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 and310 steels and stress corrosion cracking(SCC)of type 321 steel in boiling MgCl_2solution were investigated.The tracing observation showed that the plastic zoneahead of a loaded notched tip of austenitic stainless steels,whether stable or un-stable,was enlarged continuously with time and then SCG would occur while thedelayed plastic deformation developed to a certain extent.However,the initiationand propagation of SCC of type 321 in boiling MgCl_2 solution was independentof the delayed plastic deformation.The threshold values of SCC of type 321 steelsin boiling MgCl_2 solution were much lower than that of SCG during dynamicalcharging of hydrogen at high fugacity.The morphologies of the fracture surfacesof SCC of type 321 steel differed from that of SCG,which was intensely depen-dent upon the K_I at the crack tip.These results lend support to the statement thathydrogen evolved from cathodic reaction does not play a controlling part in theSCC of stainless steel in boiling MgCl_2 solution.