Related Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
ASTM A193 B16 vs ASTM A193 B7
Cr-Mo-V (Grade B16) vs Cr-Mo (Grade B7) High-Temperature Bolting - Full Side-by-Side Comparison
ASTM A193 Grade B16 and Grade B7 are the two most commonly specified alloy-steel bolting grades under ASTM A193. Both grades hit the same 125 ksi (860 MPa) minimum tensile strength and 105 ksi (725 MPa) minimum yield strength in the standard 2-1/2 in and under diameter class - so the strength is the same on paper. The structural differences appear when you read past the room-temperature mechanical row into chemistry, tempering, creep, stress rupture, and companion nut selection.
Chemistry comparison
| Element | A193 B7 (% by mass) | A193 B16 (% by mass) | Why it differs |
|---|---|---|---|
| C | 0.37 - 0.49 | 0.36 - 0.47 | Near-identical Cr-Mo alloy base |
| Mn | 0.65 - 1.10 | 0.45 - 0.70 | B16 leaner Mn for tempering control |
| P (max) | 0.035 | 0.035 | Same impurity limit |
| S (max) | 0.040 | 0.040 | Same |
| Si | 0.15 - 0.35 | 0.15 - 0.35 | Same |
| Cr | 0.75 - 1.20 | 0.80 - 1.15 | Near-identical |
| Mo | 0.15 - 0.25 | 0.50 - 0.65 | B16 has 2-3x Mo for creep resistance |
| V | not specified | 0.25 - 0.35 | B16 vanadium addition - forms fine V4C3 carbides resisting tempered-martensite embrittlement at high temperature |
UNS designations: A193 B7 = UNS G41400 (AISI 4140), A193 B16 = UNS K14072 (vanadium-modified Cr-Mo-V).
Mechanical properties (room temperature)
| Property | A193 B7 | A193 B16 |
|---|---|---|
| Tensile strength, min (in. ≤2-1/2) | 125 ksi (860 MPa) | 125 ksi (860 MPa) |
| Yield strength, min | 105 ksi (725 MPa) | 105 ksi (725 MPa) |
| Elongation in 4D, min | 16 % | 18 % |
| Reduction of area, min | 50 % | 50 % |
| Hardness, max | 35 HRC | 35 HRC |
| Min. tempering temperature | 1100 °F (593 °C) | 1200 °F (650 °C) |
B16 ductility (elongation 18 % vs 16 %) is marginally higher than B7 at room temperature, reflecting the more thermodynamically stable tempered structure produced at the elevated 1200 °F temper.
Service temperature
| Service window | A193 B7 | A193 B16 |
|---|---|---|
| Continuous service (ASME B31.1 / B31.3 / B16.5) | up to 750 °F (399 °C) | up to 840 °F (450 °C) |
| Stress-rupture qualified (S12 supplement) | not available in standard | up to 1100 °F (595 °C) with B16R suffix (25 h rupture life at 20 ksi) |
| Tempered-martensite embrittlement risk | high above 750 °F (no V to pin grain boundaries) | low - V4C3 carbides resist grain-boundary coarsening |
| Practical peak / occasional | 750 - 800 °F | 1000 - 1100 °F |
| Below -20 °F | not recommended (use A320 L7) | not recommended (use A320 L7) |
Companion nut grade
| Bolt grade | Default nut grade | Notes |
|---|---|---|
| A193 B7 | A194 Grade 2H | Q&T carbon-steel nut, general-service pairing per ASME B16.5 |
| A193 B16 | A194 Grade 7 (Cr-Mo-V nut to match bolt CTE and creep behaviour); Grade 4 legacy | Pairing per ASME B16.5 + indanasteel canonical citation |
Do NOT pair B16 studs with Grade 2H nuts for sustained service above 750 °F: the Grade 2H carbon-steel nut creeps while the B16 stud retains tension, producing thread-jacking and loss of clamp force. Do NOT pair B7 studs with Grade 7 nuts either - the Grade 7 nut is over-specified and adds cost without benefit at B7 service temperatures.
