Abstract: A bolt includes a head portion on which a recess is formed. The recess has a bottom surface and a sidewall extending from a periphery of the bottom surface. The sidewall has a lower end defined by the bottom surface and includes an increased diameter portion and a reduced diameter portion. The increased diameter portion has an inner diameter larger than an inner diameter of the lower end of the side wall. The reduced diameter portion has an inner diameter smaller than the inner diameter of the increased diameter portion. The reduced diameter portion is located opposite to the bottom surface across the increased diameter portion.

Abstract: Disclosed is a friction stir welding tool (10) equipped with a probe (13) and a holder (16). At the tip of the probe (13) there is a pin part (11) and at the base there are probe-side protrusions (24) and probe-side recesses (25). At the tip of the holder (16) there are holder-side recesses (27) which mate with the probe-side protrusions (24), and holder-side protrusions (28) which mate with the probe-side recesses (25). External force applied to the probe (13) is transmitted from the probe-side protrusions (24) to the holder-side recesses (27) and from the probe-side recesses (25) to the holder-side protrusions (28).

Abstract: Disclosed is a friction stir welding tool (10) equipped with a probe (13) and a holder (16). At the tip of the probe (13) there is a pin part (11) and at the base there are probe-side protrusions (24) and probe-side recesses (25). At the tip of the holder (16) there are holder-side recesses (27) which mate with the probe-side protrusions (24), and holder-side protrusions (28) which mate with the probe-side recesses (25). External force applied to the probe (13) is transmitted from the probe-side protrusions (24) to the holder-side recesses (27) and from the probe-side recesses (25) to the holder-side protrusions (28).

Abstract: There is provided a screw treated by a chemical conversion coating of trivalent chromium having a torque characteristic approximately equivalent to a screw of a hexavalent chromate treatment. For the purpose, on a surface of a bolt, a zinc plating layer is formed by a zinc plating treatment. On the layer, a chemical conversion coating of trivalent chromium is also formed by a treatment of chemical conversion coatings of trivalent chromium. The coating is formed to be thin and soft so that the zinc-based plating layer can be exposed on the surface due to rubbing off of the chemical conversion coating of trivalent chromium at a contact portion to a fastened member of the bolt by tightening the bolt.

Abstract: Disclosed is a high-strength bolt having a tensile strength of 1,200 N/mm2 or more and superior in delayed fracture resistance and relaxation resistance, prepared by wire-drawing a bolt steel containing the following elements: C: 0.5 to 1.0% (mass %, the same shall apply hereinafter), Si: 0.55 to 3%, Mn: 0.2 to 2%, P: 0.03% or less (but not 0%), S: 0.03% or less (but not 0%), and Al: 0.3% or less (but not 0%), and containing proeutectoid ferrite, proeutectoid cementite, bainite and martensite at a total areal rate of less than 20% and pearlite in balance; cold-heading the wire into a bolt shape; and then bluing the bolt in a temperature range of 100 to 500° C.

Abstract: A high-strength bolt having excellent delayed fracture resistance and stress relaxation resistance in addition to a tensile strength of 1200 N/mm2 or higher is disclosed. A steel material for the high-strength bolt includes C: 0.50 to 1.0% by mass (hereinafter, referred to simply as “%”), Si: 0.5% or less (not including 0%), Mn: 0.2 to 1%, P: 0.03% or less (including 0%) and S: 0.03% or less (including 0%). The steel material has pro-eutectoid ferrite, pro-eutectoid cementite, bainite and martensite structures at less than 20% in total and a pearlite structure as the remainder. The high-strength bolt is produced by drawing the steel material severely to obtain a steel wire, forming the steel wire into a bolt shape through a cold heading, and subjecting the shaped steel wire to a blueing treatment at a temperature within a range of 100 to 400° C.