[플랜트교육] Bolt B7 과 B7M 차이점 쉽게 설명드립니다. 두개의 차이점은 무엇일까요? Bolt B7 and B7M Differences Easy Understand 

Stud and Nut Combinations
B7 Studs and 2H Nuts
ASTM A193 Grade B7 bolts are made of chromium-molybdenum steel. The bolts are quenched and tempered (a.k.a heat treated) to develop the desired tensile strength (mechanical properties).
Grade B7 Stud 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. However, these bolts can have many different types of finishes, including:
Size and strength:
ASTM A193 B7 stud bolts with a diameter of 2.5 inches or less will have a yield strength of 105,000 PSI.
Grade B7 fasteners with a stud diameter of 2-5/8″ to 4″ diameter have a lower yield strength of 95,000 PSI.
4″ to 7″ inch stud bolts have an even lower yield strength of 75,000 PSI.
The nut material for Grade B7 bolts is typically ASTM A194 heavy hex nuts (2H nuts).
A194 Grade 2H Nuts
2H nuts 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.
Grade B7M Studs and Nuts
ASTM A193 Grade B7M studs are identical in chemistry to Grade B7, as they are quenched and tempered carbon steel to achieve a lower hardness. However, they have a lower tensile strength than B7 studs.
We typically see Grade B7M bolts in hydrogen stress corrosion cracking (SCC) applications such as hydrofluoric acid or in Floating Head Heat Exchangers.
ASTM A194 GRADE 2HM are similar to 2H nuts, except this grade is recommended for use in stress corrosion cracking environments.
Grade B16 Studs
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,
Although A193 Grade B16 bolts and studs have similar strength requirements as Grade B7, the fasteners retain strength under high temperatures, and also experience less relaxation at those high temperatures.
There are two nut combinations you can use for B16 stud bolts. They are:
ASTM A194 GRADE 7: These are also 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.
ASTM A194 GRADE 4: These were taken out of ASTM A194 in 2017, but were heat treated molybdenum steel nuts.
It is imperative that you use Grade 7 (or 4) nuts with B16 studs, because they have similar properties. 2H Nuts, on the other hand, will relax more. So if you have high temperature and 2H nuts you will see increased relaxation, or loss of bolt load. (Not sure how to handle bolt relaxation? Contact us. We can help.)
ASTM A194 GRADE 8 Stainless steel nuts required for these fasteners.
B8M Studs
ASTM A193 Grade B8M fasteners are manufactured the same way as B8 fasteners. The difference is in the materials that they are made of.
B8M fasteners are manufactured from AISI 316 stainless steel as opposed to AISI 304 stainless steel. The 316 form of stainless steel is better for corrosion resistance because it has more molybdenum.
ASTM A193 Grade B8M Class 1 fasteners require a carbide solution treatment, while Class 2 fasteners require an additional strain hardening just like B8 fasteners.
ASTM A194 GRADE 8M Stainless steel nuts are required for these fasteners.
ASTM A320, Grade L7 and Grade L7M
ASTM A320 Grade L7 and L7M fasteners are recommended for use in low temperature environments typically found to be -50F to -150F. These fasteners also require Grade 4 or Grade 7 nuts.
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