Overview: A Nickel-Copper Alloy Against a Nickel-Iron-Chromium Alloy
Alloy K500 (UNS N05500) and Alloy 718 (UNS N07718) are both precipitation-hardenable nickel alloys, yet they solve completely different engineering problems. Alloy K500 is a nickel-copper grade strengthened by titanium and aluminium, and it is selected wherever seawater, brine and reducing chemical environments would destroy carbon steel or stainless steel. Alloy 718 is a nickel-iron-chromium grade strengthened by niobium, molybdenum and titanium; it holds high strength up to roughly 700 °C and is a workhorse of aerospace, power generation and sour-gas hardware. Because both are age-hardenable and both are supplied in bar, rod and forged forms, they are frequently compared in specifications, even though their operating envelopes barely overlap.
Chemical Composition Compared
The two alloys have almost nothing in common at the alloying level. Alloy K500 is built on nickel and copper; Alloy 718 is built on nickel, iron and chromium with a niobium addition.
| Element, % | Alloy K500 (UNS N05500) | Alloy 718 (UNS N07718) |
|---|---|---|
| Nickel, Ni | 63.0–70.0 | 50.0–55.0 |
| Copper, Cu | 27.0–33.0 | ≤0.30 |
| Iron, Fe | ≤2.0 | Balance |
| Chromium, Cr | ≤0.50 | 17.0–21.0 |
| Molybdenum, Mo | – | 2.80–3.30 |
| Niobium, Nb | – | 4.75–5.50 |
| Titanium, Ti | 0.35–0.85 | 0.65–1.15 |
| Aluminium, Al | 2.30–3.20 | 0.20–0.80 |
| Manganese, Mn | ≤1.50 | ≤0.35 |
| Silicon, Si | ≤0.50 | ≤0.35 |
| Carbon, C | ≤0.18 | ≤0.08 |
| Sulphur, S | ≤0.010 | ≤0.015 |
The copper content of Alloy K500 is what gives it outstanding resistance to seawater, hydrofluoric acid and alkaline media. In Alloy 718, chromium supplies oxidation resistance and passivation, molybdenum contributes to pitting resistance, and niobium forms the gamma double prime phase that carries the strength after ageing. The aluminium and titanium levels in Alloy K500 play the same strengthening role there, which is why the two grades must be compared on properties rather than on chemistry.
Standards, Product Forms and Heat Treatment
| Item | Alloy K500 (UNS N05500) | Alloy 718 (UNS N07718) |
|---|---|---|
| Bar, rod and forgings | ASTM B865 | ASTM B637 |
| Sheet, plate and strip | Not a standard wrought form | ASTM B670 |
| Seamless pipe and tube | Produced to mill specification | ASTM B983 |
| Aerospace material specs | – | AMS 5662, AMS 5663 |
| Sour service qualification | NACE MR0175 / ISO 15156, API 6ACRA | NACE MR0175 / ISO 15156 |
| Werkstoff number | 2.4375 | 2.4668 |
| Typical heat treatment | Solution treat near 980 °C, then age 480–595 °C | Solution treat 955–980 °C, then two-step age 720 °C and 620 °C |
Alloy K500 is supplied mainly as round bar, hexagonal bar, wire and forgings because its main duty is machined components such as valve stems, pump shafts and fasteners. Alloy 718 is available across the full wrought range, including large forgings, sheet, plate and seamless tube, and it is also widely produced by additive manufacturing for aerospace parts. Ageing is mandatory for both grades if the specified strength is to be achieved; shipping in the annealed or solution-treated condition and ageing after machining is common practice.
Mechanical Properties, Temperature Limits and Corrosion Behaviour
| Property | Alloy K500, aged | Alloy 718, aged |
|---|---|---|
| Tensile strength | 965–1100 MPa | 1240–1375 MPa |
| 0.2 % yield strength | ≥690 MPa | ≥1035 MPa |
| Elongation | ≥20 % | ≥12 % |
| Hardness | 32–38 HRC | 36–42 HRC |
| Density | 8.44 g/cm³ | 8.19 g/cm³ |
| Practical maximum service temperature | About 480–530 °C | About 650–705 °C |
Seawater and brine: Alloy K500 is the better choice. It resists flowing seawater, high-velocity brine and marine atmospheres with very low corrosion rates.
High-temperature oxidation: Alloy 718 is the better choice. Its chromium-rich oxide remains protective to roughly 700 °C, while Alloy K500 has no chromium and oxidises rapidly above about 530 °C.
Reducing and acidic media: Alloy K500 performs well in hydrofluoric acid and in reducing conditions; Alloy 718 covers a broader range of oxidising acids at elevated temperature.
Hydrogen sulphide service: Alloy K500 is qualified to NACE MR0175 / ISO 15156 for sour oil and gas hardware, and Alloy 718 is also accepted in sour service where higher strength is required.
Galling and wear: Alloy K500 work-hardens slowly and is preferred for moving contact surfaces; Alloy 718 is usually selected where strength, not wear, governs the design.
Typical Applications and Selection Guidance
Alloy K500: seawater valve stems and trim, pump shafts and impellers, marine propeller shafts, oil and gas wellhead components, springs, fasteners, heat exchanger parts for weakly acidic streams, and electronic connectors requiring corrosion resistance.
Alloy 718: jet engine and gas turbine discs, compressor blades, casings and fasteners, cryogenic tankage and piping, nuclear reactor internals, downhole and wellhead hardware, high-strength springs, and tooling for hot forming.
The selection rule is simple. If the governing requirement is seawater or reducing-chemical corrosion and the metal temperature stays below roughly 500 °C, Alloy K500 gives the best balance of corrosion resistance, strength and machinability. If the requirement is high strength at 600–700 °C, fatigue resistance or a combination of strength with moderate oxidation resistance, Alloy 718 is the correct grade. Only rarely is either alloy a direct substitute for the other, and a change of material should always be validated by a corrosion test in the actual process stream.
Frequently Asked Questions
Q: Which alloy is stronger, K500 or 718?
Alloy 718 is substantially stronger in the aged condition. It typically reaches 1240–1375 MPa tensile strength against 965–1100 MPa for aged Alloy K500, and it keeps that strength to much higher temperatures.
Q: Can Alloy K500 be used at 700 °C?
No. Alloy K500 contains no chromium, so it oxidises quickly above roughly 530 °C and it also overages and loses strength. Alloy 718 is the appropriate grade for 650–705 °C service.
Q: Is Alloy K500 suitable for sour gas service?
Yes. Alloy K500 is certified to NACE MR0175 / ISO 15156 and API 6ACRA, which is why it is widely used for valve stems, seals and downhole tools in hydrogen sulphide bearing wells.
Q: Does Alloy 718 resist seawater?
It resists seawater reasonably well but it is not the first choice, because it can suffer crevice attack in stagnant chloride conditions. Alloy K500 is preferred where continuous seawater contact is involved.
Q: How are the two alloys hardened?
Both are age-hardenable, not quench hardened. Alloy K500 is solution treated and aged at 480–595 °C; Alloy 718 uses a two-step age at about 720 °C followed by 620 °C to precipitate the gamma double prime phase.
Q: Which grade is more cost effective for marine valves?
Alloy K500 is normally the lower-cost and better-performing option for marine valve stems, trim and pump components, while Alloy 718 is reserved for valves that must also withstand high temperature or very high mechanical loading.
