نقش عمليات سطحي در بروز ترك SCC در فولاد زنگ نزن آستنيتي L ۳۱۶ عمليات حرارتي شده در محلول كلريد منيزيم

The Effect of Surface Treatment on Postponement of SCC Crack in Heat Treated 316L Austenite Stainless Steel in MgCl2 Solution

Austenite stainless steel are inclined susceptible to cracking due to stress corrosion in Cl-ion environment; the mode of cracking can be in the form of Trans granular, Inter granular or both of them. The type of crack depends on the concentration of corroding solution, applied stress on specimen, test temperature, chemical composition and metallurgical mode of alloy. In this study, stress corrosion in sensitized 316L austenite stain­less steel and the role of surface preparation on postponement of SCC crack initiation has been investigated. Test specimens were made according to ASTM G30 standard by using constant strain technique and then ex­posed to pure MgCl2 solution with 20, 30, and 40 wt% concentrations at 90oC. For a week, the specimens were taken out of solution everyday and their surface were studied using stereo microscope. With surface treatment such as sand blast and shot blast on specimens, it was found that residual stresses on the specimen surfaces caused crack initiation to significantly postpone in this specimens compared to specimens with untreated surface. Also, the number of cracks in surface-treated specimens were less than untreatment specimens and with an increase in chloride ion concentration in solution, crack initiation time decreases, and the number of cracks and crack depth increase. At the end of tests, the specimens were examined with electronic and opti­cal microscopy and the effect of surface preparing on crack initiation and propagation were investigated. It was observed that, stress corrosion cracks of sensitized AISI 316l alloy initiate from specimen surface and gradually propagate in sensitized grain boundary and change fracture mechanism from Trans granular stress corrosion to inter granular.