Impact Energy/Specimen Thickness Relationship for Two Turbine Steels

Abstract

Abstract A steam turbine rotor and blading steel manufactured to chemical composition per ASTM A470, Class 8, and AISI Class 422, respectively, were impact load tested at room temperature in the hardened and tempered, as well as in the annealed, condition with a view to correlate energy levels with specimen thickness. The V-notch Charpy impact specimens used for this purpose included the standard size of 10 by 10 mm and various sub-sizes permitted by ASTM Standard A 370. Results indicate that the impact energy reduced linearly with the decreasing size of the specimen in two regimes, suggesting that two different mechanisms are operating in the fracture initiation for the corresponding specimen thickness ranges. It is, however, established that the energy levels obtained with the lowest thickness specimens (2.5 mm) correlate to the standard energy levels by a multiplying factor of 3 to 4 for all materials except the hardened-and-tempered blading steel, for which the multiplying factor is found to be around 2. A brief account of the fractographic features of both steels for 10, 5, and 2.5 mm thick impact specimens is presented. The relationship obtained between impact energy and specimen thickness is expected to be useful for estimating the impact toughness of service components and assessing their condition, since limitations in component size and/or codes and statutory regulations sometimes preclude removal of large amounts of material for making standard V-notch Charpy specimens.