高張力鋼における応力腐食き裂の発生および成長に及ぼす前オーステナイト粒径の影響(<小特集>X線材料強度小特集)

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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% N.aC1 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~, the threshold stress intensity factor, K _ <ISCC>, the stress corrosion fracture toughness, Ksc, 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 Ko, the yield strength σ_Y and the notch root radius p as t_n = C [2K_0/(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 K_0=K_<ISCC> and t_n=240 hrs 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 45 kg/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 Klscc 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.
1979-03-15

高張力鋼における応力腐食き裂の発生および成長に及ぼす前オーステナイト粒径の影響(<小特集>X線材料強度小特集)

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