The Mechanism of Slow Crack Growth and Stress Corrosion Cracking in Austenitic Stainless Steel

The mechanism of hydrogen induced slow crack growth (SCG) of Type 321 and Type 310 steels and stress corrosion cracking (SCC) of Type 321 steel in boiling MgCl2 were investigated. The tracing observation showed that the plastic zone ahead of a loaded notched tip of austenitic stainless steel, whether stable or unstable, was enlarged continuously with time, and SCG would occur while the delayed plastic deformation developed to a certain extent. However, the initiation and propagation of SCC of Type 321 in boiling MgCl2 was independent of the delayed plastic deformation.The threshold values of SCC of Type 321 steels in boiling MgCl2 were much lower than those of SCG during dynamic charging of hydrogen at high fugacity. The morphologies of the SCC fracture surface of Type 321 steel differed from those of SCG, which were heavily dependent on the KI at the crack tip. These results lend support to the statement that hydrogen evolved from cathodic reaction does not play a controlled part in the SCC of stainless steel in boiling MgCl2.