Effects of Microstructure on Fracture Behavior of Hydrogenated Notched 4340 Steel: A Quantitative Study

Abstract

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. 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. Atomic hydrogen can be generated at the surface of metals and alloys by thermal dissociation of gaseous diatomic hydrogen or by electrochemical decomposition of hydrogen bearing aqueous solutions and vapor condensates. Once inside hydrogen-sensitive materials, e.g., high strength steel, hydrogen atoms can interact with various microstructural features resulting in enhanced tendency for brittle fracture. The focus of this work was to study the embrittling effects of hydrogen on low alloy 4340 steel introduced from two different sources (i) vapor condensate of vaporized hydrogen peroxide and (ii) electrolytically generated from electrochemical solution of 0.5 M H2SO4 + 5mg/l As2O3. The compatibility of vaporized hydrogen peroxide treatments with high strength 4340 steel was studied in the first phase of the work. Embrittlement of high strength AISI 4340 steel was observed as a result of condensation of the vapor during exposure to the vaporized hydrogen peroxide. Notched four point bending samples of AISI 4340 steel were tested using the standard test methods of ASTM F519-06 to quantify susceptibility to hydrogen embrittlement in this aggressive service environment. No embrittlement effects were observed for samples exposed to strictly vapor phase hydrogen peroxide for concentrations up to 1000 ppm H2O2 and exposure times of 4.8 h. Higher concentrations of 1300 and 1600ppm H2O2 led to the condensation of the vapor throughout the process chamber and brittle fracture of samples. These results were confirmed by examination of the fracture surfaces of samples using scanning electron microscopy. Samples that were not considered embrittled possessed dimpled fracture surfaces consistent with ductile failure. Embrittled samples exhibited intergranular fractures along prior austenitic grain boundaries near the root of the notch. In the second phase of the work, the effects of hydrogen on embrittlement characteristics of low alloy 4340 steel was studied using double-notched tensile samples that were electrochemically charged in-situ with hydrogen in a 0.5 M H2SO4 + 5 mg/l As2O3 solution. The mechanical response of samples with prior austenitic grain sizes of 10, 40 and 100 μm with martensitic hardness of 41-52 HRC were examined after hydrogen charging times of 0-40 min. As expected, increases in hydrogen charging time and hardness resulted in decreased failure strains and decreased evidence of ductile fracture. Harder samples showed predominant intergranular fracture close to the notch and a combination of ductile and apparent cleavage fracture away from the notch. Softer samples showed quasi-cleavage fracture close to the notch and predominant ductile fracture as the distance from the notch increased. Increasing the strain rate for hydrogen charged samples resulted in decreased failure strains and increased evidence of brittle fracture. Finally, increasing the size of the prior austenitic grains was found to lead to sample failure via brittle transgranular quasi cleavage for the large-grained softer samples while brittle intergranular fracture dominated the large-grained hardest samples.