Diffusible Hydrogen Behavior and Delayed Fracture of Zinc-electrogalvanized High Strength Steel

Diffusible hydrogen behavior in zinc electrogalvanized high strength steel has been investigated by use of a newly developed hydrogen content determining method for better understanding of delayed fracture of electrogalvanized high strength steel.For as-electrogalvanized high strength steel, there are two peaks on the hydrogen evolution rate curve at 200°C and 350°C from room temperature to 600°C. After electrogalvanizing and baking at 200°C for 4 hours, 200°C peak becomes very low but 350°C peak shows still the same as before. Hydrogen at 200°C peak is diffusible and relates to delayed fracture of high strength steel. Hydrogen at 350°C peak is undiffusible and does not relate to delayed fracture. Hydrogen diffusion remarkably falls down by electrogalvanized zinc layer. In as-electrogalvanized high strength steel with 8μ zinc layer, diffusible hydrogen content is about 0.23ppm and decreases to 0.03ppm after baking at 200°C for 4 hours.In as-electrogalvanized and carburized high strength bolt with 8μ zinc layer, there are two peaks on the evolution rate curve at 150°C and 327°C. The former is diffusible hydrogen and its content is 0.69ppm. This diffusible hydrogen decreases to 0.10 ppm after baking at 190°C for 5 hours. It is investigated to show the diagram of relationships about baking time-diameter of bolt-change of initial hydrogen content.Diffusion coefficient of carburized high strength M8 bolt with 8μ zinc layer at 190°C is about 2.36 × 10-45.91 × 10-4 mm2/sec.