Abstract
The influences of thermal aging and environment on Paris-Erdogan regime fatigue crack growth rates and mechanisms in a high strength Cr–Ni steel (300M) have been investigated. Crack growth rates were measured in inert (vacuum) and aggressive (hydrogen) environments, for quenched and tempered material before and after thermal aging at 500°C. Aging induced an acceleration of crack growth in vacuum at low values of ∆K, but a retardation at high values of ∆K. The crack path was transgranular throughout, and followed carbide/matrix interfaces. These effects were associated with the embrittlement of carbide/matrix interfaces by phosphorus segregation, which facilitated transgranular crack growth at low ∆K, but produced crack tip shielding by voids surrounding the crack at high ∆K. Hydrogen acted synergistically with phosphorus to embrittle carbide/matrix interfaces, but it also localised slip deformation at crack tips, reducing void formation in surrounding material and associated crack tip shielding. This produced an enhancement of crack growth rates at all values of ∆K significantly above the threshold regime. The role of carbide/matrix interface embrittlement constitutes a third mechanism of hydrogen embrittlement and temper embrittlement interaction in fatigue crack growth, additional to those of carbide precipitation and grain boundary embrittlement previously reported.
MST/1142
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