How to Choose High-Temperature Bolts: Material Logic for -200°C to +700°C Service
Executive Summary
From liquid hydrogen at -253°C to exhaust systems at +700°C, bolts face completely different failure mechanisms: low temperature is brittle fracture, mid temperature is relaxation, and high temperature is creep. Following the wide-temperature-range framework of GB/T 3098.8-2010, this article explains material selection, strength derating, and design essentials by temperature zone.
Table of Contents
- Temperature Is the Most Underrated Variable for Fasteners
- Divide into Temperature Zones First, Then Discuss Materials
- Low-Temperature Zone: Why Bolts Fracture "Without Warning"
- Mid-Temperature Zone: Preload Being "Slowly Eaten Away"
- High-Temperature Zone: The Double Blow of Creep and Material Degradation
- Five Essentials for Wide-Temperature-Range Design
- Conclusion: Temperature Changes the Entire Design Logic
Temperature Is the Most Underrated Variable for Fasteners
A bolt that performs excellently at room temperature often fails unexpectedly in high- or low-temperature environments: at low temperature it fractures brittlely with no plastic deformation at all; at high temperature the joint loosens on its own after some service. These two problems have completely different mechanisms, but both point to the same fact—temperature changes material behavior, while standard mechanical property data are all measured at room temperature.
GB/T 3098.8-2010 (Bolted joint parts used at -200°C to +700°C) was compiled precisely to fill this gap, providing requirements for materials, property classes, and design corrections segmented by temperature zone.
Divide into Temperature Zones First, Then Discuss Materials
The wide temperature range can be divided into four zones, each with a different dominant failure mechanism:
| Temperature Zone | Range | Dominant Failure Mechanism | Key Concern |
|---|---|---|---|
| Low-temperature / cryogenic zone | < -20°C | Low-temperature brittle fracture | Notch toughness and microstructure type |
| Room-temperature zone | -20°C to +150°C | Conventional static strength and fatigue | Conventional properties |
| Mid-temperature zone | +150°C to +400°C | Stress relaxation | Preload retention |
| High-temperature zone | +400°C to +700°C | Creep, oxidation, microstructure degradation | Creep-resistant materials |
Low-Temperature Zone: Why Bolts Fracture "Without Warning"
Steel has a ductile-brittle transition temperature (DBTT). Below this temperature, the fracture mode changes from ductile tearing to brittle cleavage fracture, and the absorbed impact energy drops sharply. The DBTT of carbon steel and low-alloy steel is typically in the -30°C to 0°C range, so conventional grade 8.8 and 10.9 bolts carry a risk of brittle fracture at low temperature.
There is an easily overlooked pattern: the higher the strength grade → the lower the toughness reserve → the higher the ductile-brittle transition temperature → the greater the low-temperature brittle fracture risk. Therefore, low-temperature service should not "upgrade the strength grade"; it is often about lowering the strength grade or switching material systems.
Low-temperature material selection: down to -40°C, low-carbon steel, grade ≤8.8 can be used; grade 10.9 and above is not advisable. Down to -100°C, use low-temperature steel (Ni-bearing steel) or A2/A4, requiring a specified impact energy ≥27 J. Down to -196°C, use austenitic stainless steel (304L, 316L) or aluminum alloy; the austenitic microstructure has no ductile-brittle transition. Down to -253°C (liquid hydrogen), use austenitic stainless steel, aluminum alloy, or titanium alloy, and medium compatibility must also be considered.
Mid-Temperature Zone: Preload Being "Slowly Eaten Away"
In the 150°C–400°C range, static strength is still sufficient, but the joint's preload decays over time; this is the main risk in this zone.
Decay comes from two sources: embedding loss (the same as at room temperature, occurring shortly after assembly) and stress relaxation (under temperature and sustained load, the material creeps, elastic elongation is replaced by plastic deformation, and preload drops). The factors affecting the relaxation rate are as follows:
Four factors affect the relaxation rate: temperature (every 10–15°C rise roughly doubles the creep rate); initial preload level (the closer the stress is to yield, the faster the relaxation); material (ferritic steel relaxes faster than austenitic steel; alloy steels containing Mo, V resist relaxation better); and bolt compliance (the more compliant, the smaller the proportional preload loss for the same relaxation amount).
High-Temperature Zone: The Double Blow of Creep and Material Degradation
Above 400°C, two processes proceed simultaneously:
The first is creep deformation: under high temperature and constant stress, the material continuously deforms slowly, manifesting as the bolt elongating and preload steadily declining; once accumulated to a certain point, the joint loses its clamping ability. The second is microstructure degradation:
- Spheroidization: lamellar cementite in pearlite spheroidizes, lowering strength and hardness.
- Graphitization: carbon steel in long-term service above 450°C decomposes cementite into graphite, sharply lowering both strength and toughness.
- Oxidation and decarburization: surface oxide scale forms while the surface layer decarburizes, lowering fatigue strength and creep resistance.
High-temperature material selection: ≤400°C, use medium-carbon alloy steel quench & tempered (35CrMo, 42CrMo), grade 10.9, with relaxation checked; 400–540°C, use chromium-molybdenum steel (25Cr2MoVA, B16), common in power-plant piping and steam turbines; 540–600°C, use high-temperature alloy steel or martensitic heat-resistant steel, where creep strength becomes the controlling factor; 600–700°C, use austenitic heat-resistant steel or nickel-based superalloys (such as the GH series, Inconel 718).
Selecting high-temperature bolts is not about looking at room-temperature tensile strength, but at the stress-rupture strength and creep limit at that temperature. These two indicators have no direct conversion to room-temperature data; you must consult the material's property curves at the corresponding temperature.
Five Essentials for Wide-Temperature-Range Design
Summarizing the zone differences into design essentials: in material selection, low temperature prefers an austenitic microstructure and avoids high-strength steel, while high temperature chooses creep-resistant heat-resistant steel or superalloys; in strength grade, low temperature should be low rather than high (≤8.8), while high temperature is set by the allowable stress at that temperature; in preload, low temperature is converted from low-temperature yield strength, while high temperature reserves relaxation compensation and allows for hot retightening; thermal expansion differences must be checked for the additional load from differing linear expansion coefficients of bolt and clamped part; in lubrication and coating, low-temperature lubricants may fail and require low-temperature-resistant types, while high-temperature lubricants may coke and require high-temperature types or dry films.
When the bolt and clamped part are of different materials (such as a steel bolt joining an aluminum housing), temperature changes produce additional loads due to differing linear expansion coefficients. Aluminum's linear expansion coefficient is about 23×10-6/°C and steel's about 12×10-6/°C, nearly a twofold difference. Taking a steel-bolt-to-aluminum joint with a 50 mm clamp length as an example, a 100°C temperature rise makes the aluminum part elongate about 0.055 mm more than the steel bolt—already beyond the reasonable range of thread elastic deformation, which may cause a significant change in preload or even plastic deformation of the thread.
Conclusion: Temperature Changes the Entire Design Logic
Selection for wide-temperature-range service cannot follow room-temperature thinking: low temperature looks at toughness (not strength—the higher the grade, the more dangerous it is); mid temperature looks at relaxation (the core is whether preload can be retained; long bolts and compliant design are most effective); high temperature looks at creep (stress-rupture strength and creep limit replace room-temperature tensile strength as the main criterion); and thermal expansion differences must be checked across the entire range, especially for dissimilar-material joints. By incorporating these differences into design early, joint reliability no longer relies on the intuition that "good material was used," but is built on a material logic matched to temperature.
