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The “Gold Dust Defect” that sometimes appears on the surface of the AISI 430 ferritic stainless steel has been related to an improper recrystallization of the material. Hot-rolling, a decisive step in the production process, seems the step where this defect is induced and temperature during this treatment plays a key role. To trace back the origin of this defect, the microstructural evolution of a sample of an AISI 430 was monitored at the different stages in the production process of the steel. In particular, a detailed study combining electron backscatter diffraction with other techniques, such as optical microscopy and scanning electron microscopy has been performed. The whole set of results evidences that after hot-rolling and subsequent annealing, the material contains a large amount of martensite in the center of the cross-section due to a fast cooling of the austenite originated during rolling. In addition, the surface shows a top-layer made up by unrecrystallized subgrains separated from the matrix by an accumulation of chromium carbides. After pickling, the top-layer of subgrains semi-detaches from the surface forming flakes which cannot recrystallize. Most of these flakes do not peel off during the subsequent stages but survive until the end of the production process, forming Gold Dust Defect flakes.

The “gold dust defect” (GDD) appears at the surface of ferritic stainless steels (FSS) and degrades their appearance. Previous research showed that this defect might be related to intergranular corrosion and that the addition of aluminium improves surface quality. However, the nature and origin of this defect are not properly understood yet. In this study, we performed detailed electron backscatter diffraction analyses and advanced monochromated electron energy-loss spectroscopy experiments combined with machine-learning analyses in order to extract a wealth of information on the GDD. Our results show that the GDD leads to strong textural, chemical, and microstructural heterogeneities. In particular, the surface of affected samples presents an α-fibre texture which is characteristic of poorly recrystallised FSS. It is associated with a specific microstructure in which elongated grains are separated from the matrix by cracks. The edges of the cracks are rich in chromium oxides and MnCr2O4 spinel. In addition, the surface of the affected samples presents a heterogeneous passive layer, in contrast with the surface of unaffected samples, which shows a thicker and continuous passive layer. The quality of the passive layer is improved with the addition of aluminium, explaining the better resistance to the GDD.

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.