Related Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- API 6A ; API 20E; API 20F
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
ASTM A193 B8M Torque Chart: Calculated Nominal Values, Class 1 and Class 2
If you have arrived here looking for a torque chart for ASTM A193 B8M stud bolts and cap screws, you are almost certainly doing one of three things: tightening a flanged joint on a stainless line, writing a bolt-up procedure for a gasket on a pressure vessel, or arguing with a contractor about why the last joint leaked. This page gives you a calculation method, a worked torque table for Class 1 and Class 2 B8M in seven common UN coarse diameters, and a plain reading of what those numbers actually mean on the spanner. It does not replace ASME PCC-1 or a gasket vendor calculated assembly stress, and you should not use it to bolt up a flange before you read those two documents.
How to use this page. The torque values below are calculated nominal estimates derived from minimum yield strength per ASTM A193, a target preload of 65 percent of yield, and friction coefficients of K=0.20 dry and K=0.15 lubed. They are a starting point for engineering review only. Final assembly torque on a pressure-boundary joint must come from the gasket manufacturer recommended seating stress, the flange rating, the bolt material hot-yield, the actual lubricant data, and the engineer-of-record stamped procedure under ASME PCC-1, Guidelines for Pressure Boundary Bolted Flange Joint Assembly. Send your bill of material to TorqBolt QA-QC if you need a heat-specific calculated torque sheet shipped with the studs.
What B8M actually is, and why torque varies with the class
B8M is the ASTM A193 designation for bolting machined from UNS S31600 stainless steel, the molybdenum-bearing 316 grade. It is the 316 cousin of B8 (304). The "M" is moly. Engineers specify it where chloride pitting, sulphidation, or moderate sour service rule out plain B8, and where the temperature is high enough that low-alloy B7 will not behave. Typical homes are seawater piping, ammonia plants, refinery sour-service flanges, and any food, dairy, or pharma joint where the bolt sits in a wash-down zone.
The thing that drives torque, however, is not the grade chemistry. It is the heat-treatment class. ASTM A193 lists two practical classes for B8M studs:
- Class 1 is carbide-solution-treated B8M. Soft, ductile, austenitic. Minimum tensile strength 75 ksi, minimum yield 30 ksi at 0.2 percent offset, per A193 Table 2.
- Class 2 is solution-treated and strain-hardened B8M. The cold work raises the yield substantially. At 3/4 in and under the minimum tensile is 110 ksi, minimum yield 95 ksi. Above 3/4 in, both numbers step down because strain hardening cannot reach the centre of larger bar.
That yield delta is why a Class 2 B8M bolt at 3/4-10 UNC can take roughly three times the torque of a Class 1 bolt of the same diameter. It also explains why people get into trouble when a procurement team substitutes one class for the other on a flange that was originally torqued for the higher class. The stud has not changed shape, but the safe preload has fallen off a cliff.
The calculation method, plainly
Every torque chart you have ever seen for stud bolts comes out of the same engineering formula, sometimes called the short-form torque equation:
T = K × F × D
where:
T = installation torque (lb-in)
K = nut factor, dimensionless friction coefficient
F = target bolt preload, also called clamp force (lbf)
D = nominal bolt diameter (in)
where:
T = installation torque (lb-in)
K = nut factor, dimensionless friction coefficient
F = target bolt preload, also called clamp force (lbf)
D = nominal bolt diameter (in)
Three things drive the answer:
- The target preload F. A common, defensible rule of thumb for pressure-boundary bolting is to torque to a preload equal to 65 percent of the bolt minimum yield strength acting on the tensile stress area. So
F = 0.65 × Sy × As, whereSyis yield in psi from A193 Table 2 andAsis the tensile stress area in square inches from ASME B1.1. Some procedures use 50 percent for cyclic service, some use 75 percent for fully-engineered joints under PCC-1 Appendix A. We have used 65 percent because it is the workhorse value, conservative enough for general industrial flange work without leaving the joint slack. - The nut factor K. This is the empirical fudge factor that captures friction in the threads and under the nut face. Published K values for stainless studs run roughly 0.20 plain dry, 0.15 with a nickel-based anti-seize, and 0.12 to 0.13 with proper PTFE-bearing lubricants. K is not a constant. It depends on the lubricant, the operator, the surface finish, the number of reuses, and whether the lubricant was applied to both the threads and the nut face. Vendor data on the specific anti-seize you use should override the rule of thumb.
