Strain Rate Effects on Hydrogen Embrittlement Characteristics of Notched 4340 Steel
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Strain Rate Effects on Hydrogen Embrittlement Characteristics of Notched 4340 Steel Mobbassar Hassan Sk, Ruel A Overfelt National Center of Excellence for Research in the Intermodal Transport Environment, Materials Engineering, Auburn University, Auburn, AL 36849, United States Tel.: +1 334 844 5940; fax: +1 334 844 3400, [email protected] , [email protected] Key words: Electrolytic hydrogen charging, 4340 steel, prior austenitic grain size, hydrogen embrittlement, intergranular fracture, quasi - cleavage
ABSTRACT The effect of strain rate on hydrogen embrittlement of low alloy 4340 steel was studied using double-notched tensile samples electrochemically charged in-situ with hydrogen in 1N H2SO4 + 5 mg/l As2O3 solution. The mechanical response of samples with prior austenitic grain sizes of 10 and 40 μm and martensitic hardness of 43-52 HRC were examined after hydrogen charging times of 0-20 min. Increasing the strain rate for hydrogen charged samples resulted in decreased failure strains and increased evidence of brittle fracture. Brittle fracture surfaces for the harder samples showed primarily intergranular fracture while softer samples exhibited predominantly quasi-cleavage. INTRODUCTION Hydrogen based fuel has been identified as a promising alternative to fossil fuel to reduce the emission of greenhouse gases and to enable sustainable energy supplies worldwide [1, 2]. In spite of such promising applications of hydrogen as a prime energy source, serious technical challenges exist due to metallurgical interactions of hydrogen with fracture sensitive metals and alloys [2]. Hydrogen has caused major failures [2] in the oil and gas industries. The uptake of hydrogen into steel tanks, pressure vessels and pipelines has induced unexpected and premature fracture of such structures. Of the microstructural characteristics of hydrogen induced failures, intergranular fracture along the prior austenitic grain boundary (PAGB) is well known where hydrogen is believed to primarily diffuse along the prior austenitic grain boundaries atomically weakening those grain boundaries and inducing their decohesion [3]. The potential mechanisms for grain boundary decohesion at the atomic level have been discussed thoroughly elsewhere [e.g., 3]. Another interesting fractographic feature often associated with hydrogen embrittlement is termed quasicleavage and displays characteristics often associated with brittle fracture and plasticity. Recent results [4, 5] discuss some mechanistic aspects associated with quasi-cleavage but the precise details and applicable models remain the subject of controversy [6]. However, the strain rate dependence of hydrogen induced failure in 4340 steel for slow strain rates is yet not clear. Krom et al [7] proposed a numerical simulation showing the effect of strain rate on hydrogen distribution and ductility in round bar tensile specimens. However no concrete criterion was reported for the change in ductility based on the hydrogen concentration and the strain rate.
Finally, hydrogen embrittlement of high
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