A property class 10.9 bolt has a minimum yield strength of 940 MPa under ISO 898-1. An A286 stud made to ASTM A453 Grade 660, Classes A through C, have a lower specified minimum yield strength of 585 MPa. Both are room-temperature specification minimums and do not provide a meaningful basis for evaluating or comparing fastener performance in an exhaust joint operating at 600°C.
ISO 898-1 establishes the mechanical properties of carbon and alloy steel fasteners at room temperature and acknowledges that these properties may not be maintained under elevated-temperature operating conditions.
Fasteners in turbocharged exhaust systems commonly experience operating temperatures ranging approximately 450 to 600 degrees C, significantly exceeding the temperature range for which conventional room-temperature mechanical properties are specified. Consequently, material selection for these applications requires a more comprehensive evaluation of elevated-temperature performance characteristics.
A286, Nimonic 80A, and Inconel 718 are among the alloys commonly used for these applications, and their elevated-temperature properties make them strong candidates where quenched-and-tempered steels may fall short. However, these materials also present greater manufacturing challenges.
MacLean-Fogg uses warm forming at its Chesterfield Township, Michigan, operation to produce specialty-alloy bolts and studs that can be difficult to manufacture by conventional cold forming.
Table of Contents
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Why Standard Bolts Can Struggle at High Temperatures
- Where Warm Forming Fits
- What Determines Clamp Load Retention at High Temperatures?
- Choosing an Alloy for High-Temperature Service
- Manufacturing High Temperature Bolts at MacLean-Fogg
- Selecting the Right Fastener for a Hot Joint
- Frequently Asked Questions
Key Takeaways
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ISO 898-1 specifies property class 8.8 and 10.9 bolts at room temperature and advises consulting an experienced fastener metallurgist for service outside minus 50 to plus 150 degrees C, up to a maximum of 300 degrees C.
- Fasteners in turbocharged exhaust systems commonly run at about 450 to 600 degrees C fastener metal temperature, so alloy selection matters more than room-temperature strength grade.
- Clamp load retention depends on elevated-temperature strength, stress relaxation and creep, differential thermal expansion, and a fastener that keeps its strength can still lose preload.
- A286, Nimonic 80A, and Inconel 718 get their high-temperature capability from the alloy and its heat treatment, and warm forming is how MacLean-Fogg makes these hard-to-form materials into bolts and studs.
Why Standard Fasteners Can Struggle at High Temperatures
Property classes 8.8 and 10.9 reach their specified mechanical properties through quenching and tempering, and those properties change as operating temperature rises, particularly during prolonged exposure. ISO 898-1 specifies them at room temperature and advises consulting an experienced fastener metallurgist for service outside the range of -50 to +300 degrees C. An informative annex in the earlier ISO 898-1:1988 edition gives approximate elevated-temperature values for property class 10.9.
| Temperature | Approximate proof stress (MPa) |
|---|---|
| 20 degrees C | 940 |
| 100 degrees C | 875 |
| 200 degrees C | 790 |
| 300 degrees C | 705 |
Source: ISO 898-1:1988 informative annex (lower yield stress or 0.2 percent proof stress). Values are guidance, not design allowables, and the table does not cover performance above 300 degrees C.
These figures show how material strength changes with temperature. They do not predict how much clamp load a particular joint will retain, because a bolt can keep adequate tensile strength at temperature and still lose preload through stress relaxation, creep, or differences in how the joint materials expand. Choosing a fastener for a hot engine joint takes more than moving up a room-temperature strength grade.
It also takes knowing the temperature the fastener itself will reach. Exhaust gas temperature and actual fastener temperature can differ significantly, and the difference can affect the material recommendation.
High-Temperature Fastener Specifications
Review alloy options, temperature guidance, and Lockthread specs for turbocharger, exhaust manifold, and EGR applications. Download the High-Temperature Fastener Spec Sheet
Where Warm Forming Fits
Conventional cold forming is an efficient manufacturing method for many carbon and alloy steel fasteners. A286, Nimonic 80A, and Inconel 718 can be more difficult to form at room temperature, particularly when the finished part requires substantial deformation. Warm forming uses controlled heating to improve the workability of these materials during manufacturing, and the optimal forming temperature depends on the alloy, part geometry, and process requirements.
Cold forming, warm forming, and hot forging differ mainly in forming temperature and in the materials each one is best suited for.
| Cold Forming | Warm Forming | Hot Forging | |
|---|---|---|---|
| Forming temperature | At or near room temperature | Controlled heating, with an adjustable forming temperature set for the alloy | Induction heating up to about 1,200 degrees C (2,200 degrees F) |
| Typical use | Special fasteners, piston pins, and other cold-formed components | Threaded fasteners, pins, rivets, and other special parts in alloys that form poorly at room temperature | Near-net forged components such as large industrial nuts, spindle nuts, and gear blanks |
| Example materials | Steel, aluminum, and copper | A286, Inconel 718, Nimonic 80A, and other exotic alloys | Steel, formed from hot-rolled bar |
Sources: MacLean-Fogg forming technologies, cold formed special fasteners, copper components, warm forming, hot forming, and Savanna forged products.
