Durehete High-Temperature Bolts

HP turbine casing flange bolts are specified in Durehete 1055 / Alloy T41 / 20CrMoVTiB4-10. The Ti+B microalloying delivers the 100,000-hour creep-rupture envelope at 540-565°C steam temperature. Bolt diameters typically 36 to 100 mm; preload calculations follow EN 10269 Annex A and Annex B.

IP turbine casing bolts run in Durehete 950 / Alloy T31 / 40CrMoV4-6 at 480-540°C. Identical heat-treatment cycle to 1055 but the simpler Cr-Mo-V chemistry (no Ti+B) reduces mill cost while still covering the IP service envelope.

Throttle valve and stop-valve bolting on the HP steam side runs in Durehete 1055. The cyclic thermal stress combined with high preload requirement makes the 1055 stress-relaxation envelope the procurement driver.

Stress-relaxation curves in EN 10269 Annex B set the re-tightening interval. For HP bolts at 540°C, residual preload drops to ~70 percent at 1,000 hours and ~50 percent at 30,000 hours. Re-tightening typically scheduled at major outages every 4-6 years.

Compressor casing bolts on industrial gas turbines run in Durehete 950 / Alloy T31 for service to 480°C. Bolt diameters typically 24-72 mm. Stress-rupture envelope covers the 100,000-hour design life at compressor exit temperature.

Heavy-duty industrial frame turbine casing bolts run in Durehete 1055 / Alloy T41 at 540-565°C casing skin temperature. The Ti+B microalloying maintains the creep-rupture envelope across the 8,000-hour fired-hour service interval typical for industrial gas turbines.

Expansion-joint bolting between the compressor and turbine modules sees both thermal cycling and high preload. Durehete 1055 is specified to handle the combined fatigue plus creep loading.

Major gas-turbine OEMs accept EN 10269 type 3.1 / 3.2 inspection certificates with optional VdTÜV Werkstoffblatt 350 dual certification when AD 2000-Merkblatt acceptance is required for German installation sites.

HP main steam temperature: 540-565°C. HP main steam pressure: 250-300 bar on subcritical units, 250-265 bar at supercritical inlet. Casing flange bolts see the casing-skin temperature (typically 5-15°C below main steam due to insulation losses) and the full preload required to prevent gasket leak under thermal cycling.

Plain Cr-Mo-V grades like 21CrMoV5-7 lose creep margin above 540°C as M23C6 carbides coarsen at grain boundaries. The Ti+B microalloying in 20CrMoVTiB4-10 (Alloy T41) pins the grain boundaries against carbide coarsening and adds 25-50°C of usable temperature. This is the procurement driver at HP service.

HP bolt preload is calculated against the EN 10269 Annex A 0.2 percent proof stress at operating temperature plus the Annex B stress-relaxation curve. Typical initial preload targets 60-65 percent of yield; residual preload at 30,000 hours is 50-55 percent of initial. Re-tightening at major outage typically every 4-6 years.

HP casing flange bolts typically M36 to M100 thread, length 200-1200 mm. TorqBolt produces from EN 10269 / VdTÜV W350 certified bar via custom machining and Q+T heat treatment per the specification.

IP main steam temperature: 480-540°C (often 540°C on reheat-cycle units). IP main steam pressure: 30-60 bar after the reheat. Casing flange bolts see the same combined preload-plus-thermal-cycle loading as HP, but at a lower envelope.

The Ti+B microalloying in Alloy T41 (1055) is not required at 540°C maximum service. Plain Cr-Mo-V chemistry in Alloy T31 (40CrMoV4-6 / Durehete 950) covers the IP envelope through 100,000 hours. Specifying T31 rather than T41 saves on mill cost without sacrificing the design margin.

For US procurement specifications that call out ASTM A193 Grade B16, Durehete 950 / Alloy T31 is the direct dual-certification candidate. Chemistry envelopes overlap; heat-treatment cycle is identical. TorqBolt supplies dual EN 10269 + A193 B16 certificates from the same heat.

IP casing flange bolts typically M30 to M80 thread, length 150-900 mm.

Subcritical units operate below 221 bar; supercritical units above. The supercritical thermodynamic cycle delivers higher plant efficiency (40+ percent versus 35 percent subcritical) but pushes bolting steels harder. Modern supercritical HP main steam targets 565°C / 250 bar; reheat targets 565-600°C.

Durehete 1055 / Alloy T41 is the HP casing flange bolt grade for supercritical units at 540-565°C. The Ti+B microalloying covers the 100,000-hour creep-rupture envelope.

Durehete 950 / Alloy T31 covers IP casing flange bolts at 540°C reheat conditions.

LP casing bolting and main-steam stop-valve bolting sit at lower temperature; Durehete 900 / Alloy T22 covers the lower-temperature service. Boiler superheater header bolting follows the HP grade selection.

USC HP main steam: 600-620°C / 280-300 bar. USC reheat: 600-625°C. Bolting steel selection sits at the upper limit of ferritic Cr-Mo-V-Ti-B capability; austenitic and nickel-base alloys take over above 620°C casing skin temperature.

For USC casing flanges that run at 565°C casing skin temperature (typical due to insulation losses below the 600°C main steam), Durehete 1055 / Alloy T41 remains the procurement specification. Above 565°C casing skin, escalate to nickel-base bolting (e.g. NiCr20TiAl).

A-USC plants targeting 700°C main steam exceed the Durehete envelope entirely. Bolting selection moves to nickel-base superalloys (Inconel 740H, Haynes 282) covered under EN 10269 nickel-base section and equivalent ASME Section II.

USC plants in Germany require VdTÜV Werkstoffblatt 350 alongside EN 10269 type 3.1 / 3.2. Indian and Chinese USC plants typically accept EN 10269 alone or with ASME Section II Part D dual certification.

Radiant-section tube hangers and casing flange bolts see metal temperatures of 540-590°C. Durehete 1055 / Alloy T41 covers the upper end; the Ti+B microalloying handles the long-duration thermal-cycle exposure between scheduled outages.

Convection-section casing flange bolts run cooler, typically 400-480°C. Durehete 950 / Alloy T31 covers the band; ASTM A193 B16 dual certification is common because US-spec refineries source against ASTM bolting standards.

Burner-mount studs see direct flame proximity. Durehete 1055 is specified at the upper temperature; oxidation resistance becomes the limiting factor above 565°C and warrants escalation to a stainless or nickel-base grade.

Refinery turnaround cycles typically 4-5 years. Bolt re-tightening is scheduled at each turnaround stress-relaxation data. Replacement of bolting occurs when residual preload drops below the gasket leak threshold or when visual inspection finds creep cracking.