Multiple etchings methodology: a new approach in multiphase steel characterization.

Abstract

Following the first and second generations, the challenge in obtaining a better balance between strength and elongation is still the main characteristic of the third generation of advanced high-strength steels (AHSS). With this, the use of multiphase microstructures has increased over the last few years. It can be difficult to characterize all the phases with only optical microscopy (OM), so the use of scanning electron microscopy (SEM) is essential for accuracy in cases where researchers lack experience. To expand the possibilities, this research proposed a new approach that would allow experienced researchers to characterize multiphase steels using only OM. A high silicon steel was austempered slightly below martensite start (Ms) temperature for three different time periods in order to obtain different quantities of martensite, bainite and retained austenite. X-ray diffraction was carried out in order to confirm and obtain retained austenite volume fractions, and the results indicated that shorter holding times were not enough to enrich and stabilize retained austenite. Then, each samples was etched with four different etchants. Results showed that the new multiple etchings methodology (MEM) allowed a better visualization of all the phases when viewed together. Beraha martensitc revealed nontempered martensitic microstructures. Sodium metabisulfite revealed retained austenite. LePera and Nital were the best at revealing the evolution of the microstructure over time, even with the changes which occurred due to martensite tempering. SEM images confirmed the results obtained via MEM. LAY DESCRIPTION: For improving safety, environmental protection, mechanical resistance and others issues, many different steel grades have been studied. These grades were named depending on their mechanical properties. The current generation is the third, which is still searching for one of the main antagonists in material science: the best balance between mechanical resistance (how strong it is) and elongation (how long it can be stretched to). In order to achieve this goal, many researchers are studying variations in the production processes. During production, the materials are able to internally change their basic microstructure, named phases. In the past, steels were usually produced to have only a few phases. Today's advanced high strength steels (AHSS) can have many. Each of these phases has its own characteristics. The main focus of this research was to give a new way of identify these phases using an optical microscope. For revealing these phases, etchants are normally used. The etchants used in this research are capable of tinting each of these phases with a different colour or tone. So the purpose of this work was to suggest a new approach in order to allow for more precise identification of the phases in the steel. The results were positive, showing that looking at the samples as a whole is better than the traditional methods. Also, different etchants' characteristics were observed during the changes obtained by this work's chosen processes.