MacLean-Fogg uses warm forming to make specialty-alloy bolts, studs, and other fasteners as net-shape parts. It is particularly useful where a fastener’s geometry or material makes conventional cold forming difficult.
The ability of a finished fastener to hold strength at elevated temperature comes from the alloy, its heat treatment, and its final material condition. Warm forming enables these materials to be manufactured into the required fastener geometries, while fatigue performance and clamp load retention remain dependent on the material’s properties and must be validated through application-specific joint testing.
What Determines Clamp Load Retention at High Temperatures?
Several factors affect how much preload remains in a bolted joint during high-temperature service, and they have to be evaluated together.
- Elevated-Temperature Strength: A fastener’s resistance to yielding generally decreases as temperature rises, with the extent of strength reduction dependent on the material composition, metallurgical condition, and heat treatment.
- Stress Relaxation and Creep: Sustained exposure to elevated temperature can produce time-dependent deformation. As the fastener loses some of its elastic elongation, preload and joint integrity can be compromised..
- Differential Thermal Expansion: The fastener and the clamped components may expand at different rates. Depending on the materials, joint geometry, and operating temperatures, these differences may either increase or decrease fastener loading, potentially affecting joint integrity and long-term reliability.
A286 and certain nickel-based alloys exhibit favorable elevated-temperature properties that make them suitable candidates for demanding exhaust applications. Material strength alone does not establish how much preload a joint will maintain over its service life, as performance is also influenced by gasket characteristics, joint stiffness, installation conditions, thermal cycling, and duration of elevated temperature exposure.
The locking method is another consideration. A mechanical locking feature can help resist rotation-driven loosening, but it cannot prevent preload loss caused by creep or stress relaxation. MacLean-Fogg’s Lockthread is a mechanical locking option for joints where conventional locking methods may be ineffective, and its suitability still depends on the fastener material and operating conditions.
Choosing an Alloy for High-Temperature Service
The best bolts for high temperature service are the ones matched to the fastener metal temperature, applied load, exposure duration, thermal cycling, and corrosion environment. MacLean-Fogg has experience manufacturing fasteners from a range of alloys suited for demanding engine and exhaust applications.
| Material | Application considerations |
|---|---|
| A286 | Precipitation-hardened iron-nickel alloy used for high-temperature fasteners, including exhaust and turbocharger applications. Screening guidance extends to about 650 degrees C, subject to application requirements. |
| MF A286 ECHELON | Higher-strength A286 originally developed for marine applications, with minimum room-temperature properties of 1,300 MPa tensile and 1,030 MPa yield strength. [Temperature guidance pending engineering confirmation.] |
| Inconel 718 | Nickel-based alloy considered where elevated-temperature strength and fatigue requirements exceed A286. Screening guidance extends to about 700 degrees C, subject to material condition, load, and exposure duration. |
| Nimonic 80A | Nickel-based alloy used in high-temperature applications, including turbocharger shaft hardware. Screening guidance extends to about 815 degrees C, subject to application requirements. |
Sources: A286 strength minimums from ASTM A453 Grade 660, Classes A through C. MF A286 PRIME properties and A286, Inconel 718, and Nimonic 80A screening guidance from the heat-resistant fasteners material selection guide.
These are starting points for material selection, not universal temperature ratings. A turbocharger mounting fastener and a turbocharger shaft fastener may see different temperatures, loading, and service demands, and even when both call for specialty alloys they may not call for the same one. Engineers also need to consider how the fastener will be installed, whether a locking feature is necessary, and how the joint will be validated. The heat-resistant fasteners material selection guide covers each alloy by temperature range.
Manufacturing High Temperature Fasteners at MacLean-Fogg
MacLean-Fogg manufactures specialty-alloy bolts and studs at its Chesterfield Township, Michigan, facility, which traces its history to Maynard Manufacturing. The operation combines cold and warm forming with experience making fasteners from A286, Inconel 718, Nimonic 80A, and other specialty alloys, including unique specialized processing of A286 for end product of MF A286 Echelons.
The facility also supports application-specific engineering and testing, including evaluations at elevated temperatures and under vibration. For customers developing a new fastener or reviewing an existing joint, MacLean-Fogg can evaluate the material, forming process, and locking requirements together.
Selecting the Right Fastener for a Hot Joint
Property class 8.8 and 10.9 bolts continue to provide reliable, cost-effective fastening solutions
for many applications, and their room-temperature strength, availability, and cost make them practical when operating conditions stay within the material’s capabilities. At higher temperatures the selection process changes. Engineers have to account for the fastener’s elevated-temperature properties, the joint’s preload requirements, and the effects of sustained exposure and thermal cycling.
Specialty alloys such as A286, Nimonic 80A, and Inconel 718 provide options where conventional carbon alloy fasteners may not be suitable, and warm forming allows these materials to be made into bolts and studs for demanding engine and exhaust applications.
Have a High-Temperature Joint to Review?
Our engineering team can help evaluate material selection, fastener design, and locking requirements for turbocharger, exhaust manifold, and EGR applications. Speak with an Engineer