Here we discuss the use of fasteners for ASME pressure vessels and piping in industrial applications. If you've ever worked with these fasteners, you may have come across some confusing names such as B7, B8, 2H, and A193. Fear not, each of these labels are meaningful and indicate the use of different materials and mechanical properties. They also indicate which fasteners perform better in certain environments, such as high-temperature and high-pressure applications.

Before diving into the technical aspects, let's start with an overview of the standards governing the use of nuts and bolts in industrial applications. Industrial fasteners can be called stud bolts or plain ol’ bolts and are governed by three main industry standards: ASTM A193, ASTM A320, and ASTM A194. In this article, we will not cover coating practices, hex bolts, or machine screw nuts and coupling nuts found in ASME B18.2.2, as these have nothing to do with industrial bolting.

Now, let's discuss several different relevant aspects of these fasteners, including tensile strength, stainless steel (grade B8) vs. carbon steel (grade B7 & B16), and heavy hex nuts (2H nuts, or Grade 4 & 7 nuts).

ASTM A193 Grade B7 bolts are made of chromium-molybdenum steel and are quenched and tempered to develop the desired tensile strength. These bolts are used in pressure vessels that do not need corrosion resistance, aren’t susceptible to stress corrosion cracking, and for temperatures typically less than 750F. They can have different types of finishes, including plain finish, hot-dip galvanized process, and zinc plated and/or have a PTFE or Xylan coating for corrosion resistance. A coated stud will have a lower temperature rating than a plain finish stud bolt.

The nut material for Grade B7 bolts is typically ASTM A194 heavy hex nuts (2H nuts), which work in combination with B7 Studs and are stronger than the stud bolt. Therefore, you should see failure on the stud before you see failure in the nut. 2H nuts, also known as Heavy Hex Nuts, are very common in the industry and easy to procure.

ASTM A193 Grade B7M studs are identical in chemistry to Grade B7, but they have a lower tensile strength. We typically see Grade B7M bolts in hydrogen stress corrosion cracking (SCC) applications such as hydrofluoric acid or in Floating Head Heat Exchangers. The nut material for Grade B7M bolts is ASTM A194 GRADE 2HM, which is recommended for use in stress corrosion cracking environments.

ASTM A193 B16 stud bolts are used primarily for high-temperature applications of 751-1100F. They are manufactured from a chromium-molybdenum-vanadium alloy steel and retain strength under high temperatures while experiencing less relaxation at those high temperatures. There are two nut combinations you can use for B16 stud bolts, ASTM A194 GRADE 7 and ASTM A194 GRADE 4. Grade 7 nuts are heat-treated chrome-molybdenum steel nuts that are also suitable for sub-zero service conditions and have minimum Charpy impact values in accordance with ASTM specifications. Grade 4 nuts were taken out of ASTM A194 in 2017, but were heat-treated molybdenum steel nuts.

In conclusion, it is essential to understand the materials and mechanical properties of each fastener, along with their suitability for different environments. We hope this discussion will help you in your industrial bolting applications.