Four Failure Modes of Standard 8.8 Bolts at High Temperature

A standard 8.8 quenched-and-tempered steel bolt is heat-treated to achieve its strength at room temperature. When exposed to sustained high temperature, four failure modes appear. First, creep: the bolt slowly deforms under constant stress at temperature, stretching and losing preload. Second, temper softening: the bolt was tempered at about 425-525 C; if service temperature approaches or exceeds the tempering temperature, the bolt softens and loses strength. Third, oxidation and scaling: the surface oxidizes, reducing cross-section. Fourth, thermal expansion mismatch: the bolt and the flange material expand differently, changing preload during heat cycles. For continuous service, 8.8 bolts should not be used above about 300 C.

What Temperature Range GB/T 3098.8 Covers

GB/T 3098.8-2010 covers bolted fasteners for service temperatures from -200 C (cryogenic) to +700 C. It specifies material groups, property classes, and the recommended materials for each temperature range. The standard recognizes that as temperature rises, the required material changes from standard carbon steel, through alloy steel, to stainless and finally to nickel-base superalloys. For applications above 700 C, GB/T 3098.24-2020 extends the range and references nickel-base and cobalt-base alloys.

Which Material for Each Temperature Range

For moderate high temperatures (300-500 C), 25Cr2MoV is the classic Chinese standard bolt material for high-temperature flanges. It has good creep resistance at up to about 500-550 C and is widely used in power plant steam flanges. Above that, austenitic stainless steels (A2-70, A4-70) retain strength to about 600-650 C, though they lose tensile strength as temperature rises. Above 650 C, only nickel-base superalloys such as GH4080A (Alloy 80A) or Inconel 718 maintain useful strength.

Above 600 C: Austenitic and Nickel-Base Alloys

Above 600 C, carbon and alloy steels have no useful strength. Austenitic stainless steels (304, 316) can be used up to about 600-650 C for intermittent service, but they creep. For continuous high-temperature service, nickel-base superalloys are required. GH4080A (Equivalent to Nimonic 80A / Alloy 80A) is a precipitation-hardened nickel-chromium alloy used in gas turbine bolts and high-temperature fasteners up to about 800 C. Inconel 718 is used up to about 650 C with higher strength. These materials are expensive and difficult to machine, but they are the only option for the highest temperatures.

Notes on High-Temperature Joint Matching

High-temperature joints require more than the right bolt material. The nut must also be made from a compatible material, usually slightly softer than the bolt to avoid thread galling at temperature. The gasket must be rated for the service temperature. Thermal expansion must be calculated: if the bolt material and the flange material have different CTEs, the preload changes as temperature rises, and the joint must be designed with this in mind. Re-torquing after heat cycling is often necessary to compensate for relaxation. For critical high-temperature joints, consider using bolts with thermal stability testing and relaxation test data from the supplier.

Quick Selection Table

Designing high-temperature joints for relaxation. Even with the correct material, high-temperature bolts relax over time as the material stress-relieves. Plan for re-torquing after the first heat cycle and periodically during service. Use Belleville washers where possible to compensate for relaxation. Specify nuts that are slightly softer than the bolt material to avoid thread galling at temperature. And always verify the selected material against the actual service temperature, including spikes: a bolt rated for 450 C should not be used at 500 C just because the average temperature is 400 C. Include the maximum temperature, not just the average, in the material selection.

Nut and washer selection at temperature. At high temperature, the nut is usually one class lower than the bolt so that thread failure occurs in the nut (which is easier to replace) rather than the bolt. The washer should be made from a compatible material and rated for the service temperature. Spring washers lose their tension at high temperature as the material relaxes, so they are not recommended for high-temperature joints. Belleville washers made from high-temperature alloy can compensate for relaxation. Always specify the nut property class and washer material for high-temperature joints rather than using standard hardware. A bolt that is correctly material-selected can still fail if the nut or washer is wrong for the temperature.

Cooling and thermal cycling effects. High-temperature fasteners also see thermal cycling: heating up during operation and cooling down during shutdown. Each cycle causes differential expansion between the bolt and the flange, and repeated cycling can lead to fatigue at the thread root. Design for fatigue by using materials with good thermal fatigue resistance, avoiding sharp stress concentrations, and specifying a preload that accounts for the expected expansion and contraction. For joints that cycle frequently, consider creep-fatigue interaction in the design. A bolt that survives steady high temperature may fail under repeated thermal cycling if the material is not chosen for fatigue resistance.

Designing high-temperature joints for relaxation. Even with the correct material, high-temperature bolts relax over time as the material stress-relieves. Plan for re-torquing after the first heat cycle and periodically during service. Use Belleville washers where possible to compensate for relaxation. Specify nuts that are slightly softer than the bolt material to avoid thread galling at temperature. And always verify the selected material against the actual service temperature, including spikes: a bolt rated for 450 C should not be used at 500 C just because the average temperature is 400 C. Include the maximum temperature, not just the average, in the material selection.

Nut and washer selection at temperature. At high temperature, the nut is usually one class lower than the bolt so that thread failure occurs in the nut (which is easier to replace) rather than the bolt. The washer should be made from a compatible material and rated for the service temperature. Spring washers lose their tension at high temperature as the material relaxes, so they are not recommended for high-temperature joints. Belleville washers made from high-temperature alloy can compensate for relaxation. Always specify the nut property class and washer material for high-temperature joints rather than using standard hardware. A bolt that is correctly material-selected can still fail if the nut or washer is wrong for the temperature.

Cooling and thermal cycling effects. High-temperature fasteners also see thermal cycling: heating up during operation and cooling down during shutdown. Each cycle causes differential expansion between the bolt and the flange, and repeated cycling can lead to fatigue at the thread root. Design for fatigue by using materials with good thermal fatigue resistance, avoiding sharp stress concentrations, and specifying a preload that accounts for the expected expansion and contraction. For joints that cycle frequently, consider creep-fatigue interaction in the design. A bolt that survives steady high temperature may fail under repeated thermal cycling if the material is not chosen for fatigue resistance.