Quantitative metallography of sigma phase precipitates in AISI 347 stainless steel – a comparison between different methods

Abstract

A reliable method to perform volume fraction measurements of sigma (σ) phase in a niobium stabilised steel (AISI 347) has been developed. The most accurate results of the tested methods were obtained using backscattered electrons in a scanning electron microscope (SEM) and samples etched with oxalic acid. Both optical microscopy (OM) and SEM either on polished samples or on etched samples have been evaluated to come to this conclusion. Several etchants were also tested and careful etching with oxalic acid gave a well defined rim. The measurement of σ-phase fraction has been performed using manual point counting and digital image analysis using manual threshold. It was concluded that image analysis is usually to be preferred since it is faster and also results in higher precision The phase boundary caused by etching was evaluated, and it was found that the boundary area should be included in the measurement when using the recommended SEM method.