RFQ Today
Certifications: EN 10204 3.1 / 3.2 material test certificates, elevated-temperature mechanical test reports, and complete export documentation packages.
High Temp.
Fasteners
A world-class technical reference for EPC contractors, process and mechanical engineers, procurement heads, and TPI inspection agencies specifying high-temperature fasteners — covering the defining challenge of elevated-temperature bolted joints (stress relaxation and creep-driven preload loss), differential thermal expansion between bolt and flange materials, temperature-zone material selection across RR Hydraulic’s full materials reference library, hot-bolting maintenance practice, and the QC and documentation discipline required for critical high- temperature bolted connection supply.
Why High-Temperature Bolting
Is Fundamentally Different
The single characteristic that distinguishes high-temperature fastener engineering from ambient-temperature bolting practice is not simply “the material must survive the heat” — it is that elevated temperature causes bolt preload to be lost over time even when the bolt itself never fails, through a mechanism entirely absent at ambient temperature.
1.1 — Stress Relaxation and Creep: The Defining High-Temperature Bolting Phenomenon
1.2 — Why Higher-Alloy Materials Resist Relaxation Better
A material’s resistance to stress relaxation correlates closely with its creep-rupture strength at the service temperature — exactly the property that drives the material selection ladder discussed throughout RR Hydraulic’s high-temperature alloy references (Incoloy 800H/800HT’s grain-size-controlled creep strength, discussed in detail in RR Hydraulic’s dedicated reference, being a clear example of a material specifically engineered for superior elevated-temperature strength retention). This is why simply selecting a material that will not melt or oxidize at the service temperature is insufficient for high- temperature bolting — the material’s specific creep-rupture and stress-relaxation resistance at the actual sustained service temperature, not merely its short-term strength or melting point, governs whether the joint will retain adequate preload over its intended maintenance interval.
1.3 — Differential Thermal Expansion: Bolt vs. Flange Material Mismatch
by Service Temperature
& Governing Standards
High-temperature fastener material selection is fundamentally a temperature-zone exercise — the following table maps typical service temperature ranges to appropriate bolting materials across RR Hydraulic’s full materials reference library.
Submit service temperature, material, size, and quantity to sales@rrhydraulics.com for a certified offer.
2.1 — Temperature-Zone Material Selection Ladder
| Approx. Service Temperature | Typical Material | Key Limiting Factor | RR Hydraulic Reference |
|---|---|---|---|
| Up to ~400°C | Standard carbon steel (A307) or A193 B7 alloy steel | Tempering/temper embrittlement range, general strength retention | Carbon Steel A307, A193 B7 references |
| ~400–540°C | ASTM A193 B7/B16 alloy steel (upper qualified range) | Creep-rupture strength begins limiting design allowable stress | A193 B7 reference |
| ~540–650°C | SS 321/347 stabilised austenitic stainless | Sensitisation resistance and moderate creep strength | SS 321 reference |
| ~650–850°C | SS 310/310S, Incoloy 800H/800HT | Oxidation resistance + documented creep-rupture allowables (Incoloy) | SS 310, Incoloy 800 references |
| ~850–1000°C+ | Inconel 600/625, Hastelloy C-22/C-276 | High-temperature oxidation resistance and mechanical strength retention | Inconel 600/625, Hastelloy references |
| Highest strength at moderate-high temp | Inconel 718 (age-hardened) | Maximum strength retention to ~650–700°C for high-stress applications | Inconel 718 reference |
2.2 — Governing Standards
ASME Section II Part D
Publishes design allowable stress values by temperature for the materials referenced in Table 2.A — the primary reference for confirming a specific material’s allowable stress at the actual service temperature, including the temperature-dependent reduction reflecting creep-rupture limitations.
ASME PCC-1
Guidance on bolted flange joint assembly, including considerations for elevated-temperature service and hot bolting/re-torque practice discussed in Part 3.
ASTM A193 / A320
Governs the primary alloy steel bolting grades (B7, B16 for elevated temperature, L7 for low temperature) discussed throughout RR Hydraulic’s dedicated fastener references.
API 660 / TEMA
Heat exchanger design standards referencing appropriate high-temperature bolting material selection for shell-and-tube and other heat exchanger equipment operating at elevated temperature.
Re-Torque Maintenance
& Practical Guidance
Because stress relaxation and creep (Section 1.1) cause gradual, undetectable-without-inspection preload loss, high-temperature bolted joint maintenance requires a deliberate, planned re-tightening practice rather than a “install once and forget” assumption appropriate for ambient-temperature bolting.
3.1 — Hot Bolting: Re-Torquing at Operating Temperature
“Hot bolting” refers to the practice of re-tightening flange bolts while the equipment remains at or near operating temperature and pressure — performed specifically to restore preload lost through the stress relaxation and creep mechanism discussed in Section 1.1, without requiring a full plant shutdown and cool-down cycle. This is a specialised maintenance activity requiring specific safety procedures (working on live, high-temperature, potentially hazardous-fluid-containing equipment) and is typically performed by specifically trained personnel following a documented procedure — the practice reflects the practical reality that scheduled preload restoration is often necessary during a plant’s operating campaign between full shutdowns, given how significant relaxation-driven preload loss can become over months of continuous high-temperature service.
