Stress corrosion crack initiation and propagation in longitudinally welded 304 austenitic stainless steel

Abstract

The stress corrosion cracking (SCC) initiation and propagation behaviour of a longitudinally welded 304 austenitic stainless steel has been investigated in an aqueous solution of 1M NaCl + 0.5M HCl. The SCC in the base metal and heat affected zone (HAZ) initiates and propagates perpendicular to the applied tensile stress and the cracks initiate and propagate, initially, in the transgranular mode. The crack growth mechanism changes into the intergranular mode when the crack size attains a particular value. The cracks in the fusion zone are likely to initiate and to propagate in planes inclined at an angle of ∼30° to the maximum principal tensile stress. Optical microscopy and SEM observations show that the cracks propagate along the boundary of dendrites where δ ferrite exists, indicating that δ ferrite tends to dissolve preferentially. In spite of the difference in microstructure, no remarkable difference is found in the kinetics of crack initiation and propagation among the three zones investigated (base metal, HAZ, and weld metal). The SCC initiation lifetime increases with decreasing applied stress. The relationship between the SCC initiation lifetime t_i and the applied stress σ can be expressed as t_i = Aσ⁻ⁿ. The SCC initiation rates expressed in terms of the change in crack density with time dD_C/dt are similar to each other in the base metal and HAZ, but higher than those in the weld metal. The stress dependence of SCC initiation rates can be empirically expressed as dD_C/dt = ασ^β. The SCC initiation rates of the material tested are independent of the applied stress if the applied stress is below the yield strength of the material. However, SCC propagation rates increase under general yielding conditions.