Comparative analysis of prior austenite grain size (PAGS) determination in high grade pipeline steels

https://doi.org/10.3221/IGF-ESIS.78.27

In high‑grade casing steels operated in H2S-saturated environments, the sulfide stress cracking (SSC) is a serious concern. SSC is driven by hydrogen-coupled mechanisms that are sensitive to the microstructure formed from primary austenite. Even though the prior austenite grains (PAG) cannot be directly observed in the final martensitic / bainitic microstructure and its quantification in microalloyed fine-grained steels is complicated, there are several ways of obtaining such information. This paper compares four approaches to the estimation of prior austenite grain size (PAGS) in high-grade pipeline steels: direct measurement of grains from metallographic images after selective grain boundary etching; the lineal intercept method with chord statistics; comparison with standard plates; and the reconstruction of the parent (austenitic) microstructure based on the martensite EBSD maps using reconstruction algorithms that implement the refinement of the orientation ratio and clustering of variants in the MTEX package. The results demonstrate that even without real PAG observed by an operator, EBSD-based parent microstructure reconstruction provides several advantages over optical PAGS evaluation methods. In particular, this approach removes the dependence on the quality of selective grain-boundary etching, reduces operator-related subjectivity in grain selection, and improves the statistical representation of fine grains that may be underestimated during manual optical measurements.