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 Grade B16 Heavy Hex Bolts
Cr-Mo-V Alloy Steel Heavy Hex Bolts for High-Temperature Refinery and Pressure-Vessel Service
ASTM A193 Grade B16 heavy hex bolts (also written A193 B16 heavy hex bolts, ASTM A193 Gr B16 heavy hex bolts, SA-193 B16 heavy hex bolts, UNS K14072 heavy hex bolts) are chromium-molybdenum-vanadium quenched-and-tempered alloy-steel fasteners manufactured to ASTM A193 / A193M Table 2 mechanical limits and ASME B18.2.1 (inch heavy hex) and B18.2.3.6M (metric heavy hex) dimensional standards for high-temperature pressure-vessel and refinery flange service. TorqBolt produces the full B16 heavy hex bolts family from its Rajkot plant with imperial diameters from 1/4 in to 3 in and metric diameters from M6 to M76, both fine and coarse threads, in stock and on indent.
Heavy Hex Bolts configurations
B16 heavy hex bolts are supplied in two standard configurations per ASME B18.2.1 (inch) and B18.2.3.6M (metric):
- Heavy hex bolts (B18.2.1 / B18.2.3.6M) - thicker head and longer wrench-engagement surface than standard hex bolts. Default for refinery hot-bolted flanges, valve bonnets, and pressure-vessel bonnet bolting.
- Heavy hex cap screws (B18.2.1 / B18.2.3.3M) - finished-head precision heavy hex with chamfered head and tight tolerances. Used in steam-turbine casings and compact-flange joints requiring higher preload accuracy.
Diameter and length range
| Series | Stock diameter range | Indent diameter range | Length range (stock) | Length range (indent) |
|---|---|---|---|---|
| Inch heavy hex bolt (B18.2.1) | 1/2 in to 2 in | 1/4 in to 3 in | up to 16 in | up to 40 in |
| Metric heavy hex (B18.2.3.6M) | M12 to M48 | M6 to M76 | up to 300 mm | up to 900 mm |
| Heavy hex cap screw (B18.2.3.3M) | 1/2 in to 2-1/2 in | 1/4 in to 3 in | up to 14 in | up to 36 in |
B16 heavy hex bolt material
B16 heavy hex bolts are forged from AISI 4140 / 4142 chromium-molybdenum-vanadium alloy steel, conforming to the ASTM A193 Table 1 chemistry envelope. Vanadium addition of 0.25 to 0.35 % is the key differentiator from the B7 grade and is responsible for the creep and stress-relaxation resistance that justifies the B16 premium at elevated service temperatures.
| Element | Cr-Mo-V (% by mass) | Product analysis variation |
|---|---|---|
| C | 0.36 - 0.47 | 0.02 |
| Mn | 0.45 - 0.70 | 0.03 |
| P (max) | 0.035 | 0.005 |
| S (max) | 0.040 | 0.005 |
| Si | 0.15 - 0.35 | 0.02 |
| Cr | 0.80 - 1.15 | 0.05 |
| Mo | 0.50 - 0.65 | 0.03 |
| V | 0.25 - 0.35 | 0.03 |
B16 heavy hex bolt mechanical properties
| Diameter, in. | 2-1/2 and under | over 2-1/2 to 4 | over 4 to 8 |
|---|---|---|---|
| Min. tempering temperature, °F | 1200 | 1200 | 1200 |
| Tensile strength, min, ksi | 125 | 110 | 100 |
| Yield strength, min, 0.2 % offset, ksi | 105 | 95 | 85 |
| Elongation in 4D, min, % | 18 | 17 | 16 |
| Reduction of area, min, % | 50 | 45 | 45 |
| Hardness, max | 321 HBW / 35 HRC | 321 HBW / 35 HRC | 321 HBW / 35 HRC |
Heat treatment
B16 heavy hex bolts are supplied in the quenched-and-tempered (Q&T) condition. The standard heat-treatment route is: austenitise at 1650 to 1700 °F (900 to 925 °C), oil quench to develop a fully martensitic structure, then temper at 1200 °F (650 °C) minimum for time sufficient to meet the ASTM A193 Table 2 mechanical limits. Each heat-treat lot is tested per A193 sect 7. When Supplement S12 is invoked, stress-rupture testing at 1100 °F / 20 ksi is performed (suffix B16R). When Supplement S4 is invoked, hardness is verified on each heavy hex bolt.
Companion nut grade for B16 heavy hex bolts
The canonical companion nut for B16 heavy hex bolts is ASTM A194 Grade 7. Do not pair B16 heavy hex bolts with A194 Grade 2H nuts for sustained service above 750 °F - the Grade 2H carbon-steel nut creeps while the B16 heavy hex bolt retains tension, producing thread-jacking and loss of clamp force. Do not pair B16 heavy hex bolts with A194 Grade 8 or 8M austenitic stainless nuts at elevated temperature due to coefficient-of-thermal-expansion mismatch.
