Study on Stress Corrosion Cracking of Austenitic Stainless Steels (Report 8)

Effect of Cold Working (Part2)

Abstract

The stress corrosion cracking of austenitic stainless steels occurs not only due to the welding stress, but also due to the working process such as pressing and bending. Especially there are many problems, when the cold-worked material is welded, that is to say, the sensitivity of the material for stress corrosion cracking will be changed by plastic deformation, while at the same time, the value of welding stress in the cold worked material is assumed to become higher.
In this reportt authors discussed the cold work dependence of the sensitivity for stress corrosion cracking in two kinds of austenitic stainless steels such as SUS 27 and SUS 32 in the boiling solution of 42% MgCl₂.
In the material like SUS 27 which transforms to quasi-martensite under cold working, the resistance to stress corrosion cracking grows through cold working. It is estimated that the quasi-martensite acts as an anode spot in the corrodent, consequently the type of corrosion changes into general corrosion.
Next, it is confirmed by experiment that the resistance to stress corrosion cracking of cold worked SUS 27 drops through reheating to over 500°C. This is due to the fact that the martensitic structure is resolved.
On the other hand, the material like SUS 32 shows no change in the time-to-fracture of stress corrosion, even though it has been cold worked. In this material, no martensitic transformation has been observed after the cold working process.
Next, authors measured. the welding stress of the pre-tensioned plates. The tensile residual stress along the weld line reachers a maximum at about 10 mm distance from weld line, and it is almost the same value as the increased yield stress of the cold worked material.
The initiation of cracks in such material is earlier than that in the material without the cold working process. This fact can be observed in both SUS 27 and SUS 32.
From this experiment, it may be said that even though the cold work is confirmed, in laboratory, to be good for stress corrosion cracking, this fact cannot be applied to welded structures for prevention of stress corrosion cracking.