The high-temperature shaping of steels is accompanied by the formation of surface scales composed of oxide layers. However, the oxidation kinetics and morphology of these scales remain poorly understood. This study analyses the formation of oxide layers on AISI 4140 steel at varying oxidation times (20, 40 and 60 min) at 1000 °C. The analysis revealed the presence of hematite, magnetite, and transformed wustite in the oxide layers, along with clusters of alloying element oxides, predominantly chromium and iron oxide (FeCr₂O₄). There was a direct correlation between the duration of the oxidation process and the thickness of the scale and the number of defects observed in the material. The coating layer of alloying element oxides demonstrated insufficient adhesion to the steel substrate. Similarly, the oxides of alloying elements within this layer exhibited low cohesion among themselves. The alloying elements are present in all oxide layers, but in greater quantity in the layer in contact with the steel substrate, where a reduction in their concentrations was observed over time. This indicates that the alloying elements tend to disperse as the thickness of the alloying element oxide layer increases over time.

O objetivo deste trabalho foi obter produtos de panificação a partir da substituição parcial da farinha de trigo pela farinha de pupunha integral. A farinha foi obtida por meio da retirada da semente dos frutos, cocção dos mesmos, secagem e moagem. Após a definição das formulações com a substituição parcial da farinha de trigo pela farinha de pupunha, os produtos foram caracterizados quanto à atividade de água (Aa), umidade, cinzas, lipídeos, proteínas, fibras, carotenoides totais, carboidratos, valor energético e aceitação sensorial. A farinha de pupunha, com atividade de água de 0,275, pode ser classificada como alimento de baixa atividade de água e, portanto, estável por um período de tempo longo. Além disso, apresenta teores consideráveis de proteínas (5,83%), lipídeos (19,45%), fibras (6,60%) e carotenoides totais (13,01 mg/100g), o que torna a mesma de grande interesse nutricional. Com relação aos resultados da caracterização físico-química, destacaram-se no bolo, biscoito e pão obtidos os teores de carboidratos (60,49%, 59,38% e 48,70%), proteínas (10,28%, 10,141% e 12,90%) e lipídeos (8,10%, 12,68% e 6,32%), respectivamente. Constatou-se ainda, boa aceitação sensorial para os produtos desenvolvidos, com nota média para a impressão global de 7,80, 7,32 e 7,15 para o bolo, o biscoito e o pão de pupunha, respectivamente. Portanto, a farinha de pupunha integral pode substituir parcialmente as farinhas tradicionais em formulações de bolos, biscoitos e pães, tanto em nível doméstico quanto industrial, oferecendo incremento no valor nutricional.

The hot forming of steels is associated with surface scales formed by layers of oxides whose oxidation kinetics and morphology are not fully understood, where there are several parameters that influence their formation. The chemical composition of carbon steel can influence the oxidation kinetics as well as the morphology of the oxide layers formed at high temperatures, which makes understanding these phenomena challenging. The fact is that each element behaves differently during steel oxidation. The alloying element Si, when present in the composition of steel, oxidizes to form silica, SiO2. This oxide, in turn, at high temperatures, reacts with wustite, FeO, forming a complex oxide, fayalite, Fe2SiO4. The fayalite film has a protective characteristic, increasing the steel's resistance to oxidation and the adhesion of the oxide layers. Other alloying elements, in addition to Si, which have a greater tendency than iron to oxidize, and which form protective oxides, such as Al and Cr, can be added to the steel composition to promote certain resistance to oxidation, reducing the thickness of the layers. of oxides. However, the formation of an alloying element oxide film will depend on its percentage in the steel composition. Alloy elements with low percentages in the steel composition tend not to form a continuous film, being precipitated in the wustite layer, close to the scale/substrate region, and in the steel, participating in internal oxidation. Low concentrations of alloying elements, even if they do not form continuous films, can influence the formation kinetics of other oxides. This review provides a comprehensive overview of the influence of alloying elements in hot oxidation regimes on the kinetics and mechanism of oxidation of carbon steels.