Assessing the Banding Degree of Martensite in the Bainite Matrix through EPMA

Abstract

Due to the difficulty of distinguishing the grain boundaries between martensite and bainite in fast-cooling microstructures, few good methods were reported to accurately assess the banding degree of martensite or bainite. In present work, a novel method has been developed to meet this challenge. The hot rolled bars of drill steel 23CrMoNi, which had martensite bands distributed in the bainite matrix, were used in this research. The banded structure in this hot rolled 23CrMoNi was closely related to the segregation of the alloying elements such as Cr, Ni and Mo. These alloying element mappings were first acquired by electron probe micro analyzer (EPMA). A new data processing method was developed to correlate the segregated element mapping with the banded structure. The banding assessment was conducted on the processed binary images of element mappings according to ASTM E1268-19. This method was well verified by assessing the banded pearlite in the ferrite matrix, whose banding degree can be easily accessed through microstructure difference. It was shown from the quantitative analysis results of the hot rolled 23CrMoNi that the banded distribution of martensite was greatly optimized by the adjustment of hot rolling process.