There is a widely repeated rule of thumb in valve procurement: small valves are forged, large valves are cast. It is repeated because it describes what usually gets bought, and what usually gets bought is decided by price.
It is not an engineering rule, and in modern high-temperature power generation it is increasingly the wrong answer. The defect risk in a casting rises with wall thickness — which means the largest, thickest, hottest valves in the plant are exactly the ones where casting is least suitable. That is why main steam and hot reheat isolation valves on supercritical and ultra-supercritical units are frequently specified as forged or forged-and-welded construction, at a cost premium that the specifying engineer accepted deliberately.
This guide explains the technical basis for that decision, and where it does and does not apply.
Why thickness is the controlling variable
A casting solidifies from liquid. As the metal cools it contracts, and the last regions to solidify are the thickest ones, because heat leaves a thick section slowest. If liquid metal cannot feed into those regions fast enough, shrinkage cavities and microporosity form.
In a valve body, the thickest sections are the shell at the body-to-bonnet transition and the flange or weld end junctions. These are also the highest-stress regions in the part. The defect and the stress concentrate in the same place, and both get worse as the valve gets bigger.
Radiography loses resolution in heavy walls. Radiographic examination is the standard NDE method for castings, but as wall thickness increases, the detectable defect size increases with it. A defect that would be found easily in a 25 mm wall can be missed in a 150 mm wall. Forgings are examined by ultrasonic testing, which does not degrade the same way with thickness and is better suited to detecting planar defects.
Weld repair becomes routine. Large castings are commonly repair-welded where indications are found. A repair is a weld in the pressure boundary, with its own heat affected zone, and it requires re-heat-treatment of the body. Many modern power plant specifications now limit or prohibit major weld repairs on critical castings, which effectively means the casting has to be right the first time in the thickest, hardest section to get right.
Creep governs at high temperature, and microstructure governs creep. Above roughly 450°C the limiting property is no longer yield strength but creep rupture life over tens of thousands of hours. Creep behaviour is highly sensitive to microstructure and internal soundness. A forged microstructure, worked and grain-flowed, performs more predictably over a design life than a cast one.
The Grade 91 casting problem
The clearest example is Grade 91 (9Cr-1Mo-V), the standard material for supercritical and ultra-supercritical main steam and reheat systems.
Grade 91 depends on a precisely controlled tempered martensitic microstructure for its creep strength. It is unforgiving of heat treatment deviation: under-tempering leaves it hard and brittle, over-tempering destroys the creep strength it was chosen for. Neither condition is visible, and neither is detected by a hydrostatic test — a mis-heat-treated Grade 91 body passes hydrotest and fails years into service.
Now combine that with heavy-section casting:
- A thick cast section cools unevenly, so achieving uniform microstructure through the wall is harder than in a wrought product
- Weld repairs introduce additional heat affected zones, each requiring correct post-weld heat treatment
- Grade 91 weldments have a documented long-term failure mode — Type IV cracking in the fine-grained heat affected zone under creep
The forged route removes the solidification variable entirely and gives tighter control over the through-thickness microstructure. This is why a growing number of USC specifications call for forged bodies in Grade 91 and Grade 92 service, and why forged construction is standard practice for nuclear safety class valves where the qualification burden is highest.
How large forged valve bodies are actually made
A common misconception is that a large forged body means machining a solid billet away to nothing. That is not how it is done, and it is why the cost premium is manageable rather than absurd.
Large bodies are built from open-die forged shells, rings and blocks, forged close to the finished profile, bored and machined, and where the geometry requires it, joined by full-penetration welds under qualified procedures with full post-weld heat treatment and NDE. The pressure boundary is wrought material throughout; only the joining welds are not, and those are examined and heat treated to the same standard as any site weld, with the advantage that they are made in a controlled shop rather than in the field.
Where large forged valves belong in a power station
| Application | Conditions | Why forged |
|---|---|---|
| Main steam isolation | 540–620°C, 17–30 MPa | Heaviest wall in the plant, Grade 91/92, creep governed |
| Hot reheat isolation | 540–620°C, 3.5–5 MPa | Large bore at high temperature, thermal cycling |
| HP and LP turbine bypass | Severe pressure drop, rapid transients | Thermal shock and cycling duty |
| Feedwater pump discharge | 250–300°C, 30–35 MPa | Class 2500 heavy wall, high cycle count |
| Boiler stop and stop-check | Drum conditions | Boiler external piping, Code item |
| Nuclear safety class isolation | Per plant class | Qualification and documentation burden favours wrought material |
The common factor is not size alone. It is size combined with either high temperature, heavy wall, or cycling — any of the three that makes casting soundness the limiting risk.
