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Influence of hydrogen on the elastic modulus of 316L and XM-19 austenitic stainless steels, The
Scott, Kevin M.
Scott, Kevin M.
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2024
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Abstract
The interest in de-carbonizing the global economy is accelerating, and the push to move away from carbon based energy will likely grow well into the future. Hydrogen is an attractive energy carrier for its abundance and specific energy content. While hydrogen is currently produced as a byproduct of the oil and gas industry, the scale required for economically viable and environmentally friendly hydrogen production, transportation and custody transfer will require signicant infrastructure development in the next one to two decades. Understanding mass flow of hydrogen through up-stream, mid-stream and down-stream processes will be instrumental for process mass and energy balances as well as custody transfer applications.
Coriolis-type mass flow meters are high accuracy, high precision vibratory flow measurement devices that are well suited for hydrogen mass flow measurement applications. They are generally built with austenitic stainless steel alloys that have minimal susceptibility to traditional hydrogen damage mechanisms. However, these vibratory devices rely on constant elastic modulus or predictable changes in elastic modulus to maintain their accuracy and precision throughout their service life. There is currently little understanding on how hydrogen aects the elastic modulus of austenitic stainless steels and the fundamental mechanism of any change.
Two fully austenitic stainless steels, 316L and XM-19, were studied for their availability and applicability to Coriolis flow meter construction. Material was received in bar form, annealed, then machined to 5 mm OD by 190 mm long bars for impulse excitation elastic modulus characterization. After thoroughly characterizing the uncharged bars, some samples were charged in gaseous hydrogen at 138 MPa and 573 K for two weeks while others were charged at 41 MPa for the same duration and temperature to achieve varying equilibrium hydrogen concentration. The measured elastic modulus after charging showed a systematic increase with hydrogen concentration up to 1.2 at.% at a rate of approximately 0.4 %/at.%H.
The mechanism of the measured increase of elastic modulus with hydrogen concentration was probed with Mossbauer spectroscopy to understand and quantify changes to Debye temperature and the electronic state of the iron atoms. 316L foils 10 m thick were acquired in the as-rolled condition, solution annealed and electrolytically charged with hydrogen. Hydrogen concentration was estimated through a correlation with lattice expansion as measured by XRD. Mossbauer trials of the charged foils were conducted at 222 K for several hours to achieve a sucient count rate to minimize uncertainty in the measurement, after which the samples were heated to 343 K and held for 30 minutes to allow for hydrogen diusion out of the foil. The foils were then aged at room temperature for at least one week to diuse all hydrogen from the foil and re-characterized with Mossbauer spectroscopy at 222 K. The results showed a positive shift in Debye temperature as well as a positive chemical isomer shift. The former is indicative of an enhancement in iron bond stiffness with hydrogen loading while the latter indicates a lowering of iron nuclear electron density.
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