Comparison of Fractographic Behaviors of Electrochemically Hydrogenated and Liquid N2 Treated 4340 Steel of Various Microstructures

Fractographic behaviors of electrochemically hydrogenated low alloy 4340 steel samples were compared with low temperature (liquid N₂) treated samples of same grain sizes and hardnesses. 4340 samples of grain size range of 10-100 μm and hardness range of 41-52 HRC were electrochemically charged with hydrogen in 0.5 M H₂SO₄ + 5 mg/l As₂O₃ solution over 0-40 min charging time. Liquid N₂ treatment of the samples of same microstructural range was conducted by encapsulating the sample inside an insulating container (thermos) while the temperature (-136^oC) of the sample was tracked in-situ during the tensile test. Fracture surfaces of liquid N₂ treated samples showed exactly same fractographic characteristics (quasi cleavage) irrespective of the grain sizes and hardnesses while electrochemically hydrogenated samples showed range of fractographic features depending on the grain sizes and hardnesses.