A Fracture Mechanics Study of Stress-Corrosion Cracking of Some Austenitic and Austeno-Ferritic Stainless Steels

Abstract

Crack propagation and plane-strain rupture by stress-corrosion cracking (SCC) have been studied on austenitic stainless steels Grades 304, 316, 316L and a austeno-ferritic steel. Tests were carried out in 45 MgCl2 at 154°C at constant load on fatigue precracked constant tension specimens. These tests show that the presence of macrocrack branching is related to the size of the cyclic plastic zone generated at the fatigue crack tip. A crack propagation threshold KIscc (for a life of 500 h) has been determined. We have noticed the particularly good behavior of the austeno-ferritic steel. The crack propagation rates have been determined for different steel grades and, in the case of the 304 and 316 steels, compared with the rates of anodic dissolution. Fractographic studies have elucidated the crack propagation modes: Up to the end of a region of constant propagation rate, transgranular cracking occurs, and at higher propagation rates, mixed and intergranular cracking is observed. In conclusion, the difficulties of applying fracture mechanics concepts to SCC of austenitic and austeno-ferritic stainless steels (difficulties due to the problem of macrocrack branching) have been overcome by eliminating the plastic zone at the fatigue crack tip (by annealing) such that planar crack propagation is favored.