Research On Stress Corrosion Failure And Failure Probability Of Austenite Stainless Steel

Stress corrosion cracking caused by chloride ions is frequent with the wide application of austenitic stainless steel in pressure equipment. The fracture caused by the stress corrosion cracking (SCC) is a type of brittle break with low stresses, which usually causes disastrous consequences without any warning, and seriously affects people’s lives and the property safety. Due to affected by the environment, stress and material, the failure mechanism and process of the stress corrosion cracking are complex, and the stress corrosion cracking failure is of great uncertainty. There are still many problems need to be solved although a lot of studies have been carried out on the mechanism, influencing factors and crack growth rate of the stress corrosion cracking. In order to further deepen the understanding of the stress corrosion process of austenitic stainless steel, the author takes several critical issues of the stress corrosion to study. The main research topics are as follows:the influencing factors of the siress corrosion cracking, crack initiation and propagation, and failure probability. Only these questions are solved, we can take measures to prevent the stress corrosion damage.The effects of the medium pressure on the stress corrosion sensitivity were studied through the slow tensile experiment of Type S32168 austenitic stainless steel in the acidic chloride environment. The results show that the higher temperature is, the greater the impact of the medium pressure on the stress corrosion sensitivity. The correlation, between the medium pressure and temperature and the stress corrosion sensitivity, were analyzed by the grey correlation theory. The influence of temperature on the stress corrosion sensitivity is more significant than that of the medium pressure. At the same time, the author analyzed the effects of the macro and micro structures on the stress corrosion. The results show that chloride ions are enriched in the gap, and the concentration of chloride ion increases with the increase of the distance from the gap mouth. The martensite structure in austenitic stainless steel caused by welding reduces the pitting potential and improves the stress corrosion sensitivity.The initiation and growth of pitting corrosion were analyzed based on the electrochemical theory. The electrochemical reaction of pitting corrosion process was analyzed. The corrosion potential was calculated and the critical pitting potential was measured. The criterion of pitting initiation was established. The statistical analyses of the corresponding parameters in the criterion were done. The results show that temperature, pH, and the passive current density are all satisfied with the normal distribution. Based on the stress-intensity interference model, the randomness of pitting initiation was analyzed. Taking the test data of Type S30403 stainless steel in FeCl3 solution as an example, the random process of pitting initiation was studied. The results show that the Non Homogeneous Poisson Process can describe the random process of pitting initiation very well. Based on the theory of pitting initiation at MnS inclusion, the formula for calculating the growth dimension of semi ellipsoidal pit in the depth direction is given, and the randomness of the parameters in the formula were analyzed.The sizes of the pits were measured. Both the influence of pitting morphology on the crack initiation and the mechanism of the crack initiation in the pit were analyzed, and the crack propagation rate and its randomness were also studied. The degree of the stress concentration in the pit varies with the depth-width ratio and aspect ratio of the pit. Affected by the stress concentration, most cracks initiate at the pit mouths or pit shoulder. However, due to the presence of secondary pitting pits, the crack initiation location may be changed. Based on the Clark model, the crack growth rate was determined in high temperature and low concentration chloride environment, and the crack propagation probability which is caused by the uncertainty of temperature and yield strength was analyzed.Based on the objective and subjective uncertainty considered, the probability models of stress corrosion failure were established. The failure modes of crack initial propagation, leakage and fracture were analyzed. Taking the main parameters in failure functions as random variables, the randomness of the failure was analyzed. In the further consideration of the fuzziness of the stress corrosion failure, the fuzzy failure probability model of the stress corrosion was established based on the fuzzy probability fracture mechanics.In a word, the author studied the influencing factors of the stress corrosion, pitting corrosion and its randomness, crack initiation and growth, and failure probability. The influence rule of the media pressure on stress corrosion was obtained. The criterion of pitting initiation is proposed, and the fuzzy probability model of stress corrosion failure was established. The research can promote the improvement and development of the stress corrosion theory and the risk analysis theory. It is meaningful for the austenitic stainless steel equipment safety running. However, the mechanism of the stress corrosion cracking is so complicated that a large number of failure data are needed in order to calculate the failure probability accurately.