Bolting materials:

Bolting materials, as per ASME B16.5, are classified as High strength, intermediate strength and low strength materials.

The material grades can be found be found in Table 1B of ASME B16.5.

It is important to consider the temperature limits while selecting the materials for bolting. However, when we are dealing with the fasteners especially, it is important to see how they are being manufactured.

Bolts/Studs can be manufactured using Thread rolling and the tolerances are defined as per ISO 965. Usually, if not specifically mentioned, for threads above and including M1.6, tolerance will be 6H/6g.

The important factor is to see, which bar material is to be used for manufacturing. For high strength bolting, one will observe “B” grades and “L” grades, typically as per ASTM A193 or A320.

The “L” grades are recommended to be used in case of low temperature service. AISI 4140,4142 or 4340 are the typical bar materials used for these various ferritic grades. The important factor here to see, what makes them special to handle low or high temperature services.

AISI 4140 is typically a Chromium-Molybdenum (CrMo) steel, which is excellent for creep resistance and hence used for high temperature bolting like A193 Gr.B7. High temperature application is due to the addition of Cr which gives strength due to formation of Cr carbides and Mo, which impedes coarsening of these carbides at high temperature. However, if one sees, material chemical composition of L7 grade, then it is also the same. So to differentiate for low temperature service, one has to have the impact tested raw material as specified by ASTM standards, like A320.

Another factor that contributes to low temperature service is that, FCC microstructure, which is achieved through the addition of nickel. Hence, usually for low temperatures, the bolting materials like A320 Gr. L43, manufactured from AISI 4340 (CrMoNi) steel, can be used, which uses nickel as an alloying element.