Study On Stress Corrosion And Safety Assessment Of X80 Steel Pipe Welded Joints

The stress corrosion cracking (SCC) behaviors of X80 pipeline steel in analogysoil solutions(the solution of 0.5mol/L Na2CO3 and 1mol/L NaHCO3 and NS4solution) were studied using electrochemical measurement and slow strain ratetesting(SSRT) and scanning electron microscopy。The susceptibility to stresscorrosion cracking (SCC) of the welded joint of X80 pipeline steel in solution wasanalyzed. The electrochemical corrosion behaviors of the X80 steel and its weldedjoint in the analogy soil solutions were investigated.The stress corrosion cracking (SCC) behaviors of X80 pipeline steel in solutionof 0.5mol/L Na2CO3 and 1mol/L NaHCO3 and NS4 solutions were studied using slowstrain rate testing(SSRT) and scanning electron microscopy. At cathodic potentials,obvious quasi-cleavage fracture was observed in the fracture area of specimens insolution of 0.5mol/L Na2CO3 and 1mol/L NaHCO3. At open circuit potential andanodic potential, ductile fracture was the main fracture pattern. The SEM crackingmode was transgranular with the feature of quasi-cleavage and hold typical feature ofstress corrosion in NS4 solutions when applied negative potential. At anodic potential,fracture appearance was typical ductile fracture and obtained abundant dimples. SSRTresult showed that the general tendency in the studied potential range was that withnegative increasing of potential, fracture time, elongation and reduction in area ofspecimens decreased, mean the susceptibility to SCC increased. When appliedpotentials of welded joint and base metal homology are the same, welded joint ismore sensitive than base metal in SCC, the results showed that all tensile testspecimens cracked in weld metal and heat affected zone (HAZ).The critical stress intensity factor KISCC and crack propagation velocity wereobtained by the Wedge-open loading constant displacement specimens. The resultsshowed that the heat affected zone (HAZ) had the minimal KISCC, largest crackpropagation velocity and hold poor resistant properties of stress corrosion crack. Theinfluential factor of the SSCC property of X80 pipeline steel was analysed throughmetallographic structure,fracture appearance observation and hardness examination .By the numerical analysis of the FEM, stress field and concentration of hydrogendistribution property from the crack tip were obtained. The experimental results showmostly concordance with the FEA results.A three-dimensional finite element numerical simulation is performed on the material physical absorption variables and temperature nonlinear relationship of theX80 steel pipes with SYSWLED program. Three-dimensional FEM using mobile heatsource for analysis transient temperature filed and welding stress filed incircumferential joint of pipes is founded. Influence law of the phase transition andadherent wire energy of axial and hoop residual stresses on inner and outer surface ofpipes are investigated.The primary and second safety evaluation was proceed to toe external crack andweld deep crack of X80 pipe joints with the principle of"fit utilize",according toBS7910 test, based on the examination of X80 steel and bonding joints fracturetoughness, adopted FAD evaluation method. Considering the effect of stressconcentration and residual stress, the external allowable crack size and deep allowablecrack under the condition of given loading.