The short answer: ASTM A193 specifies alloy-steel and stainless-steel bolting; ASTM A194 specifies the companion nuts. Start by selecting the bolt material and strength condition, then define the nut and the complete assembly. B7 and B7M are chromium-molybdenum alloy steels; B8 is Type 304 stainless and B8M is Type 316 stainless.[1] [2] [3]
Start with the question you need to answer
- Choosing a bolt material? Start with the B7/B8 comparison below and the four grade references.
- Looking up a nut grade? Read ASTM A194 nuts: 2H, 2HM, 8 and 8M for hardness, proof load and marking.
- Matching a nut to a bolt? Use the A193/A194 compatibility guide for pairings and complete assembly orders.
B7 vs B8: strength or stainless material?
B7 develops strength through quenching and tempering. B8 supplies the corrosion behavior of Type 304 stainless, with strength determined by its class. The decision therefore starts with the service environment and required mechanical properties, rather than a ranking of grade numbers.[1] [2]
- B7: alloy steel and strength by diameter. Quenched-and-tempered chromium-molybdenum steel.
- B7M: controlled hardness and inspection. The related alloy-steel grade with modified mechanical requirements.
- B8: Type 304 stainless and strength classes. Class 1, Class 2 and Class 2B have separate property schedules.
- B8M: Type 316 stainless, classes, B8C and B8T. Molybdenum-bearing stainless, with context on the stabilized grades.
A small-diameter comparison shows why the class matters. Through 3/4 in, B7 and B8 Class 2 both have a 125 ksi minimum tensile strength; their minimum yield strengths are 105 ksi and 100 ksi respectively. B8 Class 1 has 75 ksi minimum tensile and 30 ksi minimum yield strength. The grade references give the full diameter-dependent schedules.[1] [2]
Bare B7 can corrode in wet service, so the finish needs its own specification. B8 requires an environment compatible with Type 304. Where the project calls for Type 316, consider B8M; see 304 vs 316 stainless fasteners for the corrosion comparison. Allowable stress at operating temperature comes from the governing design code, not from the grade name alone.
What does the M mean in B7M and B8M?
The suffix has different meanings in these two families. B7M modifies the mechanical and hardness requirements of the alloy-steel bolting. Its hardness range is 200–235 HBW or 93–99 HRB, with 100% hardness inspection; its minimum strength is lower than B7 through 4 in. B8M identifies the molybdenum-bearing Type 316 stainless grade instead of B8 Type 304.[1] [2]
Keep the full grade on the drawing and certificate. The suffix does not identify metric dimensions, and B8M is not the stainless equivalent of B7M's controlled-hardness condition.
A193 and A194 have different jobs
A193 defines bolting material, processing and mechanical requirements. A194 defines nut material, hardness, proof load and identification. An A193 material certificate does not certify a separately supplied nut.[1] [3]
The common pairings are B7/2H, B7M/2HM, B8/8 and B8M/8M. The nut compatibility guide explains the complete selection checks, including stainless Class 2 bolting and the optional A194 S1 strain-hardened nut condition.[4]
From material choice to a complete order
Use the selected grade reference to confirm diameter-dependent strength, heat treatment and marking. Use the A194 guide to confirm nut properties. Finally, use the compatibility guide to specify both components with matching threads, dimensions, finish and service requirements.
This sequence leaves each document with a clear purpose: the material certificate verifies the specified grade and condition; the drawing defines the component; the governing joint design establishes whether the assembly is suitable for service.
Preparing an A193/A194 assembly? Send the bolt grade and class, nut grade, dimensions, service conditions and quantity with your drawing or RFQ.
References
- ASTM International. ASTM A193/A193M-26. Bolting scope, grade chemistry, processing and mechanical requirements.
- Portland Bolt. ASTM A193 Grades. B7, B7M, B8 and B8M material and mechanical-property summaries.
- ASTM International. ASTM A194/A194M-26. Companion nut specification and supplementary requirements.
- Portland Bolt. A193 Nut Compatibility. Common bolting/nut pairings and the optional strain-hardened nut condition.