Microstructural Mitigation of Hydrogen Environment Embrittlement of Ultra-High Strength AerMet(Trademark) 100

摘要

AerMet(exp TradeMark) 100, a secondary precipitation hardened martensitic steel, has excellent strength (Sigmays = 1725 MPa) and toughness (K(sub IC) = 130 MP square root of am) when optimally tempered at 482C. However, it is susceptible to severe hydrogen environment assisted cracking (HEAC) in near-neutral 3.5% NaCl at very low threshold stress intensities (K(sub TH)) and with rapid Stage II plateau rates for subcritical crack propagation (da/dt(sub II). Interaction between environmental hydrogen and the complex microstructure influences the HEAC response, and modifications to heat treatment could improve HEAC resistance, particularly considering H-trap interactions. The effect of aging condition on HEAC of AerMet(TradeMark) 100 was examined. Specimens were aged for 5 h at 482C, 510C and 538C or studied in the as-quenched condition; then stressed at slow constant displacement rate in 3.5% NaCl at two polarization levels. When polarized to -0.9 V(sub sce), all aged specimens showed substantial HEAC with low K(sub TH) (10'20% of K(sub IC)) and high da/dt(sub II) (up to 30nm/s); but the as-quenched condition had substantially higher KTH (70% of K(sub IC)) and slow da/dt(sub II)(1nm/s). The HEAC path was mostly transgranular for all conditions, but predominantly intergranular when aged at 538C. The aging independence of HEAC at -0.9 V(sub SCE) may, be consistent with a mechanistic explanation based on H diffusion control of crack growth rate, but material dependent terms in the model for da/dt(sub II) and the detail of the model are uncertain. It is not possible to design alloy composition and processing conditions based on such existing models. Qualitatively, increasing incoherence of M2C precipitates does not result in reduced da/dt(sub II), perhaps due to the counterbalancing effects of increased trapping causing reduced H diffusivity but increased H solubility.