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How to choose fasteners for high-temperature service: heat-resistant steels, tempering and failure control

2026-09-22

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

How hot is too hot? Start with the material's temperature ceiling

The strength values of grade 8.8, 10.9 and 12.9 bolts under GB/T 3098.1 / ISO 898-1 are only valid up to about 300 degC — the tempering temperature of carbon-steel fasteners is typically 340-430 degC, and sustained service above it softens the part, draining both strength and preload. In practice, select by range: below 150 degC a standard plated grade 8.8 bolt is fine; 150-300 degC calls for higher-tempering alloy steel such as ASTM A193 B7; 300-450 degC needs B16 (molybdenum-vanadium steel) or heat-resistant stainless; above 450 degC move to A286 precipitation-hardening stainless (about 650 degC) or the nickel alloy Inconel 718 (about 700 degC). Confirm the medium temperature and the real reading under insulation before anything else — that is step one of high-temperature fastener selection. Two field notes make this concrete. First, the reading that matters is the metal temperature, not the process temperature: on an insulated steam header, a 250 degC process can drive the bolt to 300 degC or higher because the insulation also traps heat around the flange, so a B7 set that is nominally adequate ends up above its comfort band. Second, the property class stamped on the head only tells you what the bolt was tested to at room temperature; elevated-temperature yield and stress-rupture data come from the material specification, so ask for the A193 or equivalent figures rather than extrapolating from the 8.8 table. A worked comparison for a typical Hong Kong application: an industrial boiler flue-gas duct running at 220 degC with cyclic start-stop uses A193 B7 bolts with A194 2H nuts; the same duct after a burner upgrade to 380 degC must be re-bolted to B16 with A194 4 nuts, because B7 loses preload too quickly under creep at that temperature and flange leaks begin within a few thermal cycles. Getting the range right at the design stage avoids a costly hot re-bolt later.

Three high-temperature failure modes: tempering softening, creep relaxation and thermal mismatch

Watch for three things in hot service. Tempering softening: once service temperature exceeds the material's tempering temperature, hardness and strength keep dropping — a bolt graded 8.8 can fall below grade 5. Creep relaxation: above about 350 degC steel slowly stretches under sustained tensile load, and flange preload can decay 20-40% within weeks; most joint leaks trace back to this, which is why power and petrochemical plants hot-torque flanges 24 hours after start-up and re-tighten on a schedule. Thermal expansion mismatch: pairing an aluminium housing with steel bolts, or stainless bolts with carbon-steel flanges, combines metals whose expansion coefficients differ 1.5-2x, so every thermal cycle swings the preload — compensate with Belleville washers (DIN 2093) or match materials within the same family to reduce the gap. Each failure mode has its own visual signature, and knowing them speeds up diagnosis. Tempering softening usually shows up as bolt elongation or threads stripped out of the nut with no corrosion present. Creep relaxation shows as a weeping flange with no visible damage to the bolt, and re-torquing appears to fix it temporarily before it weeps again a few weeks later. Thermal mismatch shows as fretting marks or a polished ring under the washer face, and sometimes as nut loosening on the hot side of a joint only. Consider the numbers: a DN200 steam flange with M20 B7 bolts torqued to 450 Nm may retain only 300-330 Nm after two months at 400 degC of continuous service, which is often below the gasket seating requirement. The standard countermeasures are to specify a creep-resistant material so the relaxation rate itself falls, to use disc springs to maintain a store of elastic energy, and to write a documented hot re-torque step into the commissioning procedure. Vibration adds a further dimension: in hot vibrating service, such as an exhaust manifold or a turbine casing, the combination of thermal cycling and vibration loosens a joint far faster than either alone, so anti-loosening hardware should be selected at the same time as the material rather than as an afterthought.

Coatings and acceptance: don't ruin a heat-resistant bolt with the wrong finish

Coatings have temperature ceilings too: electroplated zinc is generally unsuitable for long service above 200 degC; hot-dip galvanized layers tend to flake above 200 degC; zinc-flake Dacromet withstands about 300 degC and is the common anti-corrosion choice for hot parts; beyond that, switch to black oxide or let a heat-resistant material go uncoated. As a selection comparison: below 200 degC, clear or blue-white zinc plating is economical and adequate; 200-250 degC, use a high-temperature zinc-flake coating rated for the duty rather than a standard grade; 250-300 degC, Dacromet or a similar inorganic zinc-flake system; above 300 degC, black oxide, phosphate plus a high-temperature anti-seize, or bare heat-resistant alloy. For stainless and nickel alloys above 400 degC, leave the part uncoated and rely on the alloy itself, since most organic or metallic coatings will burn off and leave residues in the joint. For acceptance, request EN 10204 3.1 material certificates and check the chemistry and elevated-temperature yield data; procure petrochemical flange bolting as matched sets of ASTM A193 B7 / B16 with A194 2H / 4 nuts so the nut never fails before the bolt. Verify also the marking on the bolt head — A193 B7 and A194 2H should be legible and consistent with the certificate — and confirm that no re-machining or re-threading has been done after heat treatment, because that removes the tempered surface layer exactly where the thread flank carries load. Packaging and storage matter more than they appear: keep B7 and B16 sets dry and segregated from carbon steel to avoid both corrosion and galvanic contamination, and never store high-temperature bolting in the same open bin as standard galvanised hardware.

Common pitfalls in high-temperature bolting

The first pitfall is specifying by process temperature alone. A furnace rated at 300 degC with a burner flame impinging on the casing can drive local metal temperatures 50-80 degC higher, and the bolt nearest the flame fails first. Wherever there is a hot spot, specify for the hot spot. The second pitfall is mixing grades in one joint: B7 bolts with commercial grade 8 nuts, or B7 with 2H nuts from a different heat, makes the joint behave unpredictably and voids the matched-set logic. The third is reusing bolts after a hot shutdown. High-temperature bolting that has been in service under creep has already lost some of its elastic capability, and re-torquing a stretched bolt will not restore the preload; replace fasteners whenever a flange is opened after prolonged hot service. The fourth is ignoring the tightening method: torque alone on a large B7 flange leaves a wide scatter from friction variation, so the torque-plus-angle method, hydraulic tensioning or a stud tensioner should be used, with lubricant applied consistently and the friction coefficient recorded on the job card. The fifth, and the one that causes the most disputes, is accepting a bolt on the strength grade alone. A high-temperature fastener is defined by its material specification, its heat-treatment record and its elevated-temperature properties, not by a room-temperature proof load. At goods-in, check the material certificate against the order, confirm the grade marking, verify the bolt and nut are a matched set from the same supplier, and where the duty is critical ask for a hardness check to confirm the heat treatment has not been compromised. Need help specifying high-temperature fasteners or complete B7 / B16 sets? Contact JIALI Industrial (164.hk) with your temperature and medium for matched materials and certificates.

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