Mecanismos da formação da superfície heterogênea no aço inoxidável ferrítico 439

Abstract

The aim of this work was to study the inhomogeneous surface mechanisms on 439 ferritic stainless steel grade, which corresponds to ASTM UNS S43932 (Fe-17wt% CrNb-Ti) steel. This steel has been used in a lot of final applications. The surface is a visible part of the 439 grade. It is a design item and it is the main limiting factor to the increase of the development of new applications and the volume of sales. Understanding the mechanisms is understanding the transformations of the surface during the production process. The work was carried out through observations, understanding the phenomena of hot oxidation and removal of oxides layers by chemical pickling, relating science to industrial practice. The surface heterogeneity has been caused by parallel, whitish and sinuous longitudinal strips defect, with or without the visual porous appearance. The heterogeneous surface is a degradation of the stainless steel finishing. The oxidation progress at interface of the oxide/metal is corrugated, with gulfs and protrusions aspects. The mechanism is diffusion-controlled and begins in the formation of the de-chromed layer and a silicon oxides formation at the interface region. The 2nd phase particles, the oxides residues and some mechanical marks, among other superficial discontinuities, act like cathodes and the adjacent regions become anodic, causing the formation of cavities when HNO3 and HF acids have been used in the pickling process. The main agent is hydrofluoric acid (HF). Therefore, the cavity formations on the coil surfaces increase significantly due to the use of HF acid. So, a new optimized standard for the manufacturing process was suggested.