溶融亜鉛めっき高力ボルトの遅れ破壊

抄録

At present, Hot-dip Galvanized high strength hexagon bolts having high strength levels of above 95kg/mm² are frequently used for fastening structural members of outdoor structures such as a transmission steel tower in Japan.
Though Hot-dip Galvanizing is very superior in corrosion resistance in general, delayed fracture has occurred presumably due to hydrogen embrittlement. Therefore, we thought the following two reasons for the delayed fracture occurance: Hydrogen enters into the steel by acid pickling in the Hot-dip Galvanizing process and stays in. Or the cracks occur in the Galvanized layer under fastening, and then, hydrogen enters into the steel from the environmental atmosphere.
To ascertain the above consideration, we performed the static bending test and delayed fracture test etc. on the Cr-steel specimens subjected to the same heat treatment as that for the bolt. As the result, we obtained the following conclusions.
(1) Delayed fracture occured even in a short dipping time.
(2) The thick Galvanized layer makes transmission of hydrogen atoms difficult, so that it is not possible to remove hydrogen from the steel by the usual baking treatment.
(3) Delayed fracture almost never occurs when the acid-dip process is not incorporated.
(4) Delayed fracture often occurs because cracks nucleate in the Galvanized layer by adding impact energy on a bolt.