Scope
ASTM A193 / A193M is the dominant specification for bolting used in pressure vessels, valves, flanges, and fittings operating at elevated temperature, elevated pressure, or both. It covers bars, bolts, hex cap screws, hex bolts, screws, studs, stud bolts, and threaded rod in inch and metric dimensions.
Two material families are covered:
- Ferritic and martensitic chromium-molybdenum alloy steels (B5, B6, B7, B7M, B16) — heat-treated to develop strength
- Austenitic stainless steels (B8, B8M, B8T, B8C and their variants) — solution-treated (Class 1) or strain-hardened (Class 2)
A193 does not cover low-temperature service — use A320 instead when the design temperature falls below -20°F / -29°C.
Grades at a glance
Alloy steel grades
| Grade | Material | Typical size range | Notes |
|---|---|---|---|
| B5 | 5Cr-½Mo | Limited | Uncommon; elevated-temp service |
| B6 | 13Cr martensitic (410 SS) | All | Corrosion + moderate temp |
| B7 | AISI 4140 / 4142 Cr-Mo | All common sizes | The workhorse |
| B7M | B7 chemistry, lower hardness | ≤4" | Sour service / NACE MR0175 |
| B16 | Cr-Mo-V alloy | All common sizes | Better creep resistance at high temp |
Stainless grades
| Grade | Material | Class 1 (SA) | Class 2 (strain-hardened) |
|---|---|---|---|
| B8 | 304 SS | Available | Available ≤1-1/2" |
| B8M | 316 SS | Available | Available ≤1-1/2" |
| B8T | 321 SS | Available | Available ≤1-1/2" |
| B8C | 347 SS | Available | Available ≤1-1/2" |
| B8P, B8N, B8MN, B8MLCuN | Nitrogen or N+Mo enhanced | Available | Available |
Class 1 = solution-annealed (lower strength, max corrosion resistance). Class 2 = strain-hardened (higher strength, some corrosion trade-off).
Mechanical properties — Grade B7
| Diameter | Min. Tensile | Min. Yield (0.2%) | Min. Elong. | Min. RA | Max. Hardness |
|---|---|---|---|---|---|
| Up to 2-1/2" | 125 ksi | 105 ksi | 16% | 50% | 321 HBW (35 HRC) |
| 2-5/8" through 4" | 115 ksi | 95 ksi | 16% | 50% | 321 HBW |
| 4-1/8" through 7" | 100 ksi | 75 ksi | 18% | 50% | 321 HBW |
Mechanical properties — Grade B7M
| Diameter | Min. Tensile | Min. Yield (0.2%) | Min. Elong. | Min. RA | Max. Hardness |
|---|---|---|---|---|---|
| Up to 4" | 100 ksi | 80 ksi | 18% | 50% | 235 HBW (22 HRC) |
B7M requires 100% hardness testing of every piece in the lot. The lower hardness is what delivers resistance to hydrogen-induced and sulfide stress cracking.
Mechanical properties — Grade B16
| Diameter | Min. Tensile | Min. Yield (0.2%) | Min. Elong. | Min. RA | Max. Hardness |
|---|---|---|---|---|---|
| Up to 2-1/2" | 125 ksi | 105 ksi | 18% | 50% | 321 HBW |
| 2-5/8" through 4" | 110 ksi | 95 ksi | 17% | 45% | 321 HBW |
| 4-1/8" through 7" | 100 ksi | 85 ksi | 16% | 45% | 321 HBW |
Mechanical properties — Grades B8 / B8M (common sizes)
| Class | Condition | Diameter | Min. Tensile | Min. Yield | Min. Elong. |
|---|---|---|---|---|---|
| 1 | Solution-annealed | All | 75 ksi | 30 ksi | 30% |
| 2 | Strain-hardened | ≤3/4" | 125 ksi | 100 ksi | 12% |
| 2 | Strain-hardened | 7/8" to 1" | 115 ksi | 80 ksi | 15% |
| 2 | Strain-hardened | 1-1/8" to 1-1/4" | 105 ksi | 65 ksi | 20% |
| 2 | Strain-hardened | 1-3/8" to 1-1/2" | 100 ksi | 50 ksi | 28% |
Service temperature guidance
- B7 — practical upper limit around 750°F (400°C); creep begins to dominate above this.
- B16 — used up to roughly 1100°F (593°C). Vanadium addition plus higher tempering temperature preserves strength and resists creep relaxation better than B7.
- B7M — same limit as B7; selected specifically for sour (H₂S-bearing) service per NACE MR0175 / ISO 15156.
- B8 / B8M — usable to approximately 1000°F (538°C) with declining allowable stress; B8M preferred in chloride-bearing environments.
Recommended nuts and washers
| Bolt grade | Nut grade (ASTM A194) | Washer |
|---|---|---|
| B7 | 2H or 7 | F436 (structural) or F844 (general) |
| B7M | 2HM or 7M | F436 or F844 |
| B16 | 4 or 7 (up to 1100°F); 16 preferred above 850°F | F436 or F844 |
| B8 Class 1 | 8 | F436 or F844 (matching stainless) |
| B8M Class 1 | 8M | F436 or F844 (matching stainless) |
| B8 Class 2 | 8 Class 1 or 2 | F436 or F844 (matching stainless) |
| B8M Class 2 | 8M Class 1 or 2 | F436 or F844 (matching stainless) |
Marking requirements
Studs are marked with the manufacturer's identification symbol and the grade symbol (for example, B7, B7M, B8, B8M). B7M and B8M Class 2 require additional marking protocols to confirm testing compliance.
Applications
- Refinery flanges and heat exchangers
- Process valves and piping
- Compressor and turbine casings
- Pressure vessels and boilers
- Wellhead equipment (B7M for sour service)
- Chemical and petrochemical piping (B8M for chloride exposure)
Related specifications
- A320 — Low-temperature bolting companion
- A194 — Nut pairing standard
- A182 — Companion flange / fitting material spec
- F436 — Hardened washers
- F2329 — Hot-dip galvanizing (limited applicability; not for B16 or B7M in most services)
Documentation
Every California Fastener A193 order ships with mill certificates traceable to heat number, confirming chemistry, mechanical properties, heat-treat records, and hardness. Sour-service orders (B7M, 2HM) include NACE MR0175 / ISO 15156 compliance statements on request.