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A B S T R A C T Arc welding typically generates residual tensile stresses in welded joints, leading to deteriorated fatigue performance of these joints. Volume expansion of the weld metal at high temperatures followed by contraction during cooling induces a local tensile residual stress state. A new type of welding wire capable of inducing a local compressive residual stress state by means of controlled martensitic transformation at relatively low temperatures has been studied, and the effects of the transformation temperature and residual stresses on fatigue strength are discussed. In this study, several LTTW (Low Transformation-Temperature Welding) wires have been developed and investigated to better characterize the effect of phase transformation on residual stress management in welded joints. Non-load-carrying cruciform fillet welded joints were prepared for measurement of residual stresses and fatigue testing. The measurement of the residual stresses of the three designed wires reveals a compressive residual stress near the weld toe. The fatigue properties of the new wires are enhanced compared to a commercially available wire.Keywords fatigue; residual stress; welded joints; weld metal phase transformations. N O M E N C L A T U R Eb 0 = width of the residual stress in tension or in compression e α = thermal expansion coefficient of ferrite e γ = thermal expansion coefficient of austenite h = leg length along the principal member h p = leg length along the transverse member t 1 = thickness of the principal member t 2 = thickness of the transverse member K t = stress concentration factor N = number of cycles to failure R = stress ratio S = stress level T 0 = thermodynamic equilibrium temperature T Ff = ferrite transformation finish temperature T Fs = ferrite transformation start temperature T Mf = martensite transformation finish temperature T Ms = austenite-to-martensite transformation start temperature x = distance from the weld toe T m = undercooling austenite-to-martensite transformation range temperature Correspondence: Ph. P. Darcis.

In recent years, higher safety and reliability of steel welded structures have been required as it shows growing concern about environmental problems. To prevent fatigue fracture is one of the most important challenges to improve the safety and reliability. A lot of studies how reduce stress concentration at critical areas have been carried out from the viewpoint of structural design as prevention measures while nothing has been studied from the viewpoint of material because fatigue strength of welded joints converges in limited range regardless of material strength. On the other hand, it was found that an appropriate dual phase microstructure could reduce the fatigue crack growth rate remarkably. The newly developed steel plate with high resistance to fatigue crack growth could extend the fatigue life of structures. The developed steels have already been applied to some ships and vessels, and a new bulk carrier applied the developed steels acquired the notation and descriptive note as the valuable ship with resistance to fatigue fracture by Nippon Kaiji Kyokai for the first time in the world. From further studies, it was found the developed steels had also high resistance to fatigue crack initiation as well as the growth even in welded structure. In this study, it was clarified that the fatigue strength of HAZ, where fatigue crack generally initiates, in the developed steel was higher than that in conventional steel and the stress concentration at toe of weld in the developed steel was smaller than in the conventional steel. It was considered the mechanism of suppression of fatigue crack initiation with FEM analysis and fatigue test. The newly developed steel can effectively extend fatigue fracture life of welded structure from the viewpoint of material.

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