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 (~58MPam0.5) 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 ~17MPam0.5 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 (~10MPam0.5) under the cathodic charging condition, suggesting increased susceptibility of the martensitic microstructure to HAC. However, tempering of the martensitic microstructure at 300°C for 2h 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 10MPam0.5 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.

本研究探讨了高强度钢AISI 4340的三种不同微观结构与其氢致开裂（HAC）敏感性的关系，并证明了圆周缺口拉伸（CNT）试验在区分这三种微观结构作用方面的能力。在室温下的两种条件（即在空气中以及在3.5%氯化钠溶液中原位阴极充氢）下，测定了具有不同微观结构（即球状珠光体（原始状态的钢）、未回火马氏体（油淬钢）和回火马氏体（油淬 + 回火钢））的该钢种CNT试样的临界应力强度因子（SIF）。在这两种条件下进行的CNT试验表明，原始状态的钢（具有球状珠光体微观结构）在空气中具有相当高的临界SIF（约58MPa·m⁰.⁵），并且当试验条件改为阴极充氢时，临界SIF数据下降不显著，这表明具有球状珠光体微观结构的钢对HAC的敏感性不显著。相比之下，钢的油淬处理使空气中的临界SIF大幅降低至约17MPa·m⁰.⁵，这表明高度应变的马氏体微观结构在空气中更易开裂。油淬钢的低临界SIF在阴极充氢条件下变得更低（约10MPa·m⁰.⁵），这表明马氏体微观结构对HAC的敏感性增加。然而，马氏体微观结构在300°C回火2小时使空气中的临界SIF提高（约3倍），即抗断裂能力显著提高，但未发现对HAC的抵抗能力有显著改善。具有不同微观结构的钢的硬度数据与相应的SIF数据趋势一致。该研究还提供了两条实用信息：（a）在工作载荷下，SIF低于10MPa·m⁰.⁵可避免AISI 4340钢在应用中发生HAC；（b）相对较新的CNT技术是评估高强度结构钢HAC情况下微观结构变化作用的一种可行的低成本试验选择。