抄録
In manufacturing high strength bolts, the zinc phosphate coating is generally perfomed in order to increase the effect of lubrication. However, a phospharized layer is often formed on the bolt surface by diffusion of phosphate during subsequent heat treatment. In this study, the effect of this layer on delayed fracture was investigated. The results were as follows:
(1) In high strength bolts, the phospharized layer at a thickness of about 40μm including the 5-10μm ferrite layer was formed on the surface.
(2) After the ferrite layer was corroded, the grain boundary of the phosphorized layer having a rich phosphate concentration was corroded preferentially. The corrosion pits became coupled and coalesced, growing to an initial crack.
(3) As the result of acceleration of corrosion rate and diffusivity of hydrogen with the existence of phospharized layer, the freqency of occurrence of initial cracks was increased, giving a detrimental effect on the characteristics of delayed fracture.
