Assessment of Hydrogen Embrittlement Susceptibility and Mechanism(s) in Quench and Tempered AISI 4135 Steel Using A Novel Fast Fracture Test in Bending

A newly proposed rapid fracture test in four-point bending was used to evaluate the effect of tempering on the hydrogen embrittlement (HE) susceptibility of an AISI 4135 steel, where it was tempered to four different strength (or hardness) levels. It was observed that HE susceptibility increases with the increase in hardness. It was shown that there will be minimal impact of hydrogen (H) on the fracture of materials with hardness 37 HRC and below, even if they are completely saturated with H. On the other hand, H will have similar detrimental effect on fracture properties of quench and tempered (Q and T) steels having hardness higher than 45 HRC. Ductile to brittle transition behavior was observed for a critical hardness (or strength) range as well as for a critical concentration level of H. Additionally, a critical H concentration was observed to exist for each of the strength levels. Fractography was performed in addition to microstructural characterization using transmission electron microscopy (TEM). A very good correlation was observed between the fast fracture test results and fractography. The fast fracture test was further compared with a conventional incremental step load (ISL) test for the evaluation of HE susceptibility. The ISL test results and fracture surface characteristics corroborate very well with the observations from the fast fracture test. This study successfully establishes the fast fracture test as a novel technique to study HE susceptibility and mechanism(s).