CORROSION-FATIGUE CRACK-GROWTH IN CLAD LOW-ALLOY STEELS .1. WATER-FLOW RATE EFFECTS IN HIGH-SULFUR PLATE STEEL

Corrosion fatigue crack propagation tests were conducted on a high-sul fur ASTM A302-B plate steel overlaid with weld-deposited Alloy EN82H c ladding. The specimens featured semi-elliptical surface cracks penetra ting approximately 6.3 mm of cladding into the underlying steel. The i nitial crack sizes were relatively large with surface lengths of 22.8- 27.3 mm, and depths of 10.5-14.1 mm. The experiments were initialed in a quasi-stagnant low-oxygen (O-2 < 10 ppb) aqueous environment at 243 degrees C, under loading conditions (Delta K, R, cyclic frequency), c onducive to environmentally assisted cracking (EAC) under quasi-stagna nt conditions. Following fatigue testing under quasi-stagnant conditio ns where EAC was observed, the specimens were then fatigue rested unde r conditions where active water flow of either 1.7 m/s or 4.7 m/s was applied parallel to the crack. Earlier experiments on unclad surface-c racked specimens of the same steel exhibited EAC under quasi-stagnant conditions, but water flow rates at 1.7 m/s and 5.0 m/s par allel to t he crack mitigated EAC. In the present experiments on clad specimens, water flow at approximately the same as the lower of these velocities did not mitigate EAC, and a free stream velocity approximately the sam e as the higher of these velocities resulted in sluggish mitigation of EAC. The lack of robust EAC mitigation was attributed to the greater crack surface roughness in the cladding interfering with flow induced within the crack cavity. An analysis employing the computational fluid dynamics code, FIDAP, confirmed that frictional forces associated wit h the cladding crack surface roughness reduced the interaction between the free stream and the crack cavity.