API 6A Wellhead Alloys: 718, 625 and 925

Inconel 718 (UNS N07718) is the standard material for API 6A wellhead and Christmas tree components that must hold pressure at elevated temperature while staying inside the hardness limits for sour service, and Incoloy 925 (UNS N09925) is the usual choice for studs and nuts in the same equipment. The selection is not made directly from alloy data sheets: API 6A first fixes the product specification level (PSL) and the temperature class, and those two decisions determine which alloy, which heat treatment and which test package are acceptable. A 718 component ordered for PSL 2 and the same component ordered for PSL 4 are the same alloy on paper and completely different products in the mill.

This guide sets out how the API 6A framework narrows the alloy choice, which alloy is used for which component, and how the hardness and heat treatment requirements interact for sour service. For the sour service limits themselves, see NACE MR0175 Alloys for Sour Service.

What do PSL and temperature class actually fix?

API 6A (adopted as ISO 10423) rates equipment by two independent parameters before any material is discussed.

**Product specification level (PSL)** sets how much verification the equipment carries:

**Temperature class** sets the operating window:

The practical effect is that the top of the temperature class decides whether a carbon or low-alloy body material is admissible at all. Classes P and Q sit comfortably inside the range of 4130/4140 and F6a, and those grades are still the correct answer for a large share of 6A equipment. Once the class reaches T, or once the partial pressure of H₂S puts the equipment under NACE MR0175, the body and internals move to a corrosion-resistant alloy and the nickel grades enter the picture. Confirm the current edition of API 6A before fixing a class — the tables are revised with the standard.

Which alloy is used for which API 6A component?

The equipment is designed as a set of components with different loads, exposure and hardness requirements, and the alloy choice differs across them:

Two points on that table are frequently misunderstood. First, **625 is a solid-solution alloy and 718 is precipitation-hardenable**. Substituting 625 for 718 in a pressure-containing body loses roughly half the yield strength, so it is only valid where the wall thickness is increased or the cladding route is taken. Second, **Incoloy 925 and 718 are not interchangeable as studs** even though both are precipitation-hardenable: 925 has lower strength but better stress relaxation behaviour at temperature, and it is easier to keep inside the hardness limit.

How do the mechanical properties compare?

The three alloys most often quoted against each other in a 6A enquiry differ substantially, and the difference is what drives the component-level choice:

The 718 numbers come from AMS 5662, which requires a minimum tensile strength of 1241 MPa and a minimum yield strength of 1034 MPa in the aged condition, with the grain flow controlled by the forging practice. Where a purchaser quotes a 6A part as "Inconel 718" without a material standard, the missing information is almost always the heat treatment and the acceptance level — and that omission is how a component arrives with the right alloy name and the wrong properties.

Why is the ageing treatment the decisive step for 718?

718 reaches its strength through a two-step ageing treatment after solution annealing, and the same treatment has to satisfy three separate masters at once: the mechanical property minimums, the NACE MR0175 hardness limit, and the dimensional stability of the finished part.

The standard ageing cycle for 718 is:

• 1. Solution anneal at 954–982 °C, hold, then rapid cool (water or air depending on section).
• 2. Age at 720 °C ± 5 for 8 hours.
• 3. Furnace cool at a controlled rate, typically 55 °C per hour, to 620 °C.
• 4. Hold at 620 °C for a further 8 hours.
• 5. Air cool to room temperature.

Three things go wrong in practice:

• **Ageing before final machining.** The ageing treatment moves the part, and a component finish-machined before ageing will not hold the sealing-groove tolerance afterwards. The sequence normally used is rough machine, age, then finish machine.
• **Hardness drift.** The hardness limit for 718 under NACE MR0175 is 40 HRC, and the useful window between the mechanical property minimum and the hardness maximum is narrower than it looks on a large forging where the cooling rate varies with section. A hardness survey across the part — not a single reading — is what proves compliance.
• **Cold work after ageing.** Any cold work applied after ageing, including straightening, changes the hardness locally and can put the part outside the NACE envelope. Where straightening is unavoidable it must be done before the ageing treatment and the part must be re-tested.

What does the test and documentation package contain?

For PSL 3 and PSL 4 equipment, the material package is a substantial document set and it is the part of the order most often under-specified on the enquiry:

The traceability clause deserves attention. On a 6A part the heat number has to survive machining, and the two accepted methods are hard stamping in a non-critical area or a low-stress dot-peen marking. Electric-etch marking on a nickel alloy is acceptable where the procedure has been qualified; a hand-written number in marker pen on a 718 forging is not traceability, and it is rejected at witness.

API 6A materials and forgings from Hangbo Alloy

Hangbo Alloy supplies Inconel 718 bar, billet and open-die forged stock to ASTM B637 and AMS 5662 / 5663 / 5664, Inconel 625 to ASTM B446 and ASTM B443, and Incoloy 925 to ASTM B805 — with the matching ageing treatment, hardness survey and NDE package applied before despatch rather than left to the machining shop.

Material is supplied with EN 10204 3.1 certification as standard, with third-party inspection available on request and dimension and traceability records matched to the part number.

Send us the PSL, the temperature class, the sour service status and the drawing, and we will confirm the alloy, the heat treatment route and the inspection package before the order is released.

Continue reading

• Which Alloy for a High-Temperature Furnace? 600 °C to 1250 °CFurnace alloy selection by service temperature and atmosphere — oxidation, carburisation and sulphidation — wi
• Which Alloys Comply with NACE MR0175 for Sour Service?NACE MR0175 / ISO 15156 sour service alloy selection — what counts as sour, which nickel alloys are pre-qualif
• Which Alloy Resists Seawater Best? 316L to TitaniumSeawater corrosion alloy selection ranked by pitting resistance equivalent number and critical pitting tempera
• All 4 Corrosion & Grade Selection guides

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