ASTM A193 Grade B7 — Full Technical Specification: Composition, Mechanical Properties, and Regulatory Compliance

1. Chemical Composition and Origin

ASTM A193 B7 fasteners are manufactured from AISI 4140 or AISI 4142 base steel. This alloy undergoes heat treatment involving quenching and tempering at a minimum temperature of 593 °C (1100 °F) to develop its optimal mechanical properties.

• Carbon (C): 0.37 – 0.49 %
• Chromium (Cr): 0.75 – 1.20 % (for hardness and wear resistance)
• Molybdenum (Mo): 0.15 – 0.25 % (for mechanical strength at high temperatures)

2. Mechanical Properties (Diameters ≤ 2.5 inches)

• Tensile Strength (UTS): Minimum 125 ksi (860 MPa)
• Yield Strength (0.2% Yield): Minimum 105 ksi (720 MPa)
• Maximum Hardness: 35 HRC or 321 HBW
• Minimum Elongation: 16 %

3. ⚙️ Common Applications and Threading

• Typical Use: Industrial pipe fittings, flange bolting, power plants, and pressure vessels.
• Standard Threading: For diameters greater than 1 inch, the standard by default requires an 8 threads per inch (8 TPI) pitch. UNC threads (coarse pitch) are available upon request.
• Associated Elements: B7 studs are traditionally paired with high-strength nuts according to ASTM A194 Grade 2H and hardened washers according to ASTM F436.
• An unplated ASTM A193 Grade B7 stud or screw is not systematically compliant with European regulations or critical oil & gas industry requirements without additional controls and certifications.

4. Comparative Compliance Table ASTM A193 B7

5. 🔍 Key Points to Remember for Your Orders

1. The "dual marking" ASME / ASTM: Manufacturers often certify material as ASTM A193 B7 / ASME SA-193 B7, which removes any ambiguity for projects based on American calculation codes (ASME B31.3, ASME VIII).
2. The PED pitfall: If you install ASME flanges with B7 bolts in Europe, the original 3.1 certificate is not sufficient. Explicitly demand a PED-compliant certificate from the supplier, including impact toughness (usually measured at -29 °C or -46 °C as required).
3. The API 20E standard: For the "upstream" sector (wellheads, subsea equipment), simple off-the-shelf B7 is prohibited. The required BSL level must be specified (BSL-2 being the most frequent standard in refinery/offshore).

6. 📋 Technical Requirements Table (B7 — Diameter ≤ 2.5 inches)

⚠️ Additional Requirements (PED and API)

• For PED (Europe): Mandatory addition of an Impact Test (Charpy-V Notch). The standard minimum value is 27 Joules at the minimum service temperature (often -29 °C or -46 °C depending on the need).
• For API 20E (BSL-2 / BSL-3): Mandatory addition of Non-Destructive Testing (Magnetic Particle Inspection) on 100% of parts to detect surface micro-cracks, and examination of austenitic grain size in the laboratory.

7. Technical Purchase Specification: ASTM A193 B7 Bolting

a. Purpose and Applicable Standards

• Base Material: ASTM A193 Grade B7 / ASME SA-193 Grade B7 (Latest editions in force).
• European Regulation: Compliance with Pressure Equipment Directive PED 2014/68/EU.
• Petroleum Requirement (If applicable): API 20E - BSL 2 Compliance.

b. Chemical and Metallurgical Requirements

• Original Steel: Alloy steel type AISI 4140 or 4142.
• Melting Practice: Fully killed, fine grain steel (Grain size ≥ 5 according to ASTM E112).
• Heat Treatment: Quenched and liquid tempered. Minimum tempering temperature: 593 °C (1100 °F).

c. Mechanical Characteristics (Diameters ≤ 2.5'' / 63.5 mm)

• Tensile Strength (Rm): ≥ 860 MPa (125 ksi).
• Yield Strength (Rp0.2): ≥ 720 MPa (105 ksi).
• Elongation (A%): ≥ 16 %.
• Hardness: 35 HRC maximum (321 HBW maximum).
• Impact Toughness (Mandatory PED Requirement):
  • Charpy-V impact test according to ASTM A370.
  • Test temperature: -29 °C (or -46 °C depending on project needs).
  • Minimum absorbed energy: 27 Joules (Average of 3 specimens) / minimum 20 Joules per specimen.

d. Dimensions and Threading

• Studs / Screws: According to ASME B18.31.2 or ASME B18.2.1.
• Threading: UNC type pitch for diameters ≤ 1''. 8-UN pitch (8 TPI) for diameters > 1''.
• Associated Nuts: ASTM A194 Grade 2H (or 7 / 7M if low temperature), Heavy Hex according to ASME B18.2.2.

e. Non-Destructive Testing (NDT)

• Magnetic Particle Testing (MT): 100% of batches after threading and heat treatment according to ASTM E709. No linear cracks accepted.

f. Traceability, Marking, and Certification

• Physical Marking: B7 symbol (or B7M / 20E2) and indelible Heat Number on each piece.
• Required Certificate: EN 10204 3.1 in French or English, including:
  1. Chemical analysis of the heat.
  2. Results of tensile and hardness tests.
  3. Results of impact toughness test (PED Requirement).
  4. PED 2014/68/EU conformity certificate (with approved EPM / PMA mention).