Coatings and surface finish
B16 heavy hex bolts are supplied in the following finish options:
- Plain (uncoated, as-machined) - default; protective oiling prevents shipping corrosion
- Black oxide - light protective finish for low-corrosion environments
- Hot-dip galvanised (HDG) per ASTM A153 - thick zinc coating for atmospheric service; baking required to prevent hydrogen embrittlement
- Zinc plating per ASTM B633 - thinner electroplated coating
- Cadmium plating per ASTM B766 / AMS-QQ-P-416 - aerospace and chemical-service applications
- Xylan 1424 / 1070 fluoropolymer - subsea and offshore high-temperature service
- PTFE coating (Teflon-based) - chemical-process flange bolting
- Geomet 500B / 720 - chromium-free corrosion protection for offshore
- Magni 565 / 575 - zinc-aluminium flake coating
- Phosphate and oil - shipping protection
Quality certification regime
TorqBolt supplies B16 heavy hex bolts with the following certification options:
- EN 10204 Type 3.1 - default mill test certificate covering heat analysis, product analysis, mechanical-properties test, and heat traceability
- EN 10204 Type 3.2 - third-party-witnessed certificate (DNV, Lloyd’s Register, TÜV, Bureau Veritas) for offshore and pressure-vessel projects
- NORSOK M-650 qualified test record (QTR) - five-year validity, essential-variable changes trigger re-qualification
- NACE MR0175 / ISO 15156-2 sour-service certificate - project waiver required (B16 is not a default sour-service grade; B7M is)
- PED 2014/68/EC certificate - European Pressure Equipment Directive compliance for vessel-attached bolting
- API 6A / 17D PSL-3 - subsea and surface wellhead high-temperature applications
- Charpy V-notch impact test (Supplement S6) - on request, for piping below -20 °F
- Magnetic-particle examination (Supplement S7) - for refinery and offshore wellhead service
- ISO 17782 manufacturer-of-standard-fasteners certification - subsea SUBSEA+ pressure-traceability tier (offshore project requirement)
Lead time and procurement
Stock diameters (1/2 in to 2 in / M12 to M48) ship within 3 to 5 business days of order confirmation. Indent diameters (1/4 to 1/2 in / M6 to M10 with custom length, or 2 to 3 in / M48 to M76) carry a 4 to 8 week lead time depending on heat-treatment lot consolidation and certification level (3.1 vs 3.2 vs NORSOK QTR). For NORSOK M-650 QTR-required heavy hex bolts, expect 8 to 12 weeks for first-time grade qualification or 4 to 6 weeks for repeat orders under valid QTR. MOQ on stock is 5 pieces; on indent is 25 pieces per diameter / length combination.
Why engineers specify B16 heavy hex bolts
Heavy hex differs from standard hex on two dimensions that matter under high-temperature preload: the head is thicker by approximately 50% (per ASME B18.2.1 Table 1 vs. B18.2.3.5M heavy-hex table) and the across-flats is one increment larger, giving more wrench-bearing area before galling occurs in hot service. Refinery applications drive the demand – FCC reactor and regenerator manhole flanges, hydrocracker reactor head bolting, coker drum top-head closures, and main-steam supply line flange joints in fossil and combined-cycle power plants. In these joints the cycle is bolt-up cold, heat-up to 700-840 °F service, hold for 6-18 months, cool down for shutdown, repeat. The heavy hex section gives more headroom for the torque-reaction load (often 1.5x the proof load on hot bolting) without flat distortion that would make the joint impossible to disassemble at next turnaround. B16’s vanadium-stabilised matrix holds the preload across thermal cycles in a way that B7 cannot above 750 °F.
Frequently asked questions
What is the maximum service temperature for A193 B16 heavy hex bolts?
840 °F (450 °C) for continuous service per ASME B31.1 / B31.3 / B16.5 flange-bolting tables. 1100 °F (595 °C) for stress-rupture-qualified service with Supplement S12 (suffix B16R, 25 h rupture life at 20 ksi). Some industry sources cite 1000 °F as a conservative peak.
Are B16 heavy hex bolts NACE-compliant for sour service?
B16 is not a default NACE MR0175 / ISO 15156-2 sour-service grade. For combined high-temperature and sour service, B16 requires a project-specific NACE waiver supported by stress-rupture and sulphide-stress-cracking testing. The default NACE sour-service A193 grade is B7M (impact-tested, hardness-controlled). For high-temperature sour service, an A193 B7M / B16 split assessment is often required.
Can B16 heavy hex bolts be used at cryogenic temperature?
No. B16 has no Charpy V-notch requirement below -20 °F (-29 °C). For cryogenic service, use ASTM A320 Grade L7 (UNS G41400) which is the low-temperature counterpart of B16 with mandatory Charpy testing to -150 °F (-101 °C).
When is heavy hex preferred over standard hex in B16 service?
Specify heavy hex above 1-1/2 in / M36 diameter, on any joint with sustained service above 750 °F, and on any joint that is disassembled and re-tightened at scheduled turnarounds (FCC catalyst change, hydrocracker turnaround, steam-turbine inspection). The thicker head + larger wrench flat resist the cumulative micro-distortion that turns a standard hex into a wrench-rounding hazard after 3-5 thermal cycles. Below 1 in diameter and for one-shot assemblies that never see disassembly, standard hex is acceptable and lower-cost.
Related TorqBolt resources
- ASTM A193 Grade B16 bolting specification overview
- A193 B16 vs A193 B7 - full side-by-side comparison
- B16 hex bolts
- B16 stud bolts
- B16 threaded rods
- B16 round bar (mill source)
- ASME SA-193 Grade B16 bolting specification
- B16 anchor bolts (high-temp foundation anchoring)
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