Development of High Strength Bolt for Automobile Use

For the purpose of weight reduction and improvement in fuel economy of vehicles, we developed and put in practical use the high strength bolt (following 13 T) which is excellent in delayed fracture resistance in all the environment of the bolt currently used for the automobiles, and which guarantees 1300 MPa or more of tensile strength. The points of development are in the optimization of chemical composition design and heat treatment conditions which noted the following three points.

1. 1)
   Toughness of grain-boundary be a fracture course
2. 2)
   Restraint of hydrogen invation into bolt
3. 3)
   Diffusion restraint of invaded hydrogen into bolt.

In the delayed fracture tests which simulated much corrosion environments like the salt splaying in axle parts and acid corrosion environment like the blowby gas inside engine, high reliability of the developed 13T bolt was confirmed.