8. Protective Coatings — Options A to D

Option A: Fluoropolymer / PTFE Coating (e.g., Xylan® 1014 or 1424 Blue)

• Application: Ideal for Oil & Gas, offshore, and flanges that need to be easily disassembled. Excellent chemical resistance.
• Requirements:
  • Surface preparation by blasting followed by a phosphated or zinc-nickel primer.
  • Dry film thickness: 20 to 45 microns.
  • Salt spray resistance: Minimum 1000 hours according to ASTM B117 without red rust.
  • Temperature limit: -50 °C / +200 °C max.
  • Note: The threads of associated ASTM A194 2H nuts must be oversized according to ASME B1.1 to compensate for thickness.

Option B: Electroplated Zinc with Passivation (Zinc-Nickel or Zinc-Chrome)

• Application: Standard protection for general industry and moderately corrosive environments.
• Requirements:
  • Zinc-Nickel (Zn-Ni) coating highly recommended instead of pure Zinc.
  • Thickness: 8 to 12 microns minimum.
  • ⚠️ MANDATORY DEGAUSSING: Thermal degasification treatment (Baking) within 4 hours maximum after electroplating, at 190 °C – 220 °C for 4 hours minimum (according to ASTM F1941).

🟧 Option C: Hot-Dip Galvanizing (HDG)

• Application: Heavy outdoor protection against atmospheric corrosion (civil engineering, structures).
• Requirements:
  • Conformity to ASTM A153 Class C or ISO 10684.
  • Surface preparation by mechanical pickling (shot blasting) preferred.
  • Average thickness: Minimum 50 microns.
  • Temperature limit: Risk of liquid zinc embrittlement above +300 °C.

Option D: Plain / Black Oxide Finish

• Application: Indoor use, total immersion in oil, or for very high-temperature applications (up to +450 °C).
• Requirements: Delivered as-heat-treated (cleaned mill scale) with a thin layer of neutral storage oil. No lasting protection against outdoor humidity.

💡 Practical Advice for Your Order

If your flanges operate at a temperature above 200 °C, avoid PTFE (Option A). If they operate above 300 °C, avoid galvanizing (Option C) and prefer the plain finish (Option D) or higher-grade steels (such as ASTM A193 Grade B16).

9. The 3 Blue PTFE Combinations

1. Blue PTFE on Zinc-Nickel Undercoat (High-end — Offshore Standard)

• Principle: Electrolytic deposition of Zinc-Nickel (Zn-Ni 12–15%), then baked blue PTFE over it.
• Performance: More than 1500 hours of salt spray resistance (ASTM B117).
• Purchase Requirement: Mandatory thermal degasification after Zinc-Nickel.

2. Blue PTFE on Phosphating (Economical Standard — Onshore Refinery)

• Principle: Zinc or manganese phosphating serving as a chemical anchor for blue PTFE.
• Performance: Approximately 800 to 1000 hours in salt spray.
• Caution: If the blue coating is scratched, the B7 steel may begin to rust locally.

Exact Clause to Add to Your Purchase Specification

« COATING-01: Duplex Fluoropolymer / Blue PTFE Coating (Type Xylan 1014 / 1424 or approved equivalent) »

• System: Duplex coating, comprising a base layer of [Choose: Phosphating OR Zinc-Nickel with degassing] and a finish of two layers of thermoset blue PTFE.
• Thickness: Total dry film thickness between 25 and 45 microns.
• Control: Nuts threaded oversized in accordance with ASME B1.1 Class 2B.

10. ⚠️ The Coefficient of Friction (K) — A Pitfall to Avoid

Blue PTFE is extremely lubricated (μ ≈ 0.08 to 0.12). If your teams tighten these blue bolts with the same torque as a traditional black bolt, you risk plastifying (breaking) the B7 stud due to overtightening.

📊 Comparison Table: Blue PTFE vs. Plain Black (Inches)

Note: Theoretical values based on Targett's formula (T = K · D · F). Actual values depend on your engineering department's specific calculation procedures (e.g., ASME PCC-1).

The 3 Golden Rules to Transmit to Your Assembly Teams

1. Prohibition of using old tightening sheets: If the factory switches from black bolting to blue bolting, the maintenance department must imperatively update the tightening instruction sheets.
2. No additional grease on blue PTFE: The blue coating is sufficient on its own. Adding grease (Molykote type) unpredictably alters the coefficient of friction.
3. Black bolts require controlled lubrication: A "dry" black bolt can seize instantly (galling phenomenon). Black bolts must be lightly lubricated with a standard light oil.

