Remanufacturing Technology For Liquefied Petroleum Gas (LPG) Spherical Tank

As a kind of pressure vessel used to store chemical medium,spherical tanks havebeen widely used in chemical, oil, metallurgical, urban gas industry in our country. About40% of spherical tanks are used to store liquid petroleum gas (LPG). Stress corrosioncracking would occur in the spherical tanks made of low alloy steel since LPG usuallycontains H2S over the criterion and water, which threaten the long-period and safeoperation of the spherical tanks.Based on the remanufacturing technology of LPG spherical tanks, the mechanismsof stress corrosion cracking(SCC) at wet H2S environment are analyzed, welding residualstress is measured for SPV50Q and 16MnR welded plates, the pre-fatigue crack M-WOLspecimen stress corrosion cracking tests in three H2S concentrations are performed underthe different welding conditions. The main research contents and conclusions are asfollows:1. The common forms of stress corrosion cracking in wet H2S environment aresummarized in the paper. The fact that hydrogen blister (HB) and hydrogen inducedcracking(HIC) do not depend on stress is pointed out,HB and HIC are only related to thepurity and microstructure of material, et al. While sulfide stress corrosion cracking(SSCC) and stress oriented hydrogen induced cracking (SOHIC) have something to dowith stress. Welding residual stress is one of the main factors causing SSCC and SOHIC.2. Welding residual stresses of SPV50Q and 16MnR welded plates are measured bynon-destructive magnetoelastic technique. The results show that the welding residualstress in 16MnR welded plates is lower than that in SPV50Q welded plates in as weldedconditions. Some welding residual stresses exceed 0.7σs for SPV50Q steel,while all thewelding residual stresses are less than 0.7σs for 16MnR steel. The post-weld heattreatment(PWHT)decrease the welding residual stress and gives uniformer the stressdistribution.The PWHT effect of 16MnR is better than that of SPV50Q. Welding residualstress in a SPV50Q spherical tank was measured in this paper, which indicates furtherthat welding residual stress became better by PWHT.3. After welding, the brittle and hard microstructure exists in coarsecrystalline heataffect zone for SPV50Q, which causes larger welding residual stresses. In wet H2S III摘 要environment, stress corrosion cracking occurs easily in SPV50Q welded structurewithout heat treatment. Because of the better weldability, welding residual stress in16MnR welded structure is lower. PWHT may prevent SSCC,but HIC is easily inducedin16MnR steel if high H2S concentration environment exists.4. In view of the different materials, welding conditions and concentrations ofhydrogen sulfide, stress corrosion cracking tests of modified WOL specimens werecarried out. The results show that stress corrosion crack growth rate da/dt decreases withthe decrease in the concentration of hydrogen sulfide, and the critical stress intensityfactor for SCC KISCC increases. For SPV50Q and 16MnR, the maximal growth rate andthe minimal critical stress intensity factor for SCC appear in the coarsecrystallineheat-affected zone under as-welded condition, which denotes that the resistance to sulfidestress corrosion cracking is the weakest in wet hydrogen sulfide environment. Post weldheat treatment is helpful to decrease the crack propagation rate da/dt and increase thecritical stress intensity factor KISCC for SCC, and eventually improve the resistance toSCC.5. In any case, the stress corrosion crack growth rate of SPV50Q material are morerapid than that of 16MnR material, which show that SPV50Q steel is more sensitive tostress corrosion cracking. This demonstrates that ductility of SPV50Q steel is relativelylow with the improvement of its yield and tensile strength after the quenching andtempering, especially the lath martensite exists in the coarsecrysalline heat-affected zoneafter welding. As far as the critical stress intensity factor on stress corrosion KISCC isconcerned, KISCC of the 16MnR...