Characterisation of High Temperature Oxidation Phenomena during AISI 430 Stainless Steel Manufacturing under a Controlled H2 Atmosphere for Bright Annealing

Localised-in-the-edge oxidation of four AISI 430 alloys was investigated after an industrial bright annealing process. The oxidised surface of each specimen was characterised by X-ray photoelectron spectroscopy (XPS), Raman-spectroscopy and SEM. The results showed that the selective oxidation of Cr and Mn took place at the coil edges of AISI 430. This led to the formation of an oxide scale based on Cr2O3 and MnCr2O4. On the other hand, the formation of Cr(OH)3 and MnOOH in the outer part of the oxide scale was related to the effect of the H2–N2 environment on the annealing furnace. The results concluded that the composition of Cr and Mn in the alloy determined the composition of the oxidation products. However, the effect of annealing time was minor in this oxidation mechanism, which slightly contributed to an increase in the cation diffusion from the steel to the oxide. Finally, the results obtained indicate that the colouration of the coil edges displayed in each studied material varies according to the alloy chemical composition and annealing time.

Oxidation Analysis of 304 Austenitic Stainless Steel Sheet after Intermediate Annealing

Intermediate annealing to stainless steel can eliminate the residual stress, and improve plasticity and ductility effectively, but oxidation after different annealing processes has considerable difference. In this research, three annealing processes, low temperature annealing with ordinary heat treatment furnace, high temperature annealing with ordinary heat treatment furnace and high temperature bright annealing, were used to gain insight into the mechanics of intermediate annealing. Specimens of 304 austenitic stainless steel sheets were adopted in the intermediate annealing experiments which pre-deformation was 15%. The specimens’ surface microstructure were observed by scanning electron microscope (SEM), and the specimen oxidation, which happened in the annealing process, were also analyzed. The results show that bright annealing must be adopted in high temperature annealing.

Effect of Heat Treatment Conditions on Tube Hydroformability

Tube hydroforming provides a number of advantages over conventional stamping process, including fewer secondary operations, weight reduction, assembly simplification, adaptability to forming of complex structural components and improved structural strength and stiffness. It can produce wide range of products such as sub-frame, engine cradle, and exhaust manifold. In this study, the effect of heat treatment conditions, such as post seam annealing(PSA) and bright annealing(BA), on the ovality and hydro-formability of steel tubes has been investigated. Hydroformability has been estimated as the bulging height obtained depending on the various process parameters such as an internal pressure, axial feeding, and heat treatment conditions. The ovality and forming height were found to strongly depend on material properties after heat treatments.