11. Detailed Torque Calculation Method (Targett)

The calculation is divided into 3 main steps according to the universal Targett formula: T = K · D · F

🛑 Step 1: Calculation of Resistant Area (As)

ISO 898-1 formula: As = (π/4) × (D − 0.9382 × P)²

• M16 (2.0 mm pitch): As = 156.7 mm²
• M20 (2.5 mm pitch): As = 244.8 mm²

⚙️ Step 2: Calculation of Clamping Force (F)

Metric B7 yield strength: Re = 720 MPa. ASME PCC-1 standard recommends tightening to 65% of Re.
Target stress (σ) = 720 MPa × 0.65 = 468 N/mm²

• M16 × 100: F = 156.7 × 468 = 73,476 N ≈ 73.5 kN (~7.3 tons)
• M20 × 120: F = 244.8 × 468 = 114,660 N ≈ 114.6 kN (~11.4 tons)

🔧 Step 3: Calculation of Tightening Torque (T = K · D · F)

⚠️ Important practical note

Actual values applied on-site may be 10 to 15% lower than this theoretical calculation. If your teams use copper or nickel paste on black bolts, the K factor may drop to 0.12, which would reduce the M20 torque to 275 N.m.

📊 Complete Tightening Tables — 30%, 50% and 70% of Yield Strength

🟢 M16 × 100 Bolt (As = 156.7 mm²)

🔵 M20 × 120 Bolt (As = 244.8 mm²)

All values are calculated according to the formula T = K · D · F (Targett).

Detailed calculations for the 3 dimensions (CHC shrink fit screws Class 8.8)

We use the universal torque formula T = K · D · F. The coefficient of friction used is K = 0.15 (lightly oiled class 8.8 screw).

1️⃣ CHC M8 x 25 Screw

• As — 1.25 mm pitch: 36.6 mm²
• F : 36.6 × 448 = 16,397 N → 16.4 kN (1.6 t)
• T : 0.15 × 0.008 m × 16,397 N = 19.7 N.m
• Protrusion : 25 − 22 = +3 mm ✅

2️⃣ CHC M10 x 30 Screw

• As — 1.50 mm pitch: 58.0 mm²
• F : 58.0 × 448 = 25,984 N → 26.0 kN (2.6 t)
• T : 0.15 × 0.010 m × 25,984 N = 39.0 N.m
• Protrusion : 30 − 28 = +2 mm ✅

3️⃣ CHC M12 x 35 Screw

• As — 1.75 mm pitch: 84.3 mm²
• F : 84.3 × 448 = 37,766 N → 37.8 kN (3.8 t)
• T : 0.15 × 0.012 m × 37,766 N = 68.0 N.m
• Protrusion : 35 − 34 = +1 mm ⚠️ Limit

📊 Summary Table of Values

🛠️ Assembly Method — Progressive Circular Tightening

1. Hand tighten.
2. Complete circular pass at 6 N.m (M8) / 12 N.m (M10) / 20 N.m (M12).
3. Complete circular pass at 12 N.m (M8) / 24 N.m (M10) / 40 N.m (M12).
4. Successive passes at the final torque until the torque wrench stabilizes.

12. 🧮 Full ISO 16047 Formula — Detailed M8, M10, M12 Calculations

T = F × [ (0.159 × P) + (0.577 × d₂ × μ_th) + (D_m/2 × μ_b) ]

• F : Preload force (N) — P : Thread pitch (mm) — d₂ : Pitch diameter (mm)
• D_m : Mean bearing diameter under head (mm) — μ_th = μ_b = 0.15

📐 Exact Geometries of CHC Screws (ISO 4762)

1️⃣ ISO 16047 — M8 x 25 — F = 16,397 N

1. Ramp effect: 0.159 × 1.25 = 0.19875 mm
2. Thread friction: 0.577 × 7.188 × 0.15 = 0.62239 mm
3. Under-head friction: 11.00/2 × 0.15 = 0.82500 mm

Sum = 1.64614 mm → M8 Torque = 16,397 × 0.001646 = 27.0 N.m

2️⃣ ISO 16047 — M10 x 30 — F = 25,984 N

1. Ramp effect: 0.159 × 1.50 = 0.23850 mm
2. Thread friction: 0.577 × 9.030 × 0.15 = 0.78285 mm
3. Under-head friction: 13.50/2 × 0.15 = 1.01250 mm

Sum = 2.03385 mm → M10 Torque = 25,984 × 0.002034 = 52.8 N.m

3️⃣ ISO 16047 — M12 x 35 — F = 37,766 N

1. Ramp effect: 0.159 × 1.75 = 0.27825 mm
2. Thread friction: 0.577 × 10.863 × 0.15 = 0.94184 mm
3. Under-head friction: 15.75/2 × 0.15 = 1.18125 mm

Sum = 2.40134 mm → M12 Torque = 37,766 × 0.002401 = 90.7 N.m

📊 Final Summary — ISO 16047 vs Simplified Targett

💡 Key conclusion: The simplified Targett formula consistently underestimates the actual torque by 33 to 35% for CHC screws. Always use the ISO 16047 formula for critical applications.