Effects of microstructures on hydrogen induced cracking of electrochemically hydrogenated double notched tensile sample of 4340 steel

Quantitative fractographic characteristics of 4340 steel is demonstrated for a grain size range of 10−100 µm and hardness range of 41-52 HRC. Double-notched tensile samples were electrochemically charged in-situ with hydrogen in 0.5 m H2SO4+5 mg/l As2O3 solution for 0-40 min charging time. Hydrogen induced fracture initiations were analyzed by novel metallographic investigation of the “unbroken” notch while the overall fractographic behaviors were examined by the scanning electron microscopic imaging of the fracture surfaces of the actually broken notch. Effect of hydrogen was predominantly manifested as intergranular fracture for the harder samples and quasi-cleavage fracture for the softer counterparts. 10-40 µm samples showed the maximum intensity of the hydrogen induced fracture features (intergranular and/or quasi-cleavage) close to the notch which gradually reduced with increasing distance from the notch. The largest grained samples (100 µm) however showed brittle behavior even in absence of hydrogen with similar intensity of percent fracture features at all distance from the notch, while presence of hydrogen intensified the overall percent brittle fractures with their intensities being highest close to the notch. Finally, the brittle fracture characteristics of the hydrogen embrittled samples were shown to be distinguishably different from that of the liquid nitrogen treated samples of same grain sizes and hardnesses.