Investigation of role of alloy microstructure in hydrogen-assisted fracture of AISI 4340 steel using circumferentially notched cylindrical specimens

The present study investigates the role of three different microstructures of a high strength steel AISI 4340 in relation to its susceptibility to hydrogen-assisted cracking (HAC), and demonstrates the ability of circumferential notch tensile (CNT) testing in distinguishing the role of the three microstructures. Critical stress intensity factors (SIFs) of CNT specimens of the steel with different microstructures (viz., the spheroidite (the as-received steel), the untempered martensite (the oil-quenched steel) and the tempered martensite (the oil-quenched + tempered steel)) were determined under two conditions at the room temperature, i.e., in air and under in-situ cathodic charging in 3.5% NaCl solution. The CNT tests under the two conditions show the as-received steel (with the spheroidite microstructure) to have a considerably high critical SIF (~58 MPa m^0.5) in air, and an insignificant decrease in critical SIF data when the test condition was changed to the cathodic charging, suggesting an insignificant susceptibility of the steel with the spheroidite microstructure to HAC. In contrast, the oil-quenching of the steel dramatically decreased the critical SIF to ~17 MPa m^0.5 in air, suggesting the highly strained martensitic microstructure to be more susceptible to cracking in air. The low critical SIF of the oil-quenched steel became even lower (~10 MPa m^0.5) under the cathodic charging condition, suggesting increased susceptibility of the martensitic microstructure to HAC. However, tempering of the martensitic microstructure at 300 °C for 2 h raised the critical SIF in air (by ~3 times), i.e., fracture resistance increased dramatically, but no significant improvement in the resistance to HAC was noticed. The hardness data of steels with the different microstructures are consistent with the trend in the corresponding SIF data. The study also provides two practical information: (a) an SIF below 10 MPa m^0.5 under operating load would avoid HAC in AISI 4340 steel in applications, and (b) the relatively new CNT technique is a viable low-cost testing option in assessing the role of microstructural variations in the context of HAC of high strength structural steels.