More information than you need on understanding the difference between different grade bolts, how they are identified, and the relationship between tension and shear. High strength bolts, like grade 8, are primarily through hardened to a specific core hardness that is required for that grade. Surface hardness would be much harder. Bolted joint design is based on clamping force due to the torque applied to the bolt. The higher the grade the more torque can be applied to the bolt. Enough of this. I'm beginning to feel like I'm back at work in engineering at Caterpillar where all we used were grade 8 bolts.
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