Impact Characterization of 4340 and T200 Steels by Means of Standard, Sub Size and Miniaturized Charpy Specimens

In this investigation, we performed instrumented Charpy tests in order to characterize the impact properties of three steels, used by NIST for the production of Charpy reference specimens (4340 quenched and tempered with two energy levels, and T200 18Ni maraging steel).For each of the steels, tests were performed on standard E23 Charpy specimens, sub-size specimens of three types (3/4-size, 1/2-size and 1/4-size), and miniaturized specimens of Reduced Half-Size (RHS) geometry.For every combination of steel and specimen type, full transition curves and corresponding transition temperatures were established for absorbed energy, lateral expansion and shear fracture appearance.Topics addressed in this study include:• the relationship between different measures of ductile-to-brittle transition temperature;• comparisons between, and normalization of, characteristic instrumented forces obtained from different specimen types; • the relationship between different measures of absorbed energy;• the relationship between transition temperatures and upper shelf energies calculated from different specimen types; and • the relationship between shear fracture appearance optically measured and estimated from characteristic impact forces.Among the most interesting conclusions emerging from this investigation, we mention the following.-The use of 1/4-size sub-size specimens (thickness = 2.5 mm) has pointed out several issues from the experimental and analytical point of view, and therefore, the use of miniaturized specimens might be preferable instead for the characterization of thin-walled structures or components.-The results of this investigation, in terms of the relationships between test data measured from specimens of different type (full-size Charpy, sub-size Charpy, miniaturized Charpy) and size, were partially complicated by large uncertainties in some of the measured ductile-to-brittle transition temperatures and upper shelf energies.-It looks feasible to obtain conservative predictions of shear fracture appearance (SFA) and SFA-based transition temperatures through the use of empirical formulae based on characteristic instrumented forces.-The comparison between our results and the data fromTable 9 of ASTM A370-14 (Charpy V-Notch Test Acceptance Criteria for Various Sub-Size Specimens) clearly shows that the approach proposed by the standard can work only if the different specimens tested correspond to the same material's fracture behavior (brittle/transition/ductile).If this is not the case, a completely different approach must be sought for correlating Charpy acceptance criteria between full-size and sub-size specimens.