Why hot-service joints are a different problem from room-temperature ones: the material routes and designations, the standards that do and do not cover them, and the failure modes that replace fatigue.
Room-temperature property classes say very little about hot-service behaviour.
A high temperature fastener is not simply a stronger bolt. The terms high temperature bolts, heat resistant fasteners and elevated-temperature bolting all describe the same problem: once a joint runs hot, the questions that decide whether it survives are different from the ones that decide a room-temperature joint.
| Requirement | Ordinary fastener | Fastener for hot service |
|---|---|---|
| Strength | Rated by property class at room temperature | Rated by the strength the material still carries at the service temperature |
| Preload | Elastic stretch holds the clamp load | Creep and stress relaxation bleed the clamp load — the axial force in the bolt — away over service time |
| Surface | Zinc plating is the default | Zinc is out of range at temperature; copper-based anti-seize systems are used instead |
| In-service risk | Fatigue, thread stripping | Oxidation and scaling, thread galling and seizure, thermal fatigue |
Property classes are defined and verified at room temperature. They are a measure of what the fastener can carry when it is cold — not of what it can carry after a few thousand hours at the temperature of an exhaust system or a turbine casing.
Two separate things then have to be answered. First, what the material still does at temperature. Second, whether the coating survives, because a coating that melts or embrittles takes the joint with it.
Two routes are used in practice. They trade cost against how much strength is kept at temperature.
The equivalences below are what an engineer actually searches for when a drawing quotes a grade they do not recognise.
| Designation system | Designation |
|---|---|
| China, current | 40Cr10Si2Mo |
| China, former designation | 4Cr10Si2Mo |
| China, numeric designation | S48140 |
| Europe / Germany | X40CrSiMo10-2, material number 1.4731 |
| Japan | SUH3 |
| Russia / GOST | 40Х10С2М (GOST 5632-72) |
| Principle | A low-alloy martensitic heat-resistant steel of the internal-combustion valve-steel family, supplied both as bar and, under a dedicated cold-forming standard, as cold-heading stock. |
| Designation system | A-286 |
|---|---|
| Europe / Germany | X6NiCrTiMoVB25-15-2, material number 1.4980, EN 10269 group SD |
| Condition | Solution treated and age hardened (+AT+P) |
| Principle | An austenitic precipitation-hardening alloy, and the one high-temperature material that sits inside a fastener standard rather than outside it. |
The standards that define the grade, the standards that define fasteners, and the gap between them.
| Standard | Scope | Covers 40Cr10Si2Mo |
|---|---|---|
| GB/T 1221 | Heat-resistant steel bar | Yes — primary grade standard |
| GB/T 12773 | Steel and alloy bar for internal-combustion engine valves | Yes |
| GB/T 20878 | Stainless and heat-resisting steel designations | Yes |
| GJB 2294A-2014 | Aviation steel, military specification | Yes |
| EN 10090 | Valve steels for internal-combustion engines (as 1.4731) | Yes |
| T/TCMCA 0030-2024 | Cold-forming steel technical specification | Yes — dedicated |
| GB/T 28906-2025 | Hot-rolled wire rod for cold heading — general framework | General, not grade-specific |
| GB/T 3098.8 | Mechanical properties of fasteners at elevated temperature | No |
| EN 10269 | Steels and nickel alloys for fasteners | No — but covers A-286 |
| Acceptance and inspection | Scope |
|---|---|
| GB/T 5779.3 | Surface defects and acceptance criteria for fasteners |
Why the default coating for cold service is the wrong default for hot service.
Coating choice at temperature is not primarily a corrosion question. Zinc coatings are outside their useful range once the joint runs hot, and electroplating a high-strength fastener introduces a hydrogen-embrittlement route that has to be controlled. Copper-based anti-seize systems are the usual answer for hot joints, because the practical problem is thread galling and seizure rather than atmospheric corrosion.
The failure modes that replace fatigue and stripping as the design driver.
| Mode | What happens | Why it matters |
|---|---|---|
| Creep | The material deforms slowly under sustained load at temperature | The bolt stretches and the clamp load falls — the classic hot bolt failure |
| Stress relaxation | Load is lost even without visible deformation | The joint loosens and starts to leak or fret |
| Oxidation and scaling | An oxide layer grows and can spall | Section is lost and the thread fit degrades |
| Galling and seizure | Thread surfaces weld and tear on assembly or removal | The joint cannot be serviced |
| Thermal fatigue | Repeated heating and cooling cycles the stress | Cracking after a number of cycles |
| Hydrogen embrittlement | Introduced by some plating processes | Delayed brittle failure under load |
Hot-service fasteners appear wherever a joint has to hold its clamp load through thermal cycling: the exhaust side of an engine, which means exhaust manifold bolts, turbocharger mountings and EGR fasteners; power generation equipment such as turbine, boiler and heat exchanger bolting; and industrial furnace and high-temperature process plant.
The dimensional standard for the joint is chosen separately from the material. Material and geometry are two independent decisions — the material decides what the fastener survives, the standard decides what it fits.