Effect of Microstructure at Coating Layer on Fatigue Strength in Hot-Dip Galvanized Steel

To understand the fatigue mechanism of hot-dip galvanized steel, the fatigue strength and fracture surface of hot-dip galvanized AISI 1045 steel(carbon steel) specimens were investigated. The galvanized coating layer was composed of δ1-phase, ζ-phase and η-phase, and its thickness was about 100 µm. In the low cycle region (104 cycles < Nf < 105 cycles), the fatigue strengths of both the carbon steel and the galvanized steel corresponded with the static strength. The fatigue strength of the galvanized steel was lower than that of carbon steel. As the number of cycles increased, the difference between fatigue strength of the carbon steel and that of the galvanized steel increased. Also, the morphologies of the fatigue fracture were different in low cycle region and high cycle region. In the galvanized steel, the morphology of Stage II crack on the fracture surface at low cycle region exhibited crescent shape, and multiple crack initiation sites in low cycle region were observed. Whereas the morphology at high cycle region (Nf >105 cycles) exhibited an ellipse shape, and the crack initiation site was single. At both regions, the crack initiation sites were in the coating layer. The mechanical properties of the microstructure in the coating layer had an effect on the fatigue strength. When η-phase was removed from the galvanized coating layer, the fatigue strength increased only in the high cycle region. Therefore, δ1-phase and/or ζ-phase cause the fatigue strength to decrease in low cycle region, and η-phase causes it in high cycle region.