Increase in Fatigue Strength and Charpy Absorbed Energies of TIG-Welded SS400 Steel Plates by Friction-Stir-Processing Grain-Refinement

摘要

To improve fatigue strength of high-tensile-steel welds, friction-stir-processing (FSP), which is based on the principle of friction stir welding, is an effective grain-refinement technique for the topmost layer of the welds. In this study, its effects on fatigue strength/life of the tungsten inert gas (TIG)-welded SS400 plates and Charpy absorbed energy of the FSP-modified microstructure were investigated. FSP produced microstructure consisting of ultrafine eqiaxial ferrite grains beneath the TIG-welded surface. A cross-sectional TEM image indicates that cementite phases were segregated at their grain boundary, which formed about 2 nm-thick thin layers in a certain FSP condition. The microstructure increased three-point-bending fatigue strength/life in comparison with those of TIG-welded specimens. Their Charpy absorbed energies exhibited about 110 J in the temperature range between -110°C and 40°C, which is higher than the values of TIG-welded specimens. Grain refinement provided by FSP with lower heat input could further decrease brittle-ductile transition temperature. FSP modification of a topmost area of the welds is useful to increase fracture toughness as well as fatigue strength/life of the welds.