Cost differential
B16 typically commands a 25 to 50 % premium over B7 at the bolt level, driven by:
- Vanadium alloying addition cost (V ferro-alloy is more expensive than the Mo ferro-alloy used in B7)
- Higher tempering temperature requires longer furnace time + tighter temperature control
- S12 stress-rupture qualification (B16R) adds per-lot testing cost
- Smaller production volume means less amortisation across the supply chain
Companion nut cost: A194 Grade 7 is roughly 20 to 30 % more expensive than Grade 2H. Total assembly premium for B16 + Grade 7 vs B7 + Grade 2H is in the 25 to 40 % range.
When to specify B16 over B7
- Continuous service temperature above 750 °F (399 °C)
- Steam-piping flange bolting per ASME B31.1 above 750 °F
- Refinery FCC unit reactor / regenerator flange bolting
- Steam turbine inner / outer casing splits
- Boiler header / superheater bolting
- Pressure-vessel flange bolting at design temperature above 750 °F per ASME BPVC VIII
- Any project requiring documented creep-rupture acceptance (B16R + S12)
- Combined high-temperature + cyclic-loading service where preload retention matters
When to stay with B7
- Continuous service temperature below 750 °F
- General-service flange bolting (ASME B16.5 Class 150 - 600 under 750 °F)
- Structural bolting where creep is not a design driver
- Cost-sensitive applications where the B16 premium is not justified
- Sour service (use B7M, not B16 - B16 is not a default NACE MR0175 grade)
B16 vs B7 vs B8 - three-way summary
| Property | A193 B7 | A193 B16 | A193 B8 / B8M |
|---|---|---|---|
| Material family | Cr-Mo alloy steel | Cr-Mo-V alloy steel | Austenitic stainless (304 / 316) |
| Tensile strength, min | 125 ksi | 125 ksi | 30 - 100 ksi (depends on Class 1 / 2) |
| Continuous service temp | up to 750 °F | up to 840 °F (1100 °F with S12) | up to 1500 °F (B8) |
| Corrosion resistance | low (coated only) | low (coated only) | high |
| Companion nut | A194 Gr. 2H | A194 Gr. 7 | A194 Gr. 8 / 8M |
| Cost (relative to B7) | 1.0x | 1.25 - 1.5x | 2 - 3x |
| NACE MR0175 (sour service) | NO (use B7M) | NO (project waiver) | YES (B8M qualifies) |
| Cryogenic service | NO (use A320 L7) | NO (use A320 L7) | YES (B8 retains toughness to -325 °F) |
Frequently asked questions
Are A193 B7 and A193 B16 interchangeable?
No. Above 750 °F continuous service, B7 will undergo tempered-martensite embrittlement over service life and lose preload through stress relaxation. B16 retains strength and preload through V4C3 carbide pinning. Substituting B7 for B16 in a high-temperature design is a code violation under ASME B31.1 / B31.3.
Can I use Grade 2H nuts on B16 studs?
Not for service above 750 °F. The Grade 2H carbon-steel nut creeps while the B16 stud holds tension, producing thread-jacking and loss of clamp force. Use A194 Grade 7.
Is B16 more brittle than B7 because of vanadium?
No - the opposite. B16 elongation is higher (18 % vs 16 %) and the V4C3 carbides actually improve toughness at elevated temperature by resisting grain coarsening. B16 is more ductile than B7 across the service envelope.
Why is B16 more expensive than B7?
Vanadium ferro-alloy is more expensive than the molybdenum ferro-alloy used in B7; higher temper temperature requires longer furnace time + tighter temperature control; S12 stress-rupture qualification adds per-lot testing cost; production volume is lower. Total premium is 25 to 50 % on the bolt + 20 to 30 % on the companion nut.
Related TorqBolt resources
- A193 Grade B16 bolting specification (home)
- B16 stud bolts product page
- A193 B7 sister micro-site (for B7-specific deep content)
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