A Study Of Stress Corrosion Behavior Of Sensitized 304 Stainless Steel Under Three-Point Bending

Stress corrosion cracking is one of the most common failures for 304 stainless steel.It is a delayed cracking caused by the synergetic effect of stress and corrosive environment, which has the feature of brittle fracture and may cause significant losses of production and even human lives. In this thesis, three point bending test, slow strain rate test (SSRT), metallographic analysis, energy disperse spctroscopy (EDS), scanning electron microscope (SEM) and other testing techniques were used to study the mechanism of stress corrosion cracking for 304 stainless steel. Three point bending test by using of non-standard miniature specimen was proposed to study the stress corrosion behavior of the steel. The method provides the scientific basis for semi- nondestructive evaluation of the in-service equipment.The main work and results of the thesis are as follows:(1)Electrochemical test was carried out for 304 stainless steel which had different heat treatment degrees at the sensitization temperature of 650℃. Acidic chloride solution was used in the test. From the polarization curve, corrosion potential curve and corrosion current density curve, it was found:when sensitization time increased from 0h to 100h, the corrosion potential decreased and corrosion current density increased. However, when the sensitization time reached 200h, the corrosion potential increased and the current density decreased. The results indicated that with the increasing of sensitization time the corrosion resistance decreased until the sensitization time reached a certain range.(2) Slow strain rate testing (SSRT) was carried out on standard specimens. It was shown that with the increase of sensitization time, the elongation decreased and reduction of area increased. It indicated that the susceptibility to stress corrosion cracking increased with the increasing of sensitization time. It was also observed that the pit formation that caused stress concentration was the main reason of stress corrosion cracking.(3) Three point bending test technique by using of non-standard miniature specimen was developed. When the maximum tensile stress at lower surface of specimen was 85 MPa, the corrosion rate increased with the sensitization time increased form 0h to 100h. However, when the sensitization time reached 200h, the corrosion rate decreased. Cr content was analyzed when the maximum tensile stress at lower surface of specimen was 170 MPa. The result showed that Cr element content decreased with the increasing of sensitization time and the loss of Cr element was proportional to the tensile stress distribution on the specimen surface, moreover, the deflection of the loading point of the three point bending specimen had a smooth incubation period. After incubation the deflection increased drastically. With the increasing of sensitization time, the smooth incubation period increased followed with a shorter period time to failure.