抄録
There have been many published papers about stress corrosion cracking (SCC) of austenitic stainless steels in chloride aqueous solutions. By paying too much importance to acceleration of cracking, however, many of these studies were made at stress levels high enough to cause plastic deformation. Thus, few were concerned in the nucleation mechanism and propagation characteristic of stress corrosion cracking at low stress levels that were practically important in considering the materials resistance in corrosive environments of austenitic stainless steels used for structural materials. Furthermore, to elucidate the actual mechanism of fracture, it is necessary to have a good understanding of the process of failure itself. But, for example, the fracture morphology that is related to such process has not been studied fully systematically. Moreover, in the previous studies concerning the fracture morphology, the extremely early initial stage of stress corrosion cracking that is substantially important has not been fully discussed and many points are left to be studied in future.
Therefore in this paper, stress corrosion cracking behavior at the initial stage of austenitic stainless steel SUS 304 was investigated in boiling 42% magnesium chloride aqueous solution. Nucleation and initial propagation of stress corrosion micro cracks were analyzed both in original surface and in fracture surface from microstructural aspect in a wide stress range from near threshold stress to 0.2% proof stress, where no plastic deformation occurred macroscopically.
The main results were summarized as follows:
(1) Stress corrosion cracking behavior at the initial stage was dependent upon the stress level. Namely, at higher stress levels, intercrystalline cracking originating from triple grain boundaries was observed, but as the stress level was lowered, it was replaced with transgranular one accompanied with corrosion grooves.
(2) Micro crack growth from the small corrosion grooves was an important factor determining the value of the threshold stress of material used.
(3) At a stress level near 0.2% proof stress, the stress corrosion crack nucleates from the broken point of the passive film on triple grain boundaries where the material is concentrically strained and it propagates so as to interconnect triple grain boundaries.
