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BRIEF TECHNICAL NOTE

Effect of Tempering on the Ductile-to-Brittle Transitional Behavior of Ni-Cr-Mo Low-Alloy Steel F.W. DelRio 1

&

M.L. Martin 1 & R.L. Santoyo 1 & E. Lucon 1

Received: 5 March 2020 / Accepted: 1 July 2020 # This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2020

Abstract Background About 10 years ago, super-high energy Charpy specimens at the National Institute of Standards and Technology were removed from inventory due to unacceptable variability in absorbed energy, leading to the advent of new methods and materials to reduce the variability and maintain the prescribed energy levels. Objective In this paper, we investigated the ductile-to-brittle transitional behavior of Ni-Cr-Mo low-alloy steel by testing Charpy specimens with side-grooves as a function of the final temper temperature to define the processing conditions for these super-high energy levels. Methods For each temper, absorbed energy and force-displacement data were measured as a function of test temperature; the former was used to assess transition temperature and upper-shelf energy and the latter was used to estimate shear fracture appearance (SFA). Results From the upper-shelf energy results, it was found that two of the temper conditions yielded energies in the super-high energy range and that side-grooves reduced the variability of the energy by preventing the formation of shear lips. From the SFA data, it was shown that the instrumented striker data and fractography were in excellent agreement, with the minor discrepancies attributed to difficulties with transitional fracture surfaces in the fractography and multiple crack arrest points in the instrumented striker data. Conclusions In all, the data provided clear evidence that Ni-Cr-Mo low-alloy steel is a good solution for super-high energy Charpy indirect verification specimens. Keywords Ductile-to-brittle transition . Shear fracture appearance . Impact test . Low-alloy steel . Charpy

Introduction For the last 35 years, the National Institute of Standards and Technology (NIST) has developed standard reference materials, technologies, and processes for Charpy impact testing, which is utilized worldwide to ensure the reliability and safety of structural elements in infrastructure. NIST provides certification to a traceable measurement system via indirect verification specimens at three absorbed energy levels: low energy (13 J to 20 J), high energy (88 J to 136 J), and super-high energy (176 J to 244 J) [1]. About ten years ago, the superhigh energy specimens were removed from inventory due to * F.W. DelRio [email protected] 1

Applied Chemicals and Materials Division, Material Measurement Laboratory, National Institute of Standards and Technology, Boulder, CO 80305, USA

unacceptable variability in the absorbed energy, leading NIST researchers to explore new methods and materials to (1) reduce the variability and (2) maintain the prescribed energy levels. Lucon [2] showed that both (1) and (2) could b