Hardness
An SAE Grade 8 bolt will have 6 radial lines on it as shown below. \ | / / | \
A nut and bolt combination forms a fastener .
The type of fastener commonly used to secure a nut to a bolt is a washer.
It's a measure of the strength of the bolt. The strength is measured using what's called a grade. In this case, you have a grade 8 bolt. The higher the number, the stronger the bolt.
Snap.
A bolt with no head is commonly referred to as a screw.
The homograph you are referring to is "bolt." A bolt can refer to a metal fastener used to secure objects, as well as a sudden movement or dash, typically made with fingers.
Six sided bolt.
Typically, a stud bolt refers to an un-headed fastener, like a piece of all thread rod. A hex bolt is common a six-sided fastener, available in many grades.
The homograph for a metal fastener is "bolt" (a type of screw), and for the sound made with fingers, it is "bolt" (to run away suddenly). Both words are spelled the same but have different meanings and pronunciations.
"Snap" in those senses is a homograph, not a homophone.
The Rockwell C Hardness for a grade 10.9 fastener is 32 - 39HRC or 320 - 380Hv10