Influence of the heat treatment on the hydrogen uptake and embrittlement of 42CrMo4 steel (AlSI 4140) under gaseous hydrogen charging

RIP2025-00016: To determine the microstructural influence on the hydrogen uptake and embrittlement behavior, 42CrMo4 was heat treated to achieve different microstructures with increasing martensite content and tensile strength. The diffusion kinetics and the trapping behavior of the different material conditions were characterized by the use of electrochemical permeation measurements and TDS analysis resulting in a decreasing effective diffusion coefficient with increasing martensite content. Specimens charged at 25 °C took up similar amounts of hydrogen regardless their microstructural differences with maximum amounts for specimens charged at 1000 bar being between 0.5 and 0.65 wt.-ppm. When the specimens were charged at 200 °C, an increasing hydrogen uptake with increasing material strength and martensite content to between 1.05 (low strength, no martensite) and 2.25 wt.-ppm (high strength, high martensite content) was observed at a hydrogen partial pressure of 1000 bar. Furthermore, CLTs were conducted at the materials yield strength resulting in a fracture of the highest-strength material charged at 1000 bar and 25 °C. To describe the influence of the microstructure on 42CrMo4s embrittlement behaviour in-situ charged SSRTs and the determination of the material's critical hydrogen content were conducted. The results reveal a strongly increasing embrittlement tendency with increasing material strength and martensite content. The determined critical hydrogen contents were equal to 1.84 wt.-ppm (low strength, no martensite), 0.75 wt.-ppm (medium strength, low martensite content), and 0.23 wt.-ppm (high strength, high martensite content) respectively.