Material Overview
K500 (UNS N05500, Werkstoff 2.4375) is a nickel-copper alloy that industrial users select for the combination of high mechanical strength and broad corrosion resistance. Its matrix is the same nickel-copper system as Alloy 400, but a deliberate addition of aluminium and titanium turns it into an age-hardenable grade: a solution anneal followed by an aging treatment precipitates fine intermetallic particles and lifts strength well beyond what a plain solid-solution alloy can reach.
Because the alloy keeps its ductility, toughness and non-magnetic character after aging, it is used wherever a component must carry load, resist seawater or sour gas, and remain dimensionally stable over a wide temperature range.
Chemical Composition
| Element | Content, % | Function |
|---|---|---|
| Nickel (plus cobalt) | 63.0 min | Corrosion-resistant matrix |
| Copper | 27.0 - 33.0 | Matrix, seawater resistance |
| Aluminium | 2.30 - 3.15 | Aging element |
| Titanium | 0.35 - 0.85 | Aging element |
| Iron | 2.0 max | Residual |
| Manganese | 1.5 max | Residual, deoxidation |
| Silicon | 0.5 max | Residual |
| Carbon | 0.25 max | Residual |
| Sulphur | 0.01 max | Kept low for sour service |
Roughly two thirds of the mass is nickel and about a third is copper; the aluminium and titanium contents together are usually below 4 %, yet they are responsible for the entire aging response.
Physical Properties
Density: approximately 8.44 g/cm3
Melting range: 1300 - 1350 °C
Coefficient of linear expansion: approximately 13.8 x 10-6 per °C
Magnetic behaviour: non-magnetic down to about -200 °C in the correctly heat-treated condition
Service range for mechanical properties: sub-zero temperatures up to about 480 °C
The coefficient of thermal expansion is close to that of many steels, which limits differential expansion problems in mixed-material assemblies such as pump shafts and valve trim.
Mechanical Properties and the Aging Response
| Property | Aged condition |
|---|---|
| Tensile strength | approx. 1100 MPa (160 ksi) |
| Yield strength, 0.2 % offset | approx. 790 MPa (115 ksi) |
| Elongation | approx. 20 % |
| Relative to Alloy 400 | about 3x yield, about 2x tensile |
Cold working before aging adds a further increment of strength. The usual heat treatment sequence is a solution anneal at about 980 °C with air cooling, followed by aging between 460 °C and 530 °C; exceeding this aging window causes over-aging and a measurable drop in hardness.
Processing Performance
The alloy can be turned, milled, drilled and ground with conventional machine tools, but its high strength and tendency to work harden demand rigid setups, sharp carbide tooling, generous coolant flow and moderated cutting speeds. Welding is normally performed in the annealed or solution-treated condition and is followed by re-solution treatment plus aging when full joint properties are required. After heating, a nickel-rich magnetic film may form on the surface through selective oxidation of aluminium and copper; pickling or bright dipping removes it and restores the non-magnetic surface condition.
Industrial Applications
Marine engineering: propeller shafts, pump shafts, bearings, valves and fasteners exposed to seawater and salt spray.
Oil and gas: well tools, drill collars, safety lifts and valve trim, where resistance to hydrogen sulphide and to sour gas environments is essential.
Aerospace and defence: engine and airframe components that must withstand elevated temperature and high load while remaining non-magnetic.
Chemical processing: reactors, heat exchangers and storage vessels handling salts, alkalis and non-oxidising acids.
Pulp, paper and printing: doctor blades and scrapers that need a hard, corrosion-resistant edge.
Across all of these fields the engineering argument is consistent: an age-hardenable nickel-copper alloy offers the corrosion behaviour of a solid-solution grade together with the strength of a much harder material, so parts can be made smaller and lighter without giving up service life.
Frequently Asked Questions
Q: What are the main alloying elements?
Nickel with about 27 - 33 % copper, plus 2.30 - 3.15 % aluminium and 0.35 - 0.85 % titanium as the aging additions.
Q: What is the density of the material?
Approximately 8.44 g/cm3, with a melting range of 1300 - 1350 °C.
Q: How is the strength achieved?
By precipitation hardening: a solution anneal near 980 °C, air cooling, then aging between 460 °C and 530 °C to precipitate fine Ni3(Al,Ti) particles.
Q: Is the alloy suitable for seawater service?
Yes. It resists high-velocity seawater well, which is why centrifugal pump shafts and marine hardware are among its largest applications.
Q: How difficult is it to machine?
It machines with standard methods but work hardens quickly, so rigid tooling, carbide inserts and reduced cutting speeds are recommended.
Q: What is the maximum continuous service temperature?
Mechanical properties remain stable up to about 480 °C; long exposure above that level causes over-aging and softening.
