タフトライド処理鋼の繰返し応力による組織変化と疲労強度

抄録

Fatigue tests were conducted on tufftrided low carbon steel specimens with various notch conditions. The notch radii were 13.5, 4.3, 1.9 and 0.8mm, and the values of stress concentration factor α were 1.1, 1.3, 1.6 and 2.0, respectively. Fatigue test on the smoothed specimen was also carried out. The hardness distribution and the microstructure of each specimen were investigated before and after the fatigue tests. Based on these results, the discussion was carried out on the improvement mechanism of fatigue strength in tufftrided steel. The conclusions were summarized as follows:
(1) The fatigue limits of notched specimens with α of 1.1 and 1.3 are larger than that of a smoothed specimen in case of tufftrided steel.
(2) When tufftrided specimens are subjected to reversed stress, the hardness and the hardened depth at the inner part of the specimens increase and nitrogen precipitates as Fe₄N at the same time.
(3) The increasing rate of hardness and the amount of nitrogen precipitated by reversed stressing decrease with an increase of α, and these phenomena are hardly observed in case of the notched specimen with α of 2.0.
(4) The increase in hardness and hardened depth by reversed stressing contributes to the improvement of fatigue strength in tufftrided steel, and the degree of contribution is influenced by the amount of nitrogen precipitated by reversed stressing.