Bolts are not all made of the same quality material. Bolts are also not all tempered the same. A simple way to determine the nature of a bolt is to look on the face of the hexagonal surface of the bolt end. Markings are stamped on the end of bolts which indicates the type of bolt. Actually this stamp shows the tensile strength of the bolt.
Tensile strength is the amount of pull an object will withstand before breaking. When a bolt is tightened it actually stretches about .001 per 30,000 pounds due to the tension being applied. The higher the tensile strength the more torque the bolt will accept before breaking. No markings on the bolt head indicate a low carbon steel bolt with a tensile strength of 64,000 psi or under. This is a soft bolt usually graded as a two or three grade bolt. Because of the softer steel these bolts are commonly used for simple applications.
A bolt which has three raised markings on the head is a grade five bolt made of medium tempered carbon steel. A grade five bolt has a tensile strength of 105,000 psi. The higher the markings on a bolt the "harder" the bolt, the higher the grade number the less shear resistant it becomes.
Using the correct bolt for the application is very important. Some applications require a shear bolt or a bolt that will break without breaking the equipment it is holding in place while other applications require a "harder" bolt, one that will not shear under higher torque conditions.
The process used to add tensile strength to bolts adds to the cost so the harder or higher the grade number the more expensive the bolt will be to purchase. Most of the unmarked, inexpensive standard hardware bolts are grade twos which are very soft and flexible.