The effect of microstructure on the fatigue crack growth behavior of Age-Hardened high strength steels in a corrosive environment

Fatigue crack growth rates have been measured in laboratory air and in a 3.5 pct NaCl aqueous solution under cathodic polarization of -0.85 V(Ag/AgCl reference electrode) for various agehardened, high strength steels. At low ΔK values in air, the fatigue crack growth resistance of the underaged condition was improved compared to the overaged condition. This improvement can be explained with increased crack closure and slip reversibility. The fatigue crack growth resistance of material containing both coherent precipitates and incoherent precipitates with the matrix was similar to that of the material including only incoherent precipitates because of the restriction of inhomogeneous deformation by the incoherent precipitates. Corrosion fatigue crack growth resistance in the aqueous solution was dependent on aging conditions. The environmental sensitivity was greater in the underaged materials owing to hydrogen embrittlement. The environmental sensitivity of underaged materials containing coherent precipitates with the matrix was improved by the coexistence of incoherent precipitates. The decreased amount of strain to fracture due to hydrogen diffusing into plastic zone led to decreased crack closure.