Construction details at large bore
Pressure seal bonnets. At Class 900 and above in hot service, bolted bonnets scale badly and relax under thermal cycling. Pressure seal construction is energised by line pressure itself and is the normal choice.
Butt weld ends. ASME B31.1 governs power piping. Butt weld ends avoid the gasket and crevice of a flanged joint and can be radiographed. Post-weld heat treatment is required for alloy grades.
Bypass and equalising arrangements. Large main steam gate valves are normally supplied with an integral bypass valve so the pressure across the disc can be equalised before opening. Without it, the operating force required to unseat the disc under full differential pressure is impractical.
Hardfaced seating. Stellite or equivalent hardfacing on seats and discs is standard for steam service. Specify it explicitly; it is not automatic.
Materials
| Grade | Limit | Where it belongs |
|---|---|---|
| A105 | ~425°C | Feedwater, condensate, auxiliary |
| A182 F11 / F12 | ~540°C | Auxiliary steam, moderate temperature |
| A182 F22 | ~595°C | Subcritical main steam and hot reheat |
| A182 F91 | ~620–650°C | Supercritical and ultra-supercritical steam |
| A182 F92 | ~650°C | USC where F91 is marginal |
| A182 F316H / F304H | 700°C+ | Beyond ferritic creep capability |
For the equivalent cast grades used on large bodies, see our guide to WCB, WC6 and WC9 valve materials for steam service.
Small bore matters too, and is usually neglected
The large valves get a technical evaluation. The several hundred small forged valves per unit — drains, vents, blowdown, sampling, instrument root valves — get awarded on unit price, and they generate a disproportionate share of outage findings.
Drain valves used as throttling valves. A drain valve is an isolation valve, but operators crack it open to control drainage during startup. High-pressure steam through a partly open globe valve cuts the seat by wire drawing, and the valve then passes permanently. The correct arrangement is two valves: isolation at the header, throttling downstream with hardfaced trim treated as a consumable.
Root valves at header connections. A root valve welded to a main steam header sees the full header temperature plus the gradient into the cooler branch. Thermal fatigue concentrates at that joint. Match the root valve material to the parent line — a carbon steel root valve on an F22 header creates a dissimilar metal weld with its own failure mechanism.
Boiler external piping is a Code item
Valves in boiler external piping, as defined in ASME BPVC Section I, must comply with ASME B31.1 and carry the required Code certification. This is a different regime from ASME B31.3, which governs process piping. Specifying to process plant practice and installing in boiler external piping is a compliance failure discovered at inspection, not at purchase. Mark where your line list crosses that boundary.
When cast is the right answer
- Class 600 and below, below 425°C, large bore — a quality casting with full NDE is entirely adequate and materially cheaper
- Water, condensate, cooling and general utility service at any size
- Where lead time governs and the conditions do not demand wrought material
The argument for forged construction is specific: heavy wall, high temperature, creep-governed materials, thermal cycling, or a qualification regime that makes documented material soundness the priority. Outside those conditions, paying for it is not justified.
Documentation to require
- Material test reports traceable to heat number, EN 10204 3.1 or better
- Ultrasonic examination reports on the forgings
- Weld procedure qualifications, NDE and PWHT records for any body welds
- Heat treatment charts with actual time-temperature traces, mandatory for Grade 91 and 92
- Hardness readings on base material, weld metal and heat affected zone
- API 598 shell and seat test certificates
- Positive material identification on arrival for Grade 91, as F91 and F22 are not visually distinguishable
HD Flowtech — large bore forged valves for power generation
HD Flowtech specialises in large bore forged and forged-and-welded valve bodies for high pressure, high temperature power plant service — gate, globe and check valves in A105, F11, F12, F22, F91, F92 and austenitic grades, with pressure seal bonnets, butt weld ends, hardfaced trim and integral bypass arrangements. Small bore forged valves to API 602 are supplied from the same programme.
Heat-number traceable material test reports, ultrasonic examination records, heat treatment and PWHT charts, and API 598 test certificates are supplied as standard documentation.
Send us your line list or datasheet with design and transient temperatures, and we will confirm the construction and material for each item, including where a casting is sufficient and forging is not worth paying for.