The high temperature oxidation behavior of two ferritic stainless steels type AISI 430A and AISI 430E is examined at low oxygen pressure and high temperatures. The AISI 430A steel is ferritic up to 860 °C. Above this temperature, this steel is bi-phased: presence of austenite and ferrite phases. The 430E steel is stabilized with Nb, and is ferritic at all temperatures. The oxidation experiments were performed in a thermobalance SETARAM TGDTA 92, in the range of 850-950 °C, in Ar/H 2 /H 2 O atmosphere, under oxygen partial pressures lower than 1.3 x 10 -18 atm. The microstructure and the composition of the oxide scales were analysed by scanning electronic microscopy (SEM) and energy dispersive spectroscopy (EDS). Different oxidation behaviors in AISI 430A and AISI 430E stainless steels were observed. At 850 °C, the oxidation of the 430A steel is greater than that of the 430E steel, but above 900 °C the oxidation of the 430A steels is lower than that of the 430E steel. The oxidation rate of the 430A steel shows low dependence on temperature, while the oxidation of the 430E follows an Arrhenius law, with an activation energy corresponding to the chromia scale growth.

The oxidation behavior of AISI 304 and AISI 430 stainless steels was investigated from 1100 °C up to 1200 °C. Mössbauer spectroscopy and x ray diffraction were used to access the phase composition of the formed scales. The main crystalline phases found in the oxidized materials at temperatures above 1100 °C were hematite and magnetite for AISI 430 steel, and hematite and a spinel-like phase for AISI 304 steel. Hematite was found to be the dominant oxide at lower temperatures, whereas magnetite preferentially forms at higher temperatures. The activation energy for oxidation is smaller for AISI 430 steel in relation to AISI 304 steel in the range of studied temperatures, and therefore the AISI 430 steel is less resistant towards oxidation at high temperatures

The effect of oxygen partial pressure and temperature on the oxidation behaviour of unstabilized and Nb-stabilized AISI 430 ferritic stainless steels were investigated over the temperature range 850 -950 C in air, Ar þ 1 vpm O 2 or Ar -H 2 -H 2 O atmospheres. Isothermal thermogravimetric analyses were performed for 50 h. The microstructure of the oxide films grown on the steels were characterized by SEM and chemical analyses were performed by EDS, grazing X-ray diffraction and secondary ion mass spectrometry (SIMS). Nb-stabilized AISI 430 steel is more resistant to oxidation than unstabilized AISI 430 under all tested conditions, except above 900 C in Ar-H 2 -H 2 O. While the oxidation behaviour of unstabilized AISI 430 is strongly affected by the atmosphere composition, Nbstabilized AISI 430 oxidation rates do not depend strongly on the atmosphere. For both steels, the chemical analyses show Cr 2 O 3 as the main phase in the oxide scales, but solid solutions such as FeFe 2Àx Cr x O 4 and MnCr 2 O 4 formed in almost all scales for atmospheres of Ar þ 1 vpm O 2 or in Ar -H 2 -H 2 O, and Fe 2 O 3 and Mn 1.5 Cr 1.5 O 4 , are also observed for oxidation in air.

Resumo Este trabalho tem como objetivo demonstrar a aplicação das técnicas de laminação termomecânica de aços microligados para obtenção de propriedades mecânicas elevadas em um laminador Steckel. O processamento de um aço com composição química favorável à obtenção de propriedades mecânicas compatíveis com o grau X60 especificado na norma API 5L é demonstrado. Foram alcançados limites de escoamento a 0,5% de deformação e a resistência à tração mínimos de 473MPa e 549MPa respectivamente, alongamento médio de 38%, razão elástica em 86% e temperatura de transição dúctil frágil em torno de-50°C atendendo às especificações da norma.

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