Study On Stress Corrosion Cracking Of Low-alloy High Strength Steel

Experimental and theoretical approaches towards the study on stress corrosion cracking (SCC) of pipeline steel have been performed. The subject of investigations included SCC susceptibility of API X-60 pipeline steel at various testing conditions in the soil containing water, hydrogen concentration in the steel an the role of hydrogen in cracking process. SCC tests are mainly conducted using slow strain rate testing (SSRT) and sustained load test of single-edge cracked sample. The results showed that a corrosion-induced tensile stress generated during corrosion of API X-60 pipeline steel in the soil containing water. The difference is the corrosion-induced additive stress, which can help the external stress to enhance the plastic deformation. The hydrogen concentration in the low-alloy steel is measured using hydrogen permeation (HP) method, and fracture surfaces are analyzed by scanning electron microscopy (SEM).The main conclusions are as following:1. Transgranular SCC of API X60 pipeline steel in the soil containing water can occur at SSRT withε≤4.2×10-7s-1, and the susceptibility to SCC was about 35%.2. SCC in the soil containing water can also occur under sustained load, and the threshold stress intensity factor of SCC was KISCC=0.73KC, where KC is the plane stress fracture toughness.3. SCC susceptibility (Iδ)of API X60 pipeline steel in the soil containing water increased with reduced potential.4. The corrosion-induced stressσa is tensile stress during corrosion of API X-60 pipeline steel in the soil containing water, which increased with decreased potential. 5. SCC susceptibility of API X60 pipeline steel in the soil containing water increased with increase of the corrosion-induced stressσa.6. The concentration of diffusible hydrogen in the API X-60 during charging hydrogen was increase linearly with the square root of cathodic current ( i ), i.e. C0=-1.58+1.56 i .7. When cathodic polarization ( mV), hydrogen in the soil solution has a tendency of diffusion into the steel. The concentration of diffusible hydrogen in uncharged API X-60 was equivalent to that in hydrogen-charged sample at 5mA/cm2 when mV.8. When cathodic polarization, hydrogen concentration in the steel has played the obvious function for the corrosion-induced stress. But the effect of hydrogen concentration is very low when anodic polarization.9. During charging hydrogen in the solution of 0.5mol/L H2SO4+0.25g/L As2O3, the concentration of diffusible hydrogen in the steel of 30CrMnSiA was increased with the increase of charging current .10. The impact toughness of 30CrMnSiA was decreased with the increase of the concentration of diffusible hydrogen in the steel, but it has not any change when C0≥0.42×10-4%.