Internal hydrogen embrittlement of ultrahigh-strength AERMET 100 steel

Near-peak-aged AERMET 100 is susceptible to severe internal hydrogen embrittlement (IHE) at 23 degreesC, if a sufficient diffusible hydrogen content is present, compromising the high toughness of this ultrahigh-strength steel (UHSS). Evidence includes the threshold stress intensity for subcritical IHE (K-TH) as low as 10 pct of the plane-strain fracture toughness (K-IC) and a fracture-mode transition from microvoid coalescence to brittle transgranular (TG) cracking, apparently along martensite lath interfaces and cleavage planes. The KTH value decreases from a K-IC value of 132 to 143 MParootm to 12 MParootm, and the amount of brittle TG fracture increases to nearly 100 pct as the concentration of diffusible H increases from essentially 0 to 8 wppm, with severe embrittlement in the 0 to 2 wppm H regime. The IHE is time dependent, as evidenced by increasing KTH values with increasing dK/dt and K-independent subcritical crack growth rates, and is attributed to diffusional H repartition from reversible trap sites to the stressed crack tip. The partition distance is similar to1 mum, consistent with the fine-scale microstructure of AERMET 100. The causes of the susceptibility of AERMET 100 to TG IHE are very high crack-tip stresses and a reservoir of mobile H trapped reversibly at (Fe,Cr,MO)(2)C precipitates. These factors enable repartition of H to misoriented martensite lath interfaces and interstitial sites near cleavage planes, with each prone to decohesion along a connected path. Predissolved H also reduces the ductile fracture toughness of AERMET 100 at high loading rates, perhaps due to reduced void growth caused by H trapped strongly at undissolved metal carbides.