Research On Stress Corrosion Cracking Of Typical Pressure Vessel Steels In Wet Hydrogen Sulfide Service

Stress corrosion cracking of pressure vessel in wet hydrogen sulfide service is a more often occurred damage phenomenon, which will lead to serious consequence. Especially for recent years, with the deeply oil exploiting in our country and increasing of the import oil from Middle-East area, there are more and more sulfide element in crude oil in refining industry, and it make the stress corrosion cracking in wet hydrogen sulfide service increasingly outstanding. A lot of researches on tress corrosion cracking in wet hydrogen sulfide service have been made all over the world, and some useful results are obtained, however, because of lots of factors which will effect stress corrosion cracking susceptibility, and with some limitations for all kinds of test technologies and understanding of stress corrosion cracking mechanism, there are many problems left need to be resolved and discussed.In this paper, the stress corrosion cracking susceptibilities of three typical types of pressure vessel steels: low alloy steel 16MnR, low carbon steel 20g and austenitic stainless steel 316L in wet hydrogen sulfide service are investigated, by slow strain rate test and constant strain pre-cracked specimen test. Among the environments factors, the separate and interaction effects of H2S concentration, CI-concentration, temperature and PH values on stress corrosion cracking susceptibilities are considered. The tests are designed by even design method, all parameters of medium are divided into several grade within its ranges and combined evenly. The discrepancies of stress corrosion cracking susceptibility between different types of steel, base matal and welds are analyzed. By stepwise regression analysis of test results, the notabilities of effect on stress corrosion cracking susceptive indexes of medium parameters are revealed. Based on experiment research, a micro stress corrosion cracking model of pressure vessel steel in wet hydrogen sulfide has been developed. The main conclusions of this paper are as follows: 1. The slow strain rate tests(SSRT) and the stepwise regression analysis of stresscorrosion cracking susceptibility indexes F(A) of 16MnR indicate that, in wet hydrogen sulfide service, the effect of H2S concentrations(x1) is most notable, PH value(x4) comes second, there is no notable correlativity between CF concentration(X2) and F(A), temperature(x3) and FbS(x1) play interaction role on the indexes F(A). With increasing of HS concentrations(x1), stress corrosion cracking susceptibility indexes of 16MnR steel increase. The interaction fitting equation of stress corrosion cracking susceptibility indexes F(A) of 16MnR base metal can be expressed:2. By comparing the slow strain rate test datum of 16MnR base metals with that of welds, it is found that the welds of 16MnR is more susceptible to stress corrosion cracking than the base metals. The main reasons are: the strength and rigidity of welds are higher than those of the base metals; the metallographic structures uniformities of welds are not good as the base metals, the crystals of welds are bulky; and the higher residual stress of welds region would also accelerate process of origin and propagation of stress corrosion crack.3. Constant strain pre-cracked stress corrosion tests of 16MnR steel in refining high sulfide content oil service are carried out, and it indicated that, thethreshold stress intensity factors KISCC are not higher than 32 MPa, and underthis load condition, the stress corrosion crack propagation rate is about 10-4mm/h.4. The stress corrosion cracking susceptive indexes of 20g base metal and welds in the saturated H2S solution and less saturated H2S solution of desulfuration system are low. There are lots of cavity on surface of microstructure fracture of specimen, stress corrosion cracking features are not obvious. The temperatures are more aggressive to stress corrosion cracking susceptibility than the other parameters, with the temperature increasing, susceptive indexes increase; however, the H2S concentrations of satu...