Heavy hex bolts have a head that is larger than a standard (finish) hex head. Standard heavy hex bolts are made of low to mid carbon steel. The large head offers a greater bearing surface that distributes the clamping load over a greater area.
The ASTM A307 specification covers carbon steel bolts and studs ranging from 1/4" through 4" diameter. This is your everyday, run of the mill bolt specification often manufactured using A36 round bar. There are three grades A, B, and C which denote tensile strength, configuration, and application.
The ASTM A193 standard covers alloy steel and stainless steel bolting materials for high temperature or high pressure service. This specification includes fasteners intended for use in pressure vessels, valves, flanges, and fittings. Although, this material is often available in UNC national coarse thread, diameters above one inch are typically specified 8 threads per inch (tpi).