DEPENDENCY OF THE INITIAL STAGE OF STRESS CORROSION CRACKING BEHAVIOR OF AUSTENITIC STAINLESS STEEL ON THE STRESS LEVEL

Abstract

In this paper, the initial stage of stress corrosion cracking behavior of austenitic stainless steel SUS304 was investigated in boiling 42% magnesium chloride aqueous solution. Nucleation and initial propagation of stress corrosion micro cracks were analyzed both in surface and in fracture surface from microstructural aspect in a wide range of stress from around threshold stress where no plastic deformation occurred macroscopically to 0.2% proof stress. The main results are summarized as follows;(1)Stress corrosion cracking behavior at the initial stage is dependent upon the stress level. Namely, at higher stress levels, intercrystalline cracking originating from triple grain boundaries occurred, 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 is 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 nucleats from broken point of the passive film on triple grain boundaries where the material is concentrically strained and it propagates to interconnect triple grain boundaries.