A gas turbine exhaust is not a furnace

It runs hot, but the number that shapes the order is not the temperature. It is the cycle. A peaking unit at two starts a day for twenty years accumulates roughly 14,600 thermal cycles, and each one takes the duct, the liner and the bellows through the same expansion and back again. A furnace arrives at temperature and stays; a turbine arrives and leaves twice a day, and the hardware is bought on that basis.

That is why an exhaust train is ordered in three or four materials rather than one, and why the thin parts are specified by condition and specification number instead of by grade name.

Oxidation sets the ceiling, not tensile strength

Alloy 310S is the default liner: austenitic, 24 to 26 per cent chromium, 19 to 22 per cent nickel, good oxidation resistance and strength in continuous service to 2000 F (1093 C), and intermittent service to 1900 F. The data sheet carries a condition that most enquiries skip - provided reducing sulphur gases are not present. A fuel with real sulphur content changes the answer, and no temperature rating rescues 310S from that exhaust.

Below it sit 321 and 321H, titanium stabilised and used from 1000 to 1600 F, and 347 for intermittent heating between 800 and 1650 F or for joints that cannot be post-weld annealed. 330 goes the other way, to 2200 F with carburisation and thermal shock resistance, which is why it turns up in heat treat furniture rather than in a duct run. 310S is also credited on its page with a low expansion coefficient that reduces the tendency to warp in heat service, and on a 3 mm liner that matters more than any strength figure.

Growth is measured in centimetres

310S expands 14.4 microstrain per kelvin between 0 and 100 C, and 17.5 between 0 and 649 C. A 10 m duct taken from ambient to 600 C grows about 101 mm. Alloy 625 on the same duty grows about 87 mm, since its expansion runs 13.1 at 204 C to 15.8 at 871 C. Fourteen millimetres over ten metres is not a rounding error - it is the difference between one bellows convolution and two.

The duct cannot loop its way out of this. A pipe can be taken round an offset because it is small and there is space; a two metre square duct has neither, so the movement goes into a bellows, and the bellows becomes the most heavily worked item in the train.

The bellows is an alloy order with a weld procedure attached

625 lists bellows and expansion joints among its applications, alongside jet engine exhaust systems, turbine shroud rings, thrust reversers and combustion liners. The reasons are on the data sheet: strength and toughness from cryogenic to 2000 F (1093 C) from columbium and molybdenum held in solid solution in a nickel-chromium matrix, excellent fatigue strength, and stress corrosion cracking resistance to chloride ions. Tensile strength runs 992.9 MPa at 21 C, 896.3 at 538 C and 275.8 at 871 C, elongation climbs from 44 to 117 per cent across the same range, and the modulus drops from 207 to 179 GPa at 538 C.

Read that as a failure mode. The bellows does not tear at temperature, it stretches, so the crack starts at the weld toe where the convolution meets the flange rather than in the crown. Thin 625 welded to a 321 or 310S flange is a dissimilar joint with a different expansion rate on each side of the fusion line, which makes the weld procedure, the filler and the interpass control part of the purchase.

Thin parts are bought by condition, not by grade

17-7PH is where those orders go wrong. It is semi-austenitic: austenitic as annealed, martensitic after hardening, supplied as strip, coil, foil and wire to AMS 5528 in Condition A, AMS 5529 in Condition C, ASTM A693 and MIL-S-25043. The same chemistry delivers Condition RH 950, TH 1050 or CH 900, and the mechanical properties up to 900 F (482 C) depend on which one is called out. An order that says 17-7PH strip and nothing else gets Condition A, the formable state, so a drawing that assumes hardened properties will not be met by it.

The page also credits minimum distortion on heat treatment, which is why the grade appears in honeycomb seals, clips, springs and small bellows - parts that are formed soft and harden in place, where a warp of a few tenths would scrap the assembly.

The rotor side is a different conversation

Alloy 718 is precipitation hardenable nickel-chromium with iron, niobium and molybdenum, and it holds creep-rupture strength to 1300 F (700 C) with weldability that includes resistance to postweld cracking. It is bought to ASTM B670 for plate and sheet, AMS 5589 and 5590 for tube, welded with ERNiFeCr-2 classified to AWS 5.14, and listed in NACE MR-01-75. The filler number belongs on the purchase order; a shop that substitutes a matching-composition wire without the classification has changed the joint.

Further forward the duty changes again. 422 is martensitic, hardenable to several strength levels and used to 1200 F in aircraft and power hardware where strength to weight is the constraint, while Alloy 600 covers oxidation plus chloride stress cracking in furnace components and nuclear work.

The casing and the stack are still carbon steel

Most of the tonnage in the train is not an alloy. The casing, the transition piece and the stack are plate and section: Q345 plate at 0.3 to 200 mm thick and 1000 to 1500 mm wide, or A36 where the design allows, cut, formed and welded in the same shop as any structural job, then finished with surface treatment. The frame that carries the duct is H-beam in 6 to 12 m lengths, with Q235 plate for the stack base and Z275 coil closing the weather envelope.

The interface is where the cost hides. A stainless liner fixed to a carbon steel casing needs fasteners and washers in the same family as the liner, or the joint corrodes outward from the bolt. Where the duct is a rolled spool rather than a fabricated box, large diameter stainless pipe runs 6 to 2500 mm outside diameter with 1 to 150 mm wall in 4, 5.8, 6 or 12 m lengths, and 310S tube from 8 to 680 mm, with square tube to 300 x 300 mm, cover the smaller branches.

The enquiry sheet

Maximum continuous temperature and the fuel sulphur and alkali content. Cycle count and cycle period. Duct diameter and length with anchor and bellows positions, liner thickness, fastener material, and the specification call-outs - ASTM A167, AMS 5523, AMS 5528 and 5529, ASTM B670, AWS 5.14 ERNiFeCr-2. Weld procedure and any post-weld heat treatment, with the certificates to match. Then state which parts are allowed to be carbon steel, because not all of them can be alloy.

An exhaust train looks like a high temperature order. It is a cycling order with a temperature attached, and most of the money sits in the joints between the four materials.