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,
tn, the threshold stress intensity factor,
KISCC, 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
tn was determined by the stress intensity factor
K0, the yield strength σ
Y and the notch root radius ρ as
t
n=C[2K
0/(σ
Y√πρ)]
mwhere
C and
m are constants independent of prior-austenite grain-size. The value of effective notch root radius obtained by substituting
K0=
KISCC and
tn=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
KISCC 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
KISCC 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
KISCC 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.
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