Volume 34, Issue 3 (May 2006)
Effects of Available Energy and Impact Rate on Charpy Absorbed Energy in the Upper Shelf
Advances in steelmaking technology have led to the introduction of structural steels with increased strength and toughness, to the extent that the energy absorbed by Charpy samples tested in the upper shelf region commonly approaches or exceeds the maximum available energy in conventional pendulum impact machines. In these circumstances, it becomes relevant to re-examine the validity of absorbed energy values measured in cases where the absorbed energy is a large fraction of the available energy. According to ASTM E 23 clause 10.1, “Absorbed energy values above 80 % of the scale range are inaccurate and shall be reported as approximate.” This is because “the velocity of the pendulum decreases…during fracture to the point that accurate impact energies are no longer obtained.” Although the 80 % limit has been accepted by consensus for decades, it has been challenged recently with the contention that excess capacities substantially larger than 20% are required to provide machine-independent values of impact energy. The present work was performed to test whether the amount of excess energy (i.e., the difference between available energy and absorbed energy) affects the Charpy absorbed energy in the upper shelf. Two structural steels typical of current linepipe grades have been used in the project, an experimental steel and a commercial steel, both with bainite/ferrite microstructures but with different toughnesses. Charpy samples have been tested at impact rate in pendulum and drop-weight machines. The results confirm the acceptability of absorbed energy values up to 80 % of the machine capacity, in support of the requirement stated in ASTM E 23.