Alloy K-500, UNS N05500, is the precipitation-hardening member of the nickel-copper family. It keeps the corrosion behaviour of the lower-strength nickel-copper grade while raising tensile strength to about 1100 MPa through controlled aluminium and titanium additions plus an aging treatment. That combination is why it dominates pump shafts, marine hardware and oilfield components.
Overview: Nickel-Copper Alloy K-500 (UNS N05500)
Alloy K-500 is built on a nickel-copper matrix containing roughly 63% nickel and 30% copper, with 2.30-3.15% aluminium and 0.35-0.85% titanium added to make the grade age hardenable. In the annealed condition it behaves much like the standard nickel-copper grade, forming and welding easily; after aging it develops high strength and hardness while retaining good ductility.
The alloy performs exceptionally well in hydrofluoric acid and fluorine gas, in hot concentrated alkaline solutions and in flowing seawater. It also has no ductile-to-brittle transition temperature, so it is used for low-temperature equipment and for components that must survive both cryogenic service and marine exposure. Because it keeps a low magnetic permeability, it is also specified for non-magnetic instrument housings, drill collars and survey tools.
Chemical Composition and Equivalent Grades
Composition limits for the grade, expressed in mass percent, are given below. Supply of bar, rod, wire and forgings is governed by ASTM B865 with the equivalent ASME SB865.
| Element | Limit |
|---|---|
| Nickel | 63.0 min |
| Copper | 27.0-33.0 |
| Aluminium | 2.30-3.15 |
| Titanium | 0.35-0.85 |
| Iron | 2.00 max |
| Manganese | 1.50 max |
| Carbon | 0.25 max |
| Silicon | 0.50 max |
| Sulphur | 0.010 max |
| Designation system | Value |
|---|---|
| UNS | N05500 |
| DIN / W.Nr. | 2.4375 |
| ASTM product specification | B865, bar, rod, wire and forgings |
| ASME product specification | SB865 |
| Sour service qualification | ISO 15156-3 / NACE MR0175 |
| Legacy military specification | QQ-N-286 |
Physical and Mechanical Properties
Physical data are stable across the aging treatment, because aging changes the precipitate structure rather than the matrix.
| Property | Typical value |
|---|---|
| Density | 8.44 g/cm³ |
| Melting range | 1315-1350 °C |
| Modulus of elasticity | about 179 GPa |
| Thermal conductivity at 25 °C | about 17 W/m·K |
| Magnetic permeability | low, suitable for non-magnetic components |
| Condition | Tensile strength | 0.2% yield strength | Elongation | Hardness |
|---|---|---|---|---|
| Hot finished and aged | 965 MPa (140 ksi) min | 690 MPa (100 ksi) min | 20% min | about 24-28 HRC |
| Cold drawn and aged | 1100 MPa (160 ksi) min | 830 MPa (120 ksi) min | 15% min | about 32-36 HRC |
Strength depends on the amount of cold work before aging as well as on the aging cycle, which is why the two conditions above differ. For sour service and other environments where stress corrosion cracking is a risk, the maximum hardness and strength level should be agreed with the end user rather than assumed from the higher-strength column.
Aging Heat Treatment and Microstructure
Aging precipitates fine nickel-aluminium-titanium particles throughout the matrix and is what converts the soft annealed material into a high-strength component. The standard treatment for annealed or lightly cold worked material is a hold at 605 °C for 16 hours, followed by a controlled furnace cool at 8-14 °C per hour to 480 °C, after which the material may be air cooled.
Severely cold worked material is normally aged at 538 °C for a minimum of 6 hours, then cooled under control.
A slight dimensional shrinkage occurs during aging, so final machining tolerances should be planned around the treated size.
Parts should be machined close to finished dimensions before aging, then finished after the treatment so that the shrinkage is absorbed.
Exposure to the 480-595 °C range without a completed aging treatment should be avoided, because it can produce an unstable structure.
Fabrication: Machining, Forming, Welding and Hot Working
Machining is easiest in the annealed condition. The usual approach is to rough machine annealed stock, age the part, and then take the final cuts, because the alloy work hardens rapidly, generates high cutting temperatures and tends to weld to the tool edge. Sharp carbide tooling, positive rake angles, low surface speeds and heavy, uninterrupted feeds give the best tool life.
The alloy forms readily by standard methods in the annealed condition and can also be cold worked, although cold work plus aging raises strength further. Conventional welding processes are all suitable, but welding is best performed in the annealed condition with a stress-relief anneal afterwards; parts that will be aged should complete the full aging cycle after welding rather than be exposed to the aging temperature range in two separate stages.
Hot working is carried out between 1149 °C and 871 °C, followed immediately by a water quench, and the workpiece must not be allowed to fall below 871 °C before quenching. Typical applications include:
Pump shafts, impellers and valve stems for seawater and chemical service.
Marine hardware, propeller shafting and fasteners exposed to salt spray.
Oil and gas components for high-sulphur and high-wax reservoirs, including drill collars and non-magnetic survey housings.
Springs, bolts and other high-strength parts in hydrofluoric acid alkylation and fluorine-handling equipment.
Frequently Asked Questions
Q: What is the difference between alloy K-500 and the standard nickel-copper grade?
The standard grade is a solid-solution alloy that cannot be hardened; alloy K-500 adds aluminium and titanium so that aging raises strength and hardness substantially.
Q: What aging treatment is used for alloy K-500?
A typical cycle is 16 hours at 605 °C followed by a controlled furnace cool to 480 °C, with a shorter 538 °C cycle used for severely cold worked material.
Q: Is alloy K-500 suitable for seawater service?
Yes. It resists seawater, brines and marine atmospheres, and its high strength makes it suitable for shafts, impellers and fasteners that carry load in those environments.
Q: Can alloy K-500 be welded?
Yes, with conventional processes, preferably in the annealed condition and followed by a stress-relief anneal; parts to be aged should complete the full aging cycle after welding.
Q: How should alloy K-500 be machined?
Rough machine in the annealed condition, age the part, then take final cuts with sharp carbide tooling at low surface speed and heavy feeds to manage work hardening.
Q: Is alloy K-500 magnetic?
It has low magnetic permeability and is therefore used for non-magnetic drill collars, instrument housings and similar components.
