Cap screws are generally made of higher grade material. However, some are not. You can be sure of the alloy if you use a SAE grade eight bolt. It will have six radial "hash" marks on the head (SAE bolt grade is number of "hash" marks plus two). It will be decidedly tougher to machine than softer bolts, but a properly ground high speed tool bit will cut it just fine. As above, take your time. No heat treat needed for the finished product.