The Quantitative Analysis of Fatigue Fracture Surfaces of HT50 (TMCP) and Mild Steel in Sour Crude Oil

Corrosion fatigue crack propagation tests of HT50 (TMCP) and mild steel and corrosion fatigue life test of HT50 (TMCP) were conducted in sour crude oil containing 400ppm H2S. The detailed SEM observations were carried out on these fracture surfaces obtained. The crack propagation rate of these steels in the sour crude oil was accelerated in the stress intensity factor range, ΔK higher than about 16MPa√m. The ratio of acceleration of the crack propagation rate, da/dN in the sour crude oil in the ΔK of 30MPa√m was 7 for HT50 (TMCP) and mild steel. In the higher stress range of S-N curve for the round notched bar specimens of HT50 (TMCP), the influence of sour crude oil containing 400ppm H2S on the fatigue life was observed obviously. The fatigue life ratio decreased with decreasing the stress to unity at the lower axial stress. It was found that the crack propagated predominantly on the cleavage fracture surface in association with brittle striation in the accelerated crack propagation area of fracture surface in the sour crude oil. The striation spacing per cycle, S obtained from the measured striation spacing ΔS versus ΔK curve was well coincident with the da/dN-ΔK curve in the accelerated crack propagation area. It might be concluded that the environmental enhancement of the fatigue crack propagation rate in sour crude oil is dependent on hydrogen evolved by reaction between the structural steels and hydrogen sulfide in the sour crude oil.