Fatigue crack propagation behavior of conventional steels and newly developed " OCP steels was studied. Fatigue crack propagation rate (FCPR) was measured in heat affected zone. cross bond area of weld and in base metal. The results showed that the FCPR in THCP steels was higher than that observed in conventional steels with the same strength. The welding heat input could affect the fatigue crack propagation in heat affected zone of conventional steels. while it did not affect significantly the FOGR in THCP steels. It was found also that the high heat input welding did not cause the degradation in fatigue crack propagation behavior both in conventional steels and in " OCP steels.
High heat input welding for steel structures is widely used in industries because of its advantage in construction cost. A newly developed steel for offshore structures. THCP(Thermo-mechanical Control Process) steel. has excellent weldability which permits very high heat input welding compare to the conventionally made steels. THCP steels have the microstructure composed of pearlite, ferrite and bainite which can cause the softening of heat affected zone during the welding. Therefore, it might be considered that the softened area of THCP steel weld could have lower resistance to the fatigue crack propagation than the base metal. The purpose of this work is to compare the fatigue crack propagation behavior of two kinds of offshore structural steels conventionally made steels and, newly developed THCP steels and to investigate the influence of welding heat input on the fatigue crack propagation in welds.
Four different steels were used in this study. Two steels EH36 and SK580 were made by conventional rolling process and the others. TMCP 1 and TMCP 2 were made by accelerated cooling process (one of manufacturing processes of THCP steel).