Ultrasonic fatigue testing in hot hydrogen gas

• A novel ultrasonic setup enables VHCF tests in pressurized hydrogen gas at 400 °C. • (V)HCF lifetimes of low-alloy QT steel in air and in hydrogen at 400 °C are presented. • The material shows a fatigue limit in hot air, but not in hot hydrogen gas. • Samples failed in hydrogen at 63% of the lowest stress amplitude for failures in air. • The environmental effect of hydrogen is not suppressed at the high cyclic frequency. A novel ultrasonic fatigue testing setup has been developed for performing high and very high cycle fatigue tests in hot hydrogen gas at elevated pressures. The setup consists of a hydrogen pressure chamber containing the specimen, which is mounted and sealed at a node of the ultrasonic load train. Quenched and tempered 42CrMo4 steel is tested in hydrogen gas at 400 °C and 35 bar, as well as in hot air at 400 °C for comparison. Tests were performed at a load ratio of R = –1 with specimens containing artificial surface defects of ø100 µm to reduce scatter in fatigue lives. A fatigue limit is observed in hot air but not in hot hydrogen. Failures in hot hydrogen were found at 63 % of the lowest stress amplitude leading to failure in hot air. Fracture surfaces after testing in hot hydrogen appear rough and uneven with secondary cracks and small voids, whereas they are much smoother after fracture in hot air. Hydrogen embrittlement by hot hydrogen gas is clearly visible in ultrasonic fatigue testing, making this method highly useful for rapid material screening and comparison.