In high-temperature and high-pressure services such as oil and gas, petrochemical, chemical processing, and power generation, bolting components are often among the least noticeable yet most critical elements of system safety. Numerous engineering failures have shown that equipment breakdowns do not always originate from major components or design flaws, but rather from the failure of bolts and studs due to fracture, stress corrosion, or improper material selection.
ASTM A193 bolts are resistant to high temperatures and corrosion, suitable for high-risk industrial environments. Besides being used for securing petrochemical pipelines, these high-quality bolts are also needed for installing brackets for LED high bay lights in factory workshops, to withstand corrosion and ensure the stability and longevity of the equipment.
ASTM A193 Materials and Chemical Composition
| ASTM A193 Grade | UNS | Material Type | C | Cr | Ni | Mo | N | V | Ti / Nb / Cu | Typical Application |
| B5 | — | Ferritic Cr-Mo steel | ≥0.10 | 4.0–6.0 | — | 0.40–0.65 | — | — | — | Medium-temperature service |
| B6 / B6X | S41000 | Ferritic Cr steel | 0.08–0.15 | 11.5–13.5 | — | — | — | — | — | Heat-resistant service |
| B7 | — | Ferritic Cr-Mo steel | 0.37–0.49 | 0.75–1.20 | — | 0.15–0.25 | — | — | — | High-temperature / pressure bolting |
| B7M | — | Ferritic Cr-Mo steel (controlled hardness) | 0.37–0.49 | 0.75–1.20 | — | 0.15–0.25 | — | — | — | Sour service (H₂S) |
| B16 | — | Ferritic Cr-Mo-V steel | 0.36–0.47 | 0.80–1.15 | — | 0.50–0.65 | — | 0.25–0.35 | — | Higher-temperature service |
| B8 / B8A | S30400 | Austenitic stainless steel | ≤0.08 | 18.0–20.0 | 8.0–11.0 | — | — | — | — | General corrosion resistance |
| B8C / B8CA | S34700 | Austenitic SS (Nb stabilized) | ≤0.08 | 17.0–19.0 | 9.0–12.0 | — | — | — | Nb+Ta ≥10×C | High-temperature, anti-intergranular corrosion |
| B8P / B8PA | S30500 | Austenitic stainless steel | ≤0.12 | 17.0–19.0 | 11.0–13.0 | — | — | — | — | Cold-worked fasteners |
| B8M / B8MA | S31600 | Austenitic stainless steel | ≤0.08 | 16.0–18.0 | 10.0–13.0 | 2.0–3.0 | — | — | — | Chloride-containing environments |
| B8M2 / B8M3 | S31600 | Austenitic stainless steel | ≤0.08 | 16.0–18.0 | 10.0–13.0 | 2.0–3.0 | — | — | — | Engineering sub-classes |
| B8N / B8NA | S30451 | Nitrogen-alloyed austenitic SS | ≤0.08 | 18.0–20.0 | 8.0–11.0 | — | 0.10–0.16 | — | — | Increased strength |
| B8MN / B8MNA | S31651 | Nitrogen-alloyed austenitic SS | ≤0.08 | 16.0–18.0 | 10.0–13.0 | 2.0–3.0 | 0.10–0.16 | — | — | High-strength, corrosion resistant |
| B8MLCuN / B8MLCuNA | S31254 | Super austenitic stainless steel | ≤0.020 | 19.5–20.5 | 17.5–18.5 | 6.0–6.5 | 0.18–0.22 | — | Cu 0.5–1.0 | Seawater / high-chloride service |
| B8T / B8TA | S32100 | Ti-stabilized austenitic SS | ≤0.08 | 17.0–19.0 | 9.0–12.0 | — | — | — | Ti ≤0.70 | High-temperature service |
| B8R / B8RA | S20910 | Heat-resistant austenitic SS | ≤0.06 | 20.5–23.5 | 11.5–13.5 | 1.5–3.0 | 0.20–0.40 | 0.10–0.30 | Nb+Ta | High-temperature strength |
| B8S / B8SA | S21800 | Heat-resistant austenitic SS | ≤0.10 | 16.0–18.0 | 8.0–9.0 | — | 0.08–0.18 | — | — | Elevated-temperature service |
| B8LN / B8LNA | S30453 | Low-carbon austenitic SS | ≤0.030 | 18.0–20.0 | 8.0–11.0 | — | 0.10–0.16 | — | — | Resistance to intergranular corrosion |
| B8MLN / B8MLNA | S31653 | Low-carbon austenitic SS | ≤0.030 | 16.0–18.0 | 10.0–13.0 | 2.0–3.0 | 0.10–0.16 | — | — | Resistance to intergranular corrosion |
In ASTM A193, letter suffixes are an integral and mandatory part of the grade designation and are not optional identifiers.
