Michinori Takano1 and Saburo Shimodaira1
1The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai
The effect of nitrogen on the stress corrosion cracking susceptibility of 18-8 austenitic stainless steels was investigated by transmission electron microscopy.
In the specimen to which added no nitrogen (specimen A), there was a tendency towards a cellular distribution of dislocations, but the specimen to which added nitrogen (specimen B) showed a planar arrangement of dislocations. It was confirmed from these observations that the stress corrosion cracking susceptibility of specimen B was augmented.
These thin-foil specimens were further examined after exposure to solutions which cause stress corrosion cracking, and the mechanism of the stress corrosion cracking was discussed.
When thin-foil specimen B was deformed in a boiling 42 percent magnesium chloride solution, the pitting initiated at active slip planes and advanced along these slip steps, but the chemical attack ceased if the stress was removed. Wherese, in the specimen A showing cross slip, no pitting was observed.
The mechanism of the stress corrosion cracking may be considered as follow:
When the restricted slip planes are continuously exposed to a corrosive environment during plastic deformation, the pitting is initiated at these fresh surfaces. The solute atoms are thus segregated to slip planes with the moving dislocations inside the specimen. There occures a high concentration of solute atoms on the slip planes and in their vicinity. The stress corrosion cracking, once initiated at the fresh surface, would proceed in the specimen with an electrochemical process.
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