Controlling Hydrogen Embrittlement in Precharged Ultrahigh-Strength Steels

An Fe-13Co-11Ni-3Cr-1Mo-0.2C steel alloy, processed for ultrahigh-strength and fracture toughness, exhibits three distinct hydrogen trap states in a complex precipitation-hardened martensitic microstructure and is susceptible to severe hydrogen embrittlement (HE) at threshold stress intensity levels as low as 20 MPa root m. The causes of HE susceptibility include very high crack-tip tensile stresses and a reservoir of diffusible hydrogen that is trapped reversibly with a binding energy, E-b, of 11.5 +/- 0.5 kJ/mol at (Fe,Cr.Mo)(2)C precipitates. This reversibly trapped hydrogen repartitions to interstitial sites proximate to the highly stressed crack tip and, subsequently, may retrap at martensitic lath interfaces to produce substantial local hydrogen concentrations and transgranular embrittlement. These results are pertinent to the control of HE in this modern ultrahigh-strength steel with a cadmium-plated coating and codeposited hydrogen (H). Thermal desorption spectroscopy demonstrates that 190 degrees C baking removes the detrimental hydrogen associated with (Fe,Cr,Mo)(2)C traps in both precharged but unplated steel as well as in thin, porous, cadmium-plated steel. Restoration of a high fracture toughness and a ductile fracture mode correlates directly with the removal of hydrogen from (Fe,Cr,Mo)(2)C traps as well as other low-energy trap states. However, the internal H concentration at such traps is at first intensified upon baking of cadmium-plated steel. Later, H egress is retarded by the slow H diffusivity in steel and the barrier action of the cadmium plating. Hydrogen trapped at higher trap binding energy sites is not removed by 190 degrees C baking, but cannot redistribute to the crack tip fracture process zone and does not participate in subcrittcal hydrogen cracking. Strategies for controlling hydrogen embrittlement are proposed based on the information generated.