タフトライド処理鋼の腐食疲労について（純水中および３％食塩水中での疲労試験：その１）

Abstract

It has been pointed out that the fatigue strength of steel in air is improved by tufftriding treatment. The present authors have clarified in the previous study that such improvement results from (1) the appearance of compressive residual stress, (2) the rise of hardness at the surface, (3) the strengthening of matrix by supersaturated nitrogen and (4) the increase of hardness on the fatigue process.On the other hand, a serious view is recently being taken on the corrosion fatigue fractures of machine and construction, as high strength steel is developed and utilized. However, the corrosion fatigue behavior of tufftrided steel is hardly clear, since it is influenced by many factors.Therefore, the authors carried out the static dipping corrosion test and the corrosion fatigue test of tufftrided steel. The specimen used in this investigation was a 0.15% low carbon steel, and pure water and 3% NaCl solution were used as corrosive environments.As the results, the following conclusions have been obtained:(1) The corrosion resistance of tufftrided steel is good in pure water and it results from protection to corrosion by a compound layer formed on the surface.Even when cyclic stress is applied on it, the bonding between the compound layer and the diffusion layer is very strong. So, the diffusion layer is protected from corrosion in its fatigue process until the breakdown of compound layer takes place. Therefore, the diffusion layer contributes to the fatigue in pure water as well as that in air, and the fatigue strength of steel is improved by tufftriding treatment in pure water.(2) On the other hand, the corrosion of tufftrided steel is severe in 3% NaCl solution because of formation of etch pit, which originates from the localized damage of compound layer.Such formation of etch pit is accelerated by interaction of corrosion and stress cycles. A crack initiates at the root of etch pit due to cyclic stress, and the crack propagates to become a fracture when the solution infiltrates into the tip of the crack. Therefore, the fatigue strength of tufftridedsteel decreases radically in 3% NaCl solution with the increase of stress cycles.