Etching Methods for the Microstructural Characterization of a Heat Resistant 2.25Cr-1Mo-0.25V Weld Metal

Abstract

Submerged arc welded heat resistant 2.25Cr-1Mo-0.25V weld metals are increasingly used for hydrocrackers and thick-walled pressure vessels in the petrochemical industry. They are exposed to high pressures and temperatures of more than 450 °C. In order to satisfy the high demands with respect to strength and toughness, they are typically subjected to a post-welding heat treatment at temperatures in the range of 691 – 719 °C. During heat treatment, a large number of changes take place in the weld's microstructure. These changes can partly be observed during a light microscope examination. Etching studies described in the literature are mostly concerned with determining phase fractions in multi-phase steels. This work shows how selective etching and precipitation etching allow for contrasting various microstructural features. While the etching solutions Klemm I, aqueous sodium metabisulfite, and picric acid are suitable for revealing the former austenitic grain structure, the dendritic solidification structure of samples in as-welded condition can be contrasted using color etching techniques according to LePera, Klemm I, and Beraha I. For an identification of changes in the shape of dendrites, it is recommended to perform picric acid etching. Color etchings according to Klemm I and LePera are recommended for revealing larger carbides, as they provide a good contrast between the carbides which appear bright, while the surrounding microstructure is colored.