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Impact Toughness Modification of NIST Low-Energy Charpy Verification Specimens for Testing at Room Temperature



Enrico Lucon, Raymond L. Santoyo


The possibility for NIST to certify Charpy reference specimens for testing at room temperature (21 °C ± 1 °C) instead of −40 °C has been investigated in a previous study, in which a slightly increased likelihood of specimen jamming was observed at the low-energy level (13-20 J). Moreover, it has been contended that the higher impact toughness of low-energy verification specimens at room temperature would not allow verifying the same Charpy machine features as in the case of low temperature (-40 °C) tests, namely linear elastic behavior of the sample and very high maximum forces (typically larger than 33 kN). In this paper, we report on the change of mechanical properties (hardness and absorbed energy) of the AISI 4340 steel low-energy specimens that ensues from the modification of the temperature of the final tempering heat treatment. We established that, if low-energy verification specimens are tempered at 300 °C for 2 hours and then air cooled, they exhibit at room temperature equivalent impact toughness (13 20 J) and post impact behavior (specimen halves projected backward at high speed), as compared to specimens currently on sale for testing at 40 °C. Their hardness is however increased to above 49 HRC. The minimum hardness requirement for low-energy verification specimens, currently set at 44 HRC in NIST specifications, will have to be increased to 49 HRC.
Journal of Research (NIST JRES) -


AISI 4340 steel, Charpy reference specimens, impact toughness, minimum hardness, post-impact behavior, specimen jamming, tempering heat treatment.


Lucon, E. and Santoyo, R. (2017), Impact Toughness Modification of NIST Low-Energy Charpy Verification Specimens for Testing at Room Temperature, Journal of Research (NIST JRES), National Institute of Standards and Technology, Gaithersburg, MD, [online], (Accessed May 25, 2024)


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Created March 27, 2017, Updated November 10, 2018