Suffix A indicates that the material achieves its strength through cold working; M indicates that the material is subject to maximum hardness limits to meet NACE / H₂S sour service requirements; L denotes low carbon content to reduce the risk of intergranular corrosion; N indicates that nitrogen is intentionally added to increase strength and improve pitting resistance; Cu indicates that the steel is copper alloyed to enhance corrosion resistance in acidic or high-chloride environments; and X indicates special or project-specific chemical composition control, which must be verified strictly against the specification and the Material Test Report (MTR). In engineering design, procurement, and inspection, any omission or misuse of these suffixes may result in non-compliant materials or in-service failure.
ASTM A193 – Mechanical Requirements ( Inch Products)
Ferritic Steels
| Grade | Diameter (in.) | Min Tempering Temp (°F) | Tensile Strength min (ksi) | Yield Strength min (ksi) | Elongation min (%) | Reduction of Area min (%) | Hardness Max |
| B5 (4–6% Cr) | ≤ 4 | 1100 | 100 | 80 | 16 | 50 | — |
| B6 (13% Cr) | ≤ 4 | 1100 | 110 | 85 | 15 | 50 | — |
| B6X (13% Cr) | ≤ 4 | 1100 | 90 | 70 | 16 | 50 | 26 HRC |
| B7 (Cr-Mo) | ≤ 2½ | 1100 | 125 | 105 | 16 | 50 | 321 HB or 35 HRC |
| > 2½ – 4 | 1100 | 115 | 95 | 16 | 50 | 321 HB or 35 HRC | |
| > 4 – 7 | 1100 | 100 | 75 | 18 | 50 | 321 HB or 35 HRC | |
| B7M (Cr-Mo) | ≤ 4 | 1150 | 100 | 80 | 18 | 50 | 235 HB or 99 HRB |
| > 4 – 7 | 1150 | 100 | 75 | 18 | 50 | 235 HB or 99 HRB | |
| B16 (Cr-Mo-V) | ≤ 2½ | 1200 | 125 | 105 | 18 | 50 | 321 HB or 35 HRC |
| > 2½ – 4 | 1200 | 110 | 95 | 17 | 45 | 321 HB or 35 HRC | |
| > 4 – 8 | 1200 | 100 | 85 | 16 | 45 | 321 HB or 35 HRC |
Austenitic Steels
| Class / Grade | Diameter (in.) | Heat Treatment | Tensile Strength min (ksi) | Yield Strength min (ksi) | Elongation min (%) | Reduction of Area min (%) | Hardness Max |
| Class 1 & 1D: B8, B8M, B8P, B8LN | All | Carbide solution treated | 75 | 30 | 30 | 50 | 223 HB or 96 HRB |
| Class 1: B8C, B8T | All | Carbide solution treated | 75 | 30 | 30 | 50 | 223 HB or 96 HRB |
| Class 1A: B8A, B8CA, B8MA, B8TA, B8LNA, B8MNA | All | Solution treated in finished condition | 75 | 30 | 30 | 50 | 192 HB or 90 HRB |
| Class 1B & 1D: B8N, B8MN, B8MLCuN | All | Carbide solution treated | 80 | 35 | 30 | 40 | 223 HB or 96 HRB |
| Class 1C & 1D: B8R | All | Carbide solution treated | 100 | 55 | 35 | 55 | 271 HB or 28 HRC |
| Class 1C: B8S | All | Carbide solution treated | 95 | 50 | 35 | 55 | 271 HB or 28 HRC |
| Class 1C: B8SA | All | Solution treated in finished condition | 95 | 50 | 35 | 55 | 271 HB or 28 HRC |
| Class 2: B8, B8C, B8P, B8T | ≤ ¾ | Solution treated & strain hardened | 125 | 100 | 12 | 35 | 321 HB or 35 HRC |
| > ¾ – 1¼ | ″ | 115 | 80 | 15 | 35 | 321 HB or 35 HRC | |
| > 1¼ – 1½ | ″ | 105 | 65 | 20 | 35 | 321 HB or 35 HRC | |
