A Fe-13Co-11Ni-3Cr-1Mo-0.2C steel alloy, processed for ultra-high 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 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, Eb, of 11.5 0.5 kJ/mol at (Fe,Cr,Mo)2C 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 ultra-high strength steel with a cadmium-plated coating and co-deposited hydrogen (H). Thermal Desorption Spectroscopy demonstrates that 190oC baking removes the detrimental hydrogen associated with (Fe,Cr,Mo)2C 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)2C 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 190oC baking, but cannot redistribute to the crack tip fracture process zone and does not participate in subcritical hydrogen cracking. Strategies for controlling hydrogen embrittlement are proposed based on the information generated.
Ultrahigh-strength steel (UHSS) enables high performance aerospace structures that require high tensile strength and fracture toughness.1,2 A secondary hardening UHSS, Fe-13Co-11Ni-3Cr-1Mo-0.2C (AerMet 100,(1)), was developed to provide plane strain fracture toughness (KIC) in excess of 120 MPa m, doubling that of (1) AerMet 100 is a trademark of Carpenter Technology Corp., Reading, PA 19612-4662 older steels such as AISI 4340 and 300M, each at constant yield strength ( YS) of 1750 MPa.3,4 This strength is produced by a homogeneous distribution of nanoscale coherent (Fe,Cr,Mo)2C alloy carbides in Fe-Ni martensite laths that are highly dislocated due to Co retardation of recovery.5,6 The high KIC is achieved by advanced melting to minimize S + P and inclusion contents, austenitization to control undissolved carbides and grain size, and aging to optimize austenite precipitates along martensite lath interfaces.2,4,7,8
Ultra-high strength steels are susceptible to severe internal hydrogen embrittlement (IHE) as well as hydrogen environment embrittlement (HEE), and Fe-13Co-11Ni-3Cr-1Mo-0.2C steel is no exception.9 Several studies demonstrated subcritical HEE at apparent threshold stress intensity (KTH) levels as low as 20-30 MPa m when the microstructure, optimized for high KIC (~ 130 MPa m), was stressed in neutral chloride near the free corrosion potential.10-12 UHSS is often electroplated for corrosion resistance, introducing the potential for IHE.13,14 Thomas and coworkers demonstrated that optimally aged Fe-13Co-11Ni-3Cr-1Mo-0.2C steel is susceptible to severe IHE at KTH levels as low as 20 MPa m and produced by diffusible H contents as low as 1 part-per-million by weight (wppm).15 Both IHE and HEE in Fe-13Co-11Ni-3Cr-1Mo-0.2C steel are predominantly transgranular (TG), associated with cracking of interfaces in the martensit
