High Ductility Alloy AF955® (UNS N09955) for Enhanced Resistance to Hydrogen Embrittlement
Alloy AF955® (UNS N09955) solution annealed and precipitation hardened to a condition of lower strength and higher ductility provides a much higher resistance to hydrogen embrittlement with respect to 955-120K and 955 – 140K grades of API 6ACRA.
We tested alloy 955 with about 110 ksi actual 0.2 % offset yield strength and 126 ft∙lbs TAE (average absorbed energy upon transverse impact testing at -60 °C) obtaining the following results:
- 81 % elongation ratio (ER) between elongation under cathodic polarization and elongation in inert environment;
- most of the fracture surface of specimens tested under cathodic polarization shows a ductile profile, altough several brittle initiating sites with transgranular fracture mode are observed in some peripheral area.
For comparison, 955-120K and 955-140K grades of API 6ACRA showed about 45 % and 33 % elongation ratio, respectively and several sites of the fracture surface in the peripheral area of specimens tested under cathodic polarization with brittle, transgranular (cleavage) mode.
In other words, by decreasing the mechanical strength of about 15-20 %, we obtain AF955® hydrogen embrittlement (HE) resistance corresponding to about two times the HE resistance of alloy AF955® grades of API 6ACRA specification and about five times the HE resistance of alloy UNS N07716 grades of API 6ACRA.