Every flanged joint depends on its bolting. Wrong grade, wrong coating or wrong length — and a joint can leak, seize or fail. For fastener suppliers and pipe-fitting traders, knowing stud bolt grades and how they match with nuts is what separates a professional quotation from a risky one.
Why stud bolts are used
In process and pipeline flanges, buyers usually specify fully threaded stud bolts with two heavy hex nuts rather than ordinary hex bolts. Studs are easier to tighten evenly, easier to replace and available in long lengths for thick flanges.
The main standards
- ASTM A193 — alloy and stainless steel bolting for high-temperature or high-pressure service (grades B7, B7M, B16, B8, B8M and others).
- ASTM A320 — bolting for low-temperature service (L7, L7M, L43, B8 Class 1 etc.).
- ASTM A194 — nuts for high-pressure and high-temperature service (2H, 2HM, 4, 7, 8, 8M).
- ASME B18.2.1 / B18.2.2 — dimensions of bolts and nuts.
- ASME B16.5 — gives stud lengths and diameters for each flange size and class.
Common grades and their matching nuts
| Stud grade | Material | Typical use | Common nut |
|---|---|---|---|
| A193 B7 | Chromium-molybdenum alloy steel, quenched & tempered | General high-pressure, high-temperature service | A194 2H |
| A193 B7M | Same as B7 with hardness limit | Sour service (H2S) | A194 2HM |
| A193 B8 Class 1 | Stainless 304 | Corrosive service, moderate strength | A194 8 |
| A193 B8M Class 1 | Stainless 316 | Better corrosion resistance | A194 8M |
| A193 B8M Class 2 | Stainless 316, strain hardened | Higher strength stainless | A194 8M |
| A320 L7 | Cr-Mo alloy steel, impact tested | Low temperature (often to −101 °C) | A194 Gr 4 or 7 (with impact test) |
| A193 B16 | Cr-Mo-V alloy steel | Higher temperature service | A194 Gr 4 or 7 |
B7 — the workhorse
A193 B7 is by far the most common stud grade. Up to 2½" diameter it has a minimum tensile strength of 125 ksi (860 MPa) and minimum yield of 105 ksi (720 MPa). It pairs with A194 2H heavy hex nuts.
B7M and 2HM — for sour service
In environments with hydrogen sulphide, hard materials can crack. B7M and 2HM have controlled maximum hardness to meet NACE MR0175 / ISO 15156 requirements. Each piece is usually hardness tested.
Stainless grades
B8 (304) and B8M (316) resist corrosion but have lower strength in Class 1. Class 2 (strain hardened) offers higher strength. Galling is a risk when stainless nuts and studs are tightened — use suitable lubricants.
Low-temperature grades
For LNG, refrigeration and cold-climate service, A320 L7 studs are impact tested at low temperature. Nuts must also be suitable, typically A194 grade 4 or 7 with impact testing.
Coatings and finishes
- Plain / black oxide — standard, often with oil.
- Hot-dip galvanised (HDG) — outdoor and water service; nuts are tapped oversize to fit.
- Zinc nickel and PTFE (fluoropolymer) coatings — corrosion protection for offshore and coastal use; colours vary by brand.
- Cadmium — still requested in some sectors but restricted in many regions.
Always confirm coating thickness and colour requirements.
How to describe stud bolts in a quote
Stud bolt, 7/8" × 160 mm, ASTM A193 B7, with 2 heavy hex nuts ASTM A194 2H, PTFE coated, UNC threads
Include:
- Diameter and length (inches or metric)
- Thread series (UNC or 8UN for 1" and above)
- Stud standard and grade
- Nut standard, grade and quantity per stud
- Coating
- Testing (hardness, impact) and certificate type
Calculating quantities
ASME B16.5 tables give the number of bolts and stud length for each flange size and class. For example, a 4" Class 150 RF flange pair uses 8 studs of 5/8". Multiply by the number of joints and add spares. Our flange guide explains classes and facings.
Frequently asked questions
Can I use B7 studs with grade 8 nuts?
Mixing carbon/alloy studs with stainless nuts is unusual and can cause galvanic and strength mismatches. Follow the project specification — B7 is normally paired with 2H.
What is the difference between 2H and 2HM?
2HM has a maximum hardness limit for sour service, while 2H is the standard heavy hex nut for general high-temperature service.
Why is 8UN thread used on larger studs?
For diameters of 1" and above, ASME B16.5 bolting commonly uses the 8 threads-per-inch series, which is easier to assemble and tighten.
Do stud bolts need mill test certificates?
Yes. Buyers usually require EN 10204 3.1 certificates showing heat numbers, chemistry, mechanical results and hardness. MTCMitra includes fastener grades in its library.
Key takeaways
- B7 + 2H is the standard high-temperature combination.
- B7M + 2HM for sour service; L7 for low temperature; B8/B8M for corrosion.
- Specify diameter, length, threads, grades, coating and testing in every quote.
