PURPOSE: To enhance the fixing force of a body to be tightened by performing caburized hardening to a bolt, limiting the carbon potential of a bolt surface and a complete thread part within the range of 0.35-1.00, 0.40-0.55 respectively, and making the surface hardness of caburized part in order of underhead serration part complete thread part incomplete thread part underhead R part.
CONSTITUTION: After a high strength bolt 1 is formed in the shape of bolt, caburization retarder is applied on an underhead R part 1b, an incomplete thread part 1d, and a complete thread part 1e, and brought in a caburizing furnace to perform caburizing hardening. As an example, caburization retarder made of anti-caburization stock solution, Xylene, and glyceline is used, and applied properly changing the mixing ratio of the retarder to change the hardness of respective parts in the aforementioned order. To prevent collapse of serration crest when forcibly inserted into a wheel hub, an underhead serration part 1c is made to be of maximum hardness, then, the complete thread part 1e is made hard to secure fatigue strength and prevent galling of thread ridge, and other parts are made to be of low hardness as they are most liable to be the source of delayed fracture.
JINBO YOSHIO
FUSHIMI SHINJI
