Monel K500: Density, Melting Point and Physical Data

Monel K500 is a nickel-copper alloy that is strengthened by an ageing treatment, not by its base chemistry. That one fact organises everything else on this page, because it means several of the physical properties are not single numbers. The density and the melting range are constants. The thermal conductivity, the specific heat, the electrical resistivity and the magnetic behaviour are all functions of temperature, and the resistivity and the magnetic behaviour also depend on the heat treatment condition the part was supplied in.

Getting that distinction right is the difference between a datasheet that can be used in a calculation and a list of figures that cannot. Both kinds are set out below, with the temperature or the condition stated for each one.

These are the values that hold regardless of temper or service temperature, and they are the ones that go into a mass calculation or a thermal model without further qualification. The composition is included because two of the constants below follow from it: the aluminium and titanium additions are what make the alloy age hardenable, and they are also why the alloy can develop a magnetic surface film.

The published data runs from −200 °C to 900 °C, and the columns do not all come from the same condition, which is worth stating before the numbers. Expansion is quoted as the mean from 21 °C to the temperature shown, and it was measured on age-hardened material, so the column builds cumulatively and does not give a local coefficient. Thermal conductivity, specific heat and resistivity were measured on material in the annealed condition. Those two footnotes are the difference between comparing like with like and comparing two different states of the same alloy.

Two patterns in that table are worth noting before the detail. The thermal conductivity roughly doubles between room temperature and 600 °C, so a thermal model built on the room temperature figure will understate heat flow badly at the hot end. The resistivity moves far less, from 0.615 to 0.658 µΩ·m over the same span, so a resistance or sensor calculation is much less sensitive to temperature than a heat transfer calculation is.

One footnote attached to the published resistivity column matters more than the column itself. Resistivity in this alloy is markedly influenced by thermal history, because the precipitation that hardens it also changes how electrons scatter through the lattice. The figures above were measured on material in what is equivalent to the annealed condition with a small amount of age hardening. A fully aged part will not sit exactly on that curve, and a buyer specifying a resistance value should ask for the figure for the condition actually supplied.

This is the part of the picture that a properties page usually leaves out. The same alloy, in the same size, can differ by more than a factor of two in hardness and by more than a hundred degrees Celsius of usable service temperature depending on whether it was aged. The table below sets the conditions against each other. The hardness figures are Brinell and the strength figures are the published room temperature ranges for the condition.

Hardness also falls as the service temperature rises, and it falls faster in the aged condition because the precipitate that provides the hardness starts to coarsen. The published figures below were measured on hot finished material in both conditions, and they are the reason a K500 part is not specified at the top of its temperature range when it also has to resist wear.

Monel K500 is usually described as non-magnetic, and for a buyer that description is not precise enough to design against. What the published data shows is that the alloy is weakly magnetic with a very low permeability, and that its Curie temperature lies far below room temperature. That is why it is effectively non-magnetic in service at ambient and above, and it is also why the magnetic readings change when the part is cooled.

Permeability is quoted at 200 oersted. The Curie temperature is reported as the temperature at which permeability reaches each of four threshold values, so an aged part passes 1.01 at a higher temperature than an annealed one.

The practical warning that comes with this property is on the surface of the part, not in the bulk of it. Aluminium and copper can be selectively oxidised during heating, and that leaves a nickel rich film on the outside of the piece which is magnetic. The effect is most noticeable on thin wire and thin strip, where the ratio of surface to weight is high, and it is removed by pickling or bright dipping, after which the non-magnetic behaviour is restored. A buyer whose application depends on the magnetic reading should therefore specify the surface condition as well as the alloy.

One physical property of this alloy is easy to overlook and expensive to discover at the wrong moment. Age hardening causes an initial contraction. A published measurement on an annealed rod gives a contraction of 2.5 × 10⁻⁴ in/in during the ageing treatment.

In absolute terms that is 0.025 %, which sounds small until it is applied to a long part. On a 1,000 mm bar it is a quarter of a millimetre. Once the ageing is complete, the same source reports exceptionally good dimensional stability, both over long exposure and over thermal cycling, and that stability is the reason the alloy is used in high precision devices such as gyroscopes. The distinction for a buyer is that the contraction happens once, during the treatment, so the machinist has to allow for it, while the designer does not.

