The primary application for ASTM A320 bolts and studs is for low-temperature service. Often times, these are used for pressure vessels and flanges.

Pressure Vessels

Source: Wikipedia

Pressure vessels contain pressurized liquids or gasses. They can be made in just about any shape or size and are engineered for safe operating temperatures and pressure, with a specific corrosion allowance, and with specific safety factors. These vessels are regulated by engineering authorities from country to country. Pressure vessels are used in many industries that range from mining to nuclear. 

Flanges

Source: Wikipedia

Flanges are external rims, ribs, or collars serve to connect pipes together. They come in many different shapes and sizes as well as different types of materials. Bolts used with flanges are often sold in kits along with gaskets and other connection hardware. There are different flange standards throughout the world with ANSI/ASME being the most common. The typical configurations of A320 bolts with flanges are either a stud with a nut on each end or a heavy hex bolt with one nut. Flanges are frequently used in the oil and gas industry.

Valves

Source: Wikipedia

Valves are used to control the movement and flow of liquids or gases in one direction. Many valves are controlled by using a handle. Manufacturers will make valves with a rated maximum temperature and pressure. These devices are used in just about every industry.