Industry News
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 °C — the tempering temperature of carbon-steel fasteners is typically 340–430 °C, and sustained service above it softens the part, draining both strength and preload. In practice, select by range: below 150 °C a standard plated grade 8.8 bolt is fine; 150–300 °C calls for higher-tempering alloy steel such as ASTM A193 B7; 300–450 °C needs B16 (molybdenum-vanadium steel) or heat-resistant stainless; above 450 °C move to A286 precipitation-hardening stainless (about 650 °C) or the nickel alloy Inconel 718 (about 700 °C). Confirm the medium temperature and the real reading under insulation before anything else — that is step one of high-temperature fastener selection.
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 °C 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.
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 °C; hot-dip galvanized layers tend to flake above 200 °C; zinc-flake Dacromet withstands about 300 °C and is the common anti-corrosion choice for hot parts; beyond that, switch to black oxide or let a heat-resistant material go uncoated. 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. 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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