Double influence of hydrogen on fatigue crack growth in heat-resistant steels

Abstract

We investigate fatigue crack growth in cast heat-resistant steel pipes of reforming furnaces in a vacuum, in air, and in gaseous hydrogen in the temperature range 20 – 800°C. It is shown that the character and intensity of hydrogen-induced effects depend on temperature and loading amplitude. For the crack resistance threshold, we discovered the phenomenon of temperature inversion of these effects. Namely, the value of ΔKth in hydrogen increases with temperature up to 400°C and then decreases. Under high-amplitude loading, the influence of hydrogen manifests itself only in the acceleration of crack growth. The ambiguity in the influence of hydrogen on the plastic strain resistance of the material at the crack tip is analyzed on the basis of well-known physical concepts of the influence of hydrogen on the processes of generation and displacement of dislocations. The effects discovered in this work are explained by the realization of different fracture mechanisms and different types of hydrogen-induced effects under different conditions. Thus, at low temperatures (up to 400°C) and high ΔK, one observes a decrease in the tearing strength; the case of low temperatures and low ΔK is characterized by the shear fracture mechanism and the strengthening effect of hydrogen; for high temperatures (≥ 400°C) and low ΔK, the shear fracture mechanism is combined with a decrease in the plastic strain resistance under the influence of hydrogen.