Effect of Hydrogen on Mechanical Properties of 23Co14Ni12Cr3Mo Ultrahigh Strength Steel

In order to evaluate the effect of hydrogen on mechanical properties of 23Co14Ni12Cr3Mo ultrahigh strength steel, the specimens were electrochemically hydrogen charged for different times. The tensile property, fatigue fracture behavior, fatigue crack growth (FCG) behavior, and threshold stress intensity (ΔK _th) of the samples were studied. The fracture morphology was characterized by scanning electron microscopy. It was shown that tensile strength decreases from 2300 to 2000 MPa, critical fatigue stress from 577 to 482 MPa, and ΔK _th from 27.4 to 14.3 MPam^0.5 with the increasing hydrogen contents from 0.0001 to 0.0008 wt.%. Hydrogen enhances the FCG rate from 2.4 × 10^?3 to 3.6 × 10^?3 mm/cycle at ΔK = 80 MPam^0.5 in the hydrogen-charging range. Microscopic observation showed that the tensile fracture is a combination of overload microvoids and some intergranular regions for 0 h, and isolated areas of transgranular (TG) fracture are observed with brittle cleavage for 24-72 h. The fatigue fracture is ductile for the uncharged specimens, while the hydrogen-charged specimens show mainly brittle TG fracture. These results suggest that hydrogen degrades the fracture behavior of 23Co14Ni12Cr3Mo ultrahigh strength steel.