Prior-Austenite Grain-Size Effect on Nucleation and Growth of Stress Corrosion Cracking in High Strength Steel

Abstract

Stress corrosion tests were conducted on the blunt-notch compact tension specimens of quenched-and-tempered SNCM 8 steel having several different prior-austenite grain-sizes in a 3.5% NaCl solution environment. The effects of grain-size on the nucleation and growth of stress corrosion cracks were analysed from a viewpoint of fracture mechanics. The susceptivity of material to stress corrosion cracking was characterized by five quantities, i. e., the time to crack nucleation at the notch root, t_n, the threshold stress intensity factor, K_ISCC, the stress corrosion fracture toughness, K_SC, the crack growth rate, (da/dt)_II, and the stress intensity factor, (K)_II, at the middle point of the region II in crack growth curves.
The results obtained are summarized as follows:
(1) The crack nucleation life t_n was determined by the stress intensity factor K₀, the yield strength σ_Y and the notch root radius ρ as
t_n=C[2K₀/(σ_Y√πρ)]^m
where C and m are constants independent of prior-austenite grain-size. The value of effective notch root radius obtained by substituting K₀=K_ISCC and t_n=240hrs into the above equation was found to increase with grain size.
(2) When the plastic zone ω was smaller than the grain size d, the K_ISCC value was found to take a nearly constant value of 35 to 45kg/mm^3/2. For the cases of ω/d>1, the values of K_ISCC and (K)_II increased with decreasing yield strength while the value of (da/dt)_II decreased. The larger grain sized material showed lower (K)_II and higher (da/dt)_II values when compared at the same yield strength.
(3) The values of K_ISCC and (K)_II were higher and (da/dt)_II was lower in thin specimens than in thick specimens. The amount of microbranching of cracks was suggested to be a significant clue to the effects of grain size and specimen thickness on growth kinetics.