| > 1½ | ″ | 100 | 50 | 28 | 45 | 321 HB or 35 HRC | |
| Class 2: B8M, B8MN, B8MLCuN | ≤ ¾ | Solution treated & strain hardened | 110 | 95 | 15 | 45 | 321 HB or 35 HRC |
| > ¾ – 1 | ″ | 100 | 80 | 20 | 45 | 321 HB or 35 HRC | |
| > 1 – 1¼ | ″ | 95 | 65 | 25 | 45 | 321 HB or 35 HRC | |
| > 1¼ – 1½ | ″ | 90 | 50 | 30 | 45 | 321 HB or 35 HRC | |
| Class 2: B8M2 | ≤ 2 | Solution treated & strain hardened | 95 | 75 | 25 | 40 | 321 HB or 35 HRC |
| > 2 – 2½ | ″ | 90 | 65 | 30 | 40 | 321 HB or 35 HRC | |
| Class 2C: B8M3D | ≤ 2 | Solution treated & strain hardened | 85 | 60 | 30 | 60 | 321 HB or 35 HRC |
| > 2½ – 3 | ″ | 80 | 55 | 30 | 40 | 321 HB or 35 HRC |
Important Notes
- When the Brinell hardness exceeds 200 HB, the hardness shall be 93 HRB or less.
- Classes 1A, 1C, and 1D shall be solution treated in the finished condition.
- Strain-hardened austenitic steels may exhibit non-uniform mechanical properties across large cross sections.
- For diameters 1½ in. and larger, center (core) properties may be lower than the tabulated values, as test results are typically based on values determined at one-half radius.
ASTM A193 – Mechanical Requirements (Metric Products)
Ferritic Steels
| Grade | Diameter (mm) | Min Tempering Temp (°C) | Tensile Strength min (MPa) | Yield Strength min (MPa) | Elongation min (%) | Reduction of Area min (%) | Hardness Max |
| B5 (4–6% Cr) | ≤ M100 | 593 | 690 | 550 | 16 | 50 | — |
| B6 (13% Cr) | ≤ M100 | 593 | 760 | 585 | 15 | 50 | — |
| B6X (13% Cr) | ≤ M100 | 593 | 620 | 485 | 16 | 50 | 26 HRC |
| B7 (Cr-Mo) | ≤ M64 | 593 | 860 | 720 | 16 | 50 | 321 HB or 35 HRC |
| > M64 – M100 | 593 | 795 | 655 | 16 | 50 | 321 HB or 35 HRC | |
| > M100 – M180 | 593 | 690 | 515 | 18 | 50 | 321 HB or 35 HRC | |
| B7M (Cr-Mo) | ≤ M100 | 620 | 690 | 550 | 18 | 50 | 235 HB or 99 HRB |
| > M100 – M180 | 620 | 690 | 515 | 18 | 50 | 235 HB or 99 HRB | |
| B16 (Cr-Mo-V) | ≤ M64 | 650 | 860 | 725 | 18 | 50 | 321 HB or 35 HRC |
| > M64 – M100 | 650 | 760 | 655 | 17 | 45 | 321 HB or 35 HRC | |
| > M100 – M180 | 650 | 690 | 585 | 16 | 45 | 321 HB or 35 HRC |
Austenitic Steels
Classes 1 / 1A / 1B / 1C / 1D
| Class / Grade | Diameter (mm) | Heat Treatment | Tensile Strength min (MPa) | Yield Strength min (MPa) | Elongation min (%) | Reduction of Area min (%) | Hardness Max |
| Class 1 & 1D: B8, B8M, B8P, B8LN | All | Carbide solution treated | 515 | 205 | 30 | 50 | 223 HB or 96 HRB |
| Class 1: B8C, B8T | All | Carbide solution treated | 515 | 205 | 30 | 50 | 223 HB or 96 HRB |
| Class 1A: B8A, B8CA, B8MA, B8TA, B8LNA, B8MNA | All | Solution treated in finished condition | 515 | 205 | 30 | 50 | 192 HB or 90 HRB |
| Class 1B & 1D: B8N, B8MN, B8MLCuN | All | Carbide solution treated | 550 | 240 | 30 | 40 | 223 HB or 96 HRB |
| Class 1C & 1D: B8R | All | Carbide solution treated | 690 | 380 | 35 | 55 | 271 HB or 28 HRC |
| Class 1C: B8RA | All | Solution treated in finished condition | 690 | 380 | 35 | 55 | 271 HB or 28 HRC |