3.2 — When Re-Torque/Hot Bolting Is Planned vs. Reactive
Scheduled Preventive Re-Torque
For known high-relaxation-risk joints (typically identified through the material selection and expected relaxation behaviour discussed in Section 1.2, or through documented plant experience with similar equipment), a scheduled re-torque at a defined interval after initial startup — often within the first few days to weeks of reaching operating temperature, when the majority of short-term relaxation occurs — is standard preventive practice at many facilities.
Reactive Hot Bolting in Response to Detected Leakage
Where flange leakage (visible, detected by gas detection, or identified through routine inspection) indicates inadequate remaining clamping force, reactive hot bolting restores preload without requiring an unplanned shutdown — a valuable operational flexibility, though ideally supplemented by the scheduled preventive approach to avoid relying solely on leak detection as the trigger for maintenance action.
Material Selection to Reduce Re-Torque Frequency
Specifying a higher-relaxation-resistance material (moving up the temperature-zone ladder in Table 2.A, or specifically selecting a material with superior documented stress relaxation behaviour at the actual service temperature) can meaningfully reduce the required re-torque frequency — a genuine engineering trade-off between higher initial material cost and reduced ongoing maintenance burden that should be evaluated for critical or difficult-to-access high-temperature joints.
3.3 — Correct Installation Practice for High-Temperature Bolted Joints
- Correct initial preload: Since some preload loss to relaxation is expected and normal, initial installation preload should account for anticipated relaxation over the joint’s planned maintenance interval, per the applicable ASME PCC-1 guidance and the specific gasket/joint design requirements discussed in RR Hydraulic’s PTFE reference
- Correct tightening sequence: The same star/cross pattern tightening sequence discussed in RR Hydraulic’s Carbon Steel A193 B7 reference applies equally to high-temperature bolting, ensuring even initial gasket seating stress
- Anti-seize compound rated for the service temperature: Standard anti-seize compounds may not remain effective at elevated temperature — verify the specific anti-seize product’s temperature rating is adequate for the actual service condition, since a compound that degrades at temperature loses its anti-galling and friction-consistency benefit precisely when needed most
- Documentation of initial and re-torque preload/torque values: Maintaining a clear maintenance record of initial installation torque, any subsequent hot bolting/re-torque events, and the specific values applied supports informed decision-making about future maintenance intervals and material selection for repeat applications
Industry Applications
& Documentation
RR Hydraulic maintains full traceability across the high- temperature fastener materials range, with elevated-temperature mechanical property verification standard on all project-grade supply.
4.1 — Inspection & QC Protocol
4.2 — EN 10204 / Documentation Requirements
| Certificate | Content | EPC Requirement | When Mandatory |
|---|---|---|---|
| 2.1 / 2.2 | Declaration / non-specific | Not acceptable for pressure-boundary high-temperature supply | Never for critical furnace/boiler/turbine bolting supply |
| 3.1 (EN 10204) | Heat-traceable chemical + mechanical test report | Mandatory — all EPC supply | All high-temperature bolting and general project supply |
| Elevated-temperature mechanical test report | Tensile/creep data at the actual service temperature | Conditional — critical/high-temperature applications | Applications where design allowable stress verification at temperature is required |
| 3.2 (EN 10204) | 3.1 + TPI countersign | Critical / owner-specified critical items | High-consequence high-temperature pressure equipment |
4.3 — Applications by Industry
Refinery and Petrochemical High-Temperature Piping
ASTM A193 B7 through Incoloy/Inconel bolting across the full temperature-zone ladder discussed in Section 2.1, selected per the specific process unit’s operating temperature and design life requirement, with hot bolting maintenance practice (Section 3.1) planned for known high-relaxation-risk joints.
Power Generation Boiler and Turbine Bolting
High-temperature fastener supply for boiler superheater/reheater systems and steam turbine casing bolting, discussed in detail alongside the broader ASME Section I/III framework in RR Hydraulic’s Power Plant Hardware reference — a critical application category where stress relaxation management directly affects plant availability and maintenance planning.
Furnace and Heat Treating Equipment
SS 310/310S and Incoloy fasteners for industrial furnace and heat treating equipment fixtures and flanged connections, operating at the upper end of the temperature-zone ladder where oxidation resistance and elevated-temperature strength retention are both critical selection factors.
4.4 — Export Packaging Specification
- High-temperature fasteners packed by material grade and temperature zone with clear labelling to prevent grade confusion at site receiving inspection, given the significant performance differences across the temperature-zone material ladder discussed in Section 2.1
- Heat/lot number marked or tagged on each item, cross-referenced to the accompanying material test certificate including elevated-temperature mechanical data where provided
- Anti-seize compound (temperature-rated per Section 3.3) supplied or recommended alongside fastener shipment where specified
- Documentation in a waterproof pocket: EN 10204 3.1/3.2 MTC, chemical composition report, mechanical properties report (including elevated-temperature data where applicable), grain size report (where applicable), and packing list with material/temperature-zone/size breakdown per item
- ISPM-15 timber or export cartons for international shipment, with country of origin and HS tariff code documentation matched to the specific fastener material category
Submit your service temperature, material, size, and quantity to RR Hydraulic for a complete, certified commercial offer.