1700 MPa-class ultra-high strength bolts were fabricated through quenching and tempering process after cold forming of a prototype steel with a chemical composition of Fe-0.5%C-2%Si-1%Cr-1%Mo (in mass%). Bolt shape and dimensions were optimized to enhance tensile deformation capacity. The developed bolt has a waisted shank and a thread form that are developed for reducing stress concentration at thread roots. Due to its shape and dimensions, the waisted shank precedingly yields and plastically deforms, resulting in higher bolt elongation. The specification of nut was also determined for the 1700 MPa-class shank preceding yield type bolt.
