Evaluation of a method to characterize material inhomogeneity in ferritic steels within the ductile-to-brittle transition regime

The ASTM “Standard Test Method for Determination of Reference Temperature, To, for Ferritic Steels in the Transition Range,” (ASTM E1921) has been developed to characterize the onset of cleavage cracking in the ductile-to-brittle transition regime for ferritic structural steels. This standard determines To, the reference temperature at which the median fracture toughness of a 25 mm thick specimen is 100 MPa m. Material inhomogeneity has been shown to have a strong effect on To, and suggestions on how to characterize and quantify the effects of inhomogeneity have been developed within a proposed annex for E1921.This study demonstrates that for small data sets neither the current E1921 outlier procedure nor the method in the proposed annex can effectively distinguish between homogeneous and inhomogeneous materials. This study also shows that the bimodal inhomogeneity analysis in the proposed annex only accurately characterizes materials if (1) at least 30 specimens are tested, and (2) the difference between the To values that describe each unique homogeneous population within the inhomogeneous material is greater than 20 °C, and (3) the percentage of the smallest homogeneous material population is greater than 20%.