| Class 1C & 1D: B8S | All | Carbide solution treated | 655 | 345 | 35 | 55 | 271 HB or 28 HRC |
| Class 1C: B8SA | All | Solution treated in finished condition | 655 | 345 | 35 | 55 | 271 HB or 28 HRC |
Austenitic Steels – Class 2（Solution Treated + Strain Hardened）
Class 2: B8, B8C, B8P, B8T
| Diameter (mm) | Tensile min (MPa) | Yield min (MPa) | Elongation min (%) | Reduction min (%) | Hardness Max |
| ≤ M20 | 860 | 690 | 12 | 35 | 321 HB or 35 HRC |
| > M20 – M24 | 795 | 550 | 15 | 35 | 321 HB or 35 HRC |
| > M24 – M30 | 725 | 450 | 20 | 35 | 321 HB or 35 HRC |
| > M30 – M36 | 690 | 345 | 28 | 45 | 321 HB or 35 HRC |
Class 2: B8M, B8MN, B8MLCuN
| iameter (mm) | Tensile min (MPa) | Yield min (MPa) | Elongation min (%) | Reduction min (%) | Hardness Max |
| ≤ M20 | 760 | 655 | 15 | 45 | 321 HB or 35 HRC |
| > M20 – M24 | 690 | 550 | 20 | 45 | 321 HB or 35 HRC |
| > M24 – M30 | 655 | 450 | 25 | 45 | 321 HB or 35 HRC |
| > M30 – M36 | 620 | 345 | 30 | 45 | 321 HB or 35 HRC |
Class 2B / 2C / 2D
| Class / Grade | Diameter (mm) | Tensile min (MPa) | Yield min (MPa) | Elongation min (%) | Reduction min (%) | Hardness Max |
| Class 2B: B8M2 | ≤ M48 | 655 | 515 | 25 | 40 | 321 HB or 35 HRC |
| > M48 – M64 | 620 | 450 | 30 | 40 | 321 HB or 35 HRC | |
| Class 2C: B8M3 | ≤ M48 | 550 | 380 | 30 | 40 | 321 HB or 35 HRC |
| > M48 – M72 | 585 | 450 | 30 | 60 | 321 HB or 35 HRC | |
| Class 2D: B8M3D | > M48 | 585 | 415 | 30 | 60 | 321 HB or 35 HRC |
Key Notes
- To meet the tensile strength requirements, the Brinell hardness shall be greater than 200 HB (approximately 93 HRB).
- Classes 1A, 1C, and 1D shall be solution treated in the finished condition.
- Class 2 (strain-hardened) austenitic steels may exhibit non-uniform mechanical properties at large diameters.
- For diameters ≤ M20, a maximum hardness of 241 HB (100 HRB) is permitted.
For diameters ≥ M38, center (core) properties may be lower than the tabulated values, as test results are typically based on values determined at one-half radius.
Summary
ASTM A193 is not merely a “bolt material standard,” but a comprehensive engineering safety specification designed for high-risk service conditions. Its requirements for chemical composition, mechanical properties, heat treatment, and hardness control are directly linked to the long-term reliability of bolting materials operating under high temperature, high pressure, corrosive, or sour service environments.
In engineering design, procurement, and inspection, a complete and accurate ASTM A193 material designation is essential to prevent material misuse and reduce operational risk. In many cases, getting the specification right at the beginning is far more effective than correcting problems after installation. Only by fully understanding and correctly applying ASTM A193 can its intended engineering value be realized.
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