Corrosion Fatigue Strength of Welded Joints

Abstract

The fatigue crack propagation and fatigue life tests were performed in sour crude oil containing H2S of 400 ppm and in air to study the corrosion fatigue strength of the welded jouints of ship hull structural plate which is essential for the evaluation of fatigue life of the crude oil tank components of VLCC's. The test specimens were machined out of the CO2 welded joints and boxing fillet welded joints of a 25 mm thick, ship hull structural plate KA36 (TMCP). The main results obtained follow : 1) The fatigue crack propagation rate was much higher in sour crude oil than in air. It became smaller in the order of base metal, heat affected zone (HAZ) and weld metal (WM) both in sour crude oil and in air.2) The fatigue crack propagation in HAZ and WM in sour crude oil consisted of three stages. They are Stage 1 where the fatigue crack propagation rate (da/dN) decreases with the increase in the range of stress intensity factor (ΔK) as a result of the protection of crack tip by corrosion product, Stage 2 where da/dN increases with ΔK as sour crude oil gradually reaches the crack tip through broken corrosion product, and Stage 3 where da/dN increases with ΔK in a manner of typical fatigue crack propagation in corrosive environment.3) Brittle striations were observed on the fracture surface of the specimen tested in sour crude oil particularly in the fatigue crack propagation accelerating region.4) The sour crude oil environment did not deteriorate the fatigue strength of the boxing fillet welded joint at lower stress levels.