オーステナイト系不銹鋼の応力腐食割れに関する研究(第7報)

Abstract

We investigated the effects of cold working on the stress corrosion cracking of austenitic stainless steels in the boiling solution of 42% MgCl2.In the case of an 18-8 stainless steel which is transformed partially into quasi-martensite by cold working, both induction period and crack propagation time are changed by cold working. Especially, the crack propagation time is very much lengthened by heavy cold working.But in the case of other stainless steels, both induction period and crack propagation time are hardly changed by cold working.As a result of the quasi-martensite obstructing the advance of crack, the crack propagation time is lengthened. And this effect is generally called "keying effect".The electro-chemical potential of test piece indicated at the crack-initiation time is a little changed by cold working. When the test piece is cold worked lightly such as 7% or 15%, it indicates a slightly lower potential than the original test piece. But when the test piece is cold worked heavily such as 30% or 45%, it indicates a slightly higher potential than the original one.The slip bands occurring at the time of cold working are never attacked during the stress corrosion test, but the slip bands occurring in the MgCl2 solution by the applied test load are gradually attacked during the stress corrosion test. When a slip band occurs, its fault plane has not any protective film, If this unprotected fault plane appears in the MgCl2 solution, it is attacked. But if it appears in the atmosphere, its area is suddenly oxidized and goes into passive state.The higher the applied stress is, the more of cracks appear on the surface of tested specimen. When the test piece is cold worked, this relation is shifted toward upward stress followed by the rising of yield point.