- The diameter D. The nominal bolt diameter, in inches. Worth noting that the formula uses nominal D, not the pitch diameter or the minor diameter. The nut factor K absorbs the geometry difference.
For B8M, there is a fourth consideration that almost no chart prints: galling. Austenitic stainless threads gall easily, particularly Class 2 strain-hardened material running against a Class 2 nut. A torque chart that does not specify lubricant is dangerous for B8M. The torque values below ASSUME a nickel-based or PTFE anti-seize was applied to both flanks of the thread and to the nut face. Dry assembly is shown for reference only; in practice, you should not assemble B8M dry on any joint you care about.
Calculated nominal torque, B8M Class 1, UN coarse
Calculated using F = 0.65 × 30,000 × As, then T = K × F × D, converted from lb-in to lb-ft. Yield strength of 30 ksi is the A193 Table 2 minimum for Class 1 B8M at all diameters. Tensile stress areas are standard UNC values per ASME B1.1.
| Size (UNC) | As (in²) | Target preload F (lbf) | Torque dry, K=0.20 (lb-ft) | Torque lubed, K=0.15 (lb-ft) |
|---|---|---|---|---|
| 1/4 in – 20 | 0.0318 | 620 | 2.6 | 1.9 |
| 3/8 in – 16 | 0.0775 | 1,511 | 9.4 | 7.1 |
| 1/2 in – 13 | 0.1419 | 2,767 | 23.1 | 17.3 |
| 5/8 in – 11 | 0.2260 | 4,407 | 45.9 | 34.4 |
| 3/4 in – 10 | 0.3345 | 6,523 | 81.5 | 61.2 |
| 7/8 in – 9 | 0.4617 | 9,003 | 131.3 | 98.5 |
| 1 in – 8 | 0.6057 | 11,811 | 196.9 | 147.6 |
Calculated nominal torque, B8M Class 2, UN coarse
Class 2 B8M yields step down with diameter, so the calculation uses 95 ksi yield for 3/4 in and under, and 80 ksi yield for 7/8 in and 1 in per A193 Table 2. Same 65 percent preload target, same K values, same UNC tensile stress areas.
| Size (UNC) | Sy (ksi) | Target preload F (lbf) | Torque dry, K=0.20 (lb-ft) | Torque lubed, K=0.15 (lb-ft) |
|---|---|---|---|---|
| 1/4 in – 20 | 95 | 1,964 | 8.2 | 6.1 |
| 3/8 in – 16 | 95 | 4,786 | 29.9 | 22.4 |
| 1/2 in – 13 | 95 | 8,762 | 73.0 | 54.8 |
| 5/8 in – 11 | 95 | 13,956 | 145.4 | 109.0 |
| 3/4 in – 10 | 95 | 20,660 | 258.2 | 193.7 |
| 7/8 in – 9 | 80 | 24,008 | 350.1 | 262.6 |
| 1 in – 8 | 80 | 31,496 | 524.9 | 393.7 |
Note on diameters above 1 in. ASTM A193 Class 2 B8M loses strength above 1 in. The yield falls to 65 ksi between 1 and 1-1/4 in, and 50 ksi between 1-1/4 and 1-1/2 in. We have stopped this table at 1 in deliberately. Calculate larger diameters from the A193 Table 2 yield at the actual diameter, not by extrapolating from the 1 in row.
