Hydrogen Susceptibility of Nanostructured Bainitic Steels
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INTRODUCTION
DELETERIOUS hydrogen is introduced into the microstructure of steel as a result of manufacture, welding, corrosion, or galvanic protection.[1–3] High strength steels are thought to be particularly susceptible.[4–7] Nanostructured bainitic steels possess high strength imparted by the fine bainite plates in a matrix of retained austenite.[8–13] Isothermal transformation is the only viable method of producing such a nanostructure in bulk.[10,14,15] These novel nanostructures result from transformation at around 473 K to 523 K (200 °C to 250 °C), with a colossal supersaturation of carbon in the austenite and high supersaturation of carbon in ferrite.[16–20] The large fractions of austenite possible in these steels allow improvement in mechanical properties by transformation-induced plasticity,[21] and the presence of austenite is known to result in increased trapping of hydrogen.[7,22] The diffusion of hydrogen at 300 K (27 °C) is also much slower (2 9 108 times) in austenite than in ferrite.[1,23] The percolating austenite structure has been observed to slow diffusion,[24] and the effective diffusivity of ferrite is also reduced by the large number of defects present.[24] In the current work, nanostructured steels with an ultimate tensile strength of 1.6 GPa were produced with austenite content varying from 0 to 35 vol pct. Electrolytic hydrogen charging to saturation was applied to characterize the maximum detriment to the mechanical properties. Thermal desorption measurements were applied to characterize the potency of austenite as a trapping site in these steels. MATHEW JAMES PEET, Research Associate, is with the Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge, CB3 0FS, U.K. Contact e-mail: [email protected] TOMOHIKO HOJO, Assistant Professor, is with the Department of Mechanical Engineering, Iwate University, Morioka, Iwate, Japan. Manuscript submitted April 26, 2015. Article published online November 30, 2015 718—VOLUME 47A, FEBRUARY 2016
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EXPERIMENTAL METHOD
Tensile specimens were produced with 1.1 mm thickness, 6 mm width, and 20 mm length of the parallel section by electro-discharge machining from cast and rolled ingots of two compositions, Steel A with composition Fe-0.83C-1.98Mn-1.02Cr-1.57Si-1.54Co-0.24Mo wt pct and steel B having composition Fe-0.78C-2.02Mn1.01Cr-1.60Si-3.87Co-1.37Al-0.24Mo wt pct. The materials are from the same stock as previously developed and characterized by Garcia–Mateo et al.[9,10] The tensile specimens were austenitized for 30 minutes at 1223 K (950 °C) in an argon tube furnace before transfer to fan-assisted oven at 563 K (290 °C) and held for 6 hours. A number of samples of steel A were tempered at 773 K (500 °C) to provide comparison against a microstructure with a negligible amount of retained austenite. Previous experiments reveal the high resistance to tempering of the bainitic microstructure in these steels,[25] and tempering at 573 K (300 °C) in this work was not sufficient to reduce the fraction of retai
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