Hydrogen Embrittlement Resistance of High Strength 9260 Bar Steel Heat Treated by Quenching and Partitioning

The influence of microstructure on hydrogen embrittlement of high strength steels for fastener applications is explored in this study. Space limiting applications in areas such as the automotive or agricultural industries provide a need for higher strength fasteners. Albeit, hydrogen embrittlement susceptibility typically increases with strength. Using a 9260 steel alloy, the influence of retained austenite volume fraction in a martensitic matrix was evaluated with microstructures generated via quenching and partitioning. X-ray diffraction and scanning electron microscopy were used to assess the influence of retained austenite in the matrix with different quenching parameters. The quench temperatures varied from 160 °C up to 220 °C, and a constant partitioning temperature of 290 °C was employed for all quench and partitioned conditions. The target hardness for all testing conditions was 52-54 HRC. Slow strain rate tensile testing was conducted with cathodic hydrogen pre-charging that introduced a hydrogen concentration of 1.0-1.5 ppm to evaluate hydrogen embrittlement susceptibility of these various microstructures. The retained austenite volume fraction and carbon content varied with the initial quench temperature. Additionally, the lowest initial quench temperature employed, which had the highest austenite carbon content, had the greatest hydrogen embrittlement resistance for a hydrogen concentration level of 1.0-1.5 ppm.

Chemical heterogeneity enhances hydrogen resistance in high-strength steels

The antagonism between strength and resistance to hydrogen embrittlement in metallic materials is an intrinsic obstacle to the design of lightweight yet reliable structural components operated in hydrogen-containing environments. Economical and scalable microstructural solutions to this challenge must be found. Here, we introduce a counterintuitive strategy to exploit the typically undesired chemical heterogeneity within the material’s microstructure that enables local enhancement of crack resistance and local hydrogen trapping. We use this approach in a manganese-containing high-strength steel and produce a high dispersion of manganese-rich zones within the microstructure. These solute-rich buffer regions allow for local micro-tuning of the phase stability, arresting hydrogen-induced microcracks and thus interrupting the percolation of hydrogen-assisted damage. This results in a superior hydrogen embrittlement resistance (better by a factor of two) without sacrificing the material’s strength and ductility. The strategy of exploiting chemical heterogeneities, rather than avoiding them, broadens the horizon for microstructure engineering via advanced thermomechanical processing.