Crack paths and hydrogen-Afinssisted crack growth response in AlSi 4340 steel

Abstract

A study of the correlation between crack paths and crack growth response was undertaken to define better the elemental processes involved in gaseous hydrogen embrittlement. AISI 4340 steel fractured under sustained load in hydrogen and in hydrogen sulfide over a range of temperatures and pressures, whose crack growth kinetics have been well characterized previously, was chosen for study. Fractographic results showed that crack growth followed predominantly along prior-austenite grain boundaries, with a small amount of quasi-cleavage, at low temperatures. At high temperatures, crack growth occurred primarily by microvoid coalescence. The fracture surface morphology, which is indicative of the micromechanisms for crack growth, was essentially the same for hydrogen and hydrogen sulfide. Changes in fracture morphology,i.e., crack paths, corresponded to changes in crack growth kinetics, both of which depended on pressure and temperature. There was no evidence for crack nucleation in advance of the main crack, and this suggests that the fracture process zone is located within one prior-austenite grain diameter from the crack tip. The experimental results indicate that microstructure plays an important role in determining crack growth response. The prior-austenite grain boundaries are seen to be most susceptible to hydrogen embrittlement, followed by the (110)α’ and (112)α’ cleavage planes. The martensite matrix, on the other hand, is relatively immune. The observed changes in crack growth rate with temperature and pressure in the higher temperature region are explained in terms of the partitioning of hydrogen into the different microstructural elements and the consequent changes in the micromechanisms for fracture.