The reason to separate the constants from the curves is that different design tasks need different ones, and a datasheet that presents them as one flat list invites the wrong one to be used.

8.44 g/cm³, which is 0.305 lb/in³. It is a single figure and does not change with the heat treatment condition or with the service temperature, so it is one of the few properties on this page that goes into a calculation with no condition attached to it.

The melting range is 1,315 °C to 1,350 °C, or 2,400 °F to 2,460 °F. It is a range because the alloy freezes over an interval, and for a process calculation the lower, solidus figure is the one that governs. The published range is confirmed by the conversion from the Fahrenheit figures.

Usually because the heat treatment condition is not stated. Resistivity is markedly influenced by thermal history in this alloy, hardness spans from 140 HB in the annealed condition to 370 HB in the cold drawn and aged condition, and permeability changes with ageing as well. A figure without a condition attached cannot be compared with another figure that has one.

It is weakly magnetic with a very low permeability, and its Curie temperature is far below room temperature, between about −90 °C and −134 °C depending on condition. At room temperature and above it is therefore effectively non-magnetic, with a permeability of the order of 1.002. The exception is a magnetic surface film that can form during processing if aluminium and copper are selectively oxidised, which is removed by pickling.

The part contracts. A published measurement on an annealed rod gives 2.5 × 10⁻⁴ in/in, which is about 0.025 %, or a quarter of a millimetre on a 1,000 mm length. After ageing, the alloy has exceptionally good dimensional stability over long exposure and over thermal cycling.

0.615 µΩ·m at 21 °C, rising to 0.658 at 600 °C and 0.695 at 900 °C. The change across that span is modest compared with the change in thermal conductivity, so a resistance calculation is much less temperature sensitive than a heat transfer calculation. The published curve is measured on material close to the annealed condition.

The physical constants of Monel K500 are density at 8.44 g/cm³, a melting range of 1,315 °C to 1,350 °C, a tensile modulus of about 179 GPa and a Poisson ratio of 0.32. The properties that vary are thermal conductivity, which roughly doubles between room temperature and 600 °C; specific heat, which rises by about a third over the same span; and electrical resistivity, which moves only slightly but shifts with the ageing condition as well as with temperature. Hardness and the magnetic behaviour depend on the condition the material was supplied in, and the ageing treatment itself contracts the part by about 0.025 %.

Monel K500 bar, wire, strip, plate and tube are supplied to UNS N05500 and W.Nr. 2.4375, in annealed, cold drawn and age hardened conditions, from a plant at Changzhou, Jiangsu that holds ISO 9001 certification and is audited by SGS. Send the form, the condition and the service temperature, and the physical properties that matter follow from those three answers.

1. Special Metals technical bulletin SMC-062, MONEL alloy K-500. Source for the composition limits, the nominal physical constants, the thermal and electrical property curves against temperature, the magnetic characteristics and Curie temperatures by condition, the hardness values against temperature, and the contraction measured during ageing.
2. The same bulletin, Table 3 footnote. Source for the statement that electrical resistivity in this alloy is markedly influenced by thermal history, and for the identification of the condition the published resistivity curve was measured on.
3. The same bulletin, Table 6 footnote. Source for the caution that the published property ranges are composites for various product sizes and are not suitable for specification purposes.
4. UNS designation N05500 and W.Nr. 2.4375 as published for this alloy. Source for the identifiers used in the summary.
5. Published temperature range over which the alloy retains its strength, quoted as 1,200 °F, against the cryogenic behaviour that holds to liquid hydrogen temperature. Source for the service window referred to in the condition discussion.
6. Product data held for DLX Alloy Monel K500 bar, wire, strip, plate and tube. Source for the forms and conditions supplied.

The figures quoted are published values for the standard product and are not acceptance limits. Several are composites covering a range of product sizes, and the property ranges are not suitable for specification purposes. What a particular part achieves depends on the form, the condition and the section size together, and acceptance rests on the mill test certificate for the heat supplied.