e a fração em peso das fases presentes no material. Os resultados das composições experimentais foram comparados com outros trabalhos já realizados com o aço comercial UNS S44660. Através dos resultados obtidos uma estimativa da temperatura de estabilidade da fase ferrítica foi determinada e as possíveis fases deletérias foram identificadas de acordo com variação da temperatura e da composição química.

ABSTRACT Super ferritic stainless steels are used in equipment such as heat exchangers and distillation towers operating under extreme temperature and pressure conditions and in environments containing high concentrations of chlorides, H2S and CO2 found in the new areas of oil and gas exploration on the Brazilian coast. A super ferritic alloy with composition based on commercial steels was developed for the present study, aiming to adapt the composition of super ferritic steels in order to improve them. The developed alloy contains 4.4 wt.% of nickel (Ni) to improve toughness, 25.7wt.% of chromium (Cr) so that the stability of the ferritic phase is maintained due to the addition of Ni, 4.6wt.% of Mo to increase the corrosion resistance. The differential for the study of this alloy was the addition of 0.07wt.% (700 ppm) of boron, aiming to increase hardenability, toughness and grain boundary stabilization. Thermodynamic calculations were performed using the Thermo-Calc software for the simulation of phase diagrams and the weight fraction of the phases present in the material. The results of the experimental compositions were compared with results from commercial steel UNS S44660. The results obtained an estimate of the stability temperature of the ferritic phase and determined the possible deleterious phases according to temperature variation and chemical composition.

Experiments have been conducted to study the strain induced transformation from austenite to martensite in a metastable AISI 301LN austenitic stainless steel, deformed by uniaxial tension applied along rolling direction. Samples deformed 10 and 20% have shown the presence of α´ martensite phase. Measured pole figures of martensite phase were compared to calculated ones, assuming no variant selection and selection of variants where interaction between stress and the plate of martensite adds to the driving force transformation variants. EBSD (electron back scatter diffraction) microtexture experiments and macrotexture X-ray diffraction were performed. The orientation distribution functions (ODFs) from measured pole figure data were calculated. The measured results were compared with calculated results in both polycrystalline and single crystal samples of austenite. The results showed that the calculated textures based in a process of variant selection consistent with Patel and Cohen's theory, which emphasizes a mechanical component of free energy, were in good agreement with measured texture.