Abstract
Influences of applied stress on stress corrosion cracking (SCC) and those of annealing on the corrosion morphology of α′-martensite were investigated in order to clarify the role of stress in the mechanism of SCC of 18-8 stainless steel in H2SO4-NaCl solution.
The fracture time of specimens by SCC was decreased with increasing the degree of prestraining and applied stress. Compared with a prestrained specimen, the fracture time of a solution treated specimen was dependent strongly on the level of applied stress. The deep corrosion striations were observed on a prestrained specimen without applied stress and also on the compression site of the U-bend specimen after immersion in the solution. The SCC behavior of the steel in the solution was thought to be closely related with the preferential dissolution of martensite in the austenitic matrix.
Wide corrosion grooves were found for a subzero treated specimen in the immersion test as the result of preferential dissoluiton of the α′-martensite. The harf-value width of α′-martensite peaks in X-ray diffraction patterns of subzero treated specimens decreased with increasing annealing time at 673 and 773 K. By the annealing at 673 K for 300 h prior to the immersion test, the needle-shaped protuberances remained on the side wall of the corrosion grooves. This corrosion morphology was the same as that of annealing at 1283 K for 0.5 min when the α′-martensite disappeared. These results suggest that the dissolution of α′-martensite may be related to the presence of a large number of lattice defects in it.
A prominent role of applied stress in SCC is thought to form the strain induced martensite containing a large number of lattice defects in the austenitic matrix of the steel, enhancing the SCC susceptibility.
