Study on Stress Corrosion Cracking of Austenitic Stainless Steels (Report 2)

Analysis of Stress Corrosion Process

Abstract

In almost every research which was done by the tensile stress corrosion test, the sensitivity for cracking was compared in terms of the fracture time of test piece.
The fracture time of stress corrosion cracking consists of two processes: the induction period which is the period from the start of test to the initiation of a crack and the propagation period which is the period from the initiation of crack to the fracturet of test piece.
There is no relation between induction period and propagation period, but these vary indepe-ndently of each other by the changes in stress value, material and size of test piece. Therefore, because the fracture time is long, it does not follow that the sensitivity to cracking is low.
In this report, to make clear these weak points of previous works, we tried to apply the con-tiouous microscopic observation to the stress corrosion test for type 304L stainless steel and its welded metal, and analysed the stress corrosion process: the initiation and propagation of crack. The test pieces used in this report were notched tensile plate type.
The induction period was not constant, but became longer with a decreasing stress value.
The fracture time of base metal was shorter than that of deposited metal, but the induction period of base metal was longer than that of deposited metal.
The average propagating speed in the deposited metal was lower than that in the base metal, because the crack propagation through the deposited metal was hindered by the ferritic phases dotted in the austenite matrix.
Moreover, the propagating speed of crack decreased linearly with decreasing stress.