What to do with these numbers before you pick up a torque wrench
Treat the tables above as a sanity check, not a procedure. The real assembly torque for a gasketed flange is governed by the gasket seating stress and the operating-condition stress, which depend on the gasket material, the gasket area, the temperature, and the design pressure. ASME PCC-1 sets out a target-stress method that backs into the bolt torque from the joint required gasket stress, not from the bolt yield. The two answers can be very different. On a soft graphite gasket in a low-pressure carbon-line flange the joint may need only 30 percent of the bolt yield; on a spiral-wound on a high-temperature steam header it may need 70 percent.
A safe workflow before final tightening:
- Confirm the bolt material and class on the certified mill test report. A Class 2 stamp on the stud end matters as much as the chemistry. See ASME SA-193 B8M bolting specification for class markings.
- Get the gasket manufacturer recommended seating stress and confirm it is consistent with the flange rating from ASME B16.5 flange dimensions.
- Calculate target torque from the gasket stress through PCC-1 Appendix O, or use the gasket vendor torque chart for that specific gasket.
- Compare the result against the calculated table above. If the PCC-1 torque is meaningfully higher than the table value, the joint is likely overstressed for the bolt; check that the bolt class is right.
- Use a calibrated wrench with a verified accuracy (PCC-1 calls for plus or minus 5 percent full scale). Apply torque in the standard cross pattern, in three passes, then a final round.
- Re-torque after thermal cycling if the joint sees temperature above 250 °F or process upsets, per PCC-1 Appendix N hot-tightening guidance.
Companion specifications and material data
Bolts never assemble alone. For a complete pressure-boundary joint you also need to confirm the nut grade, the gasket spec, and the flange dimensions match the bolt selection. The following pages cover the rest of the joint:
- Grade B8M Class 2 bolts: full mechanical, dimensional, and inspection requirements for the strain-hardened class used in the Class 2 column above.
- B8M UNS S31600 material data: chemistry, corrosion behaviour, sour-service notes, and PREN.
- ASTM A194 Grade 8M companion nuts: the matching 316 stainless nut grade. Do not pair B8M studs with A194 Grade 2H carbon nuts on a stainless line.
- ASME B16.5 pipe flange dimensions: bolt circle, bolt hole, and stud-length reference tables.
- ASME SA-193 B8M bolting specification: ASME Section II pressure-vessel equivalent with the additional certification requirements.
- All A193 B8M product pages and standards references.
What TorqBolt ships with the studs
Every B8M order leaves our Rajkot plant with the A193 Table 2 mechanical test results, the chemistry per Table 1, and a heat-traceable EN 10204 3.1 certificate. For pressure-vessel work and oil-and-gas jobs we ship 3.2 with third-party witness. If your assembly procedure cites a specific calculated assembly stress under PCC-1 we can pre-calculate the matching torque for the as-shipped heat using the actual yield from the MTR, not the A193 minimum. Email TorqBolt QA-QC with the joint drawing, gasket spec, and flange rating, and we will return the calculated torque sheet with the shipment.
References
- ASTM A193/A193M, Standard Specification for Alloy-Steel and Stainless Steel Bolting for High Temperature or High Pressure Service. Tables 1 and 2.
- ASME PCC-1, Guidelines for Pressure Boundary Bolted Flange Joint Assembly. Appendices A, N, and O.
- ASME B1.1, Unified Inch Screw Threads. UN coarse tensile stress area values.
- ASME B16.5, Pipe Flanges and Flanged Fittings.
Related references
Spec hub: ASTM A193 / SA-193 B8M overview (mechanical properties, NORSOK M-650 qualification, EN 10204 inspection). Source data: B8M Class 2 strain-hardened tensile and yield, UNS S31600 datasheet. Companion nut: A194 Grade 8M heavy hex nuts. Flange context: ASME B16.5 flange dimensions. ASME-route MTC: ASME SA-193 specification.
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