Monel 400 Composition and Properties Across Temperature

Monel 400 is UNS N04400, also published as W.Nr. 2.4360 and 2.4361, and it is a binary solid solution of nickel and copper that can be hardened only by cold working. That single sentence establishes both halves of what a properties page has to say: the composition decides the character of the alloy, and the amount of work put into it decides every strength figure you can buy.

The composition is nickel at 63.0 % minimum with copper between 28.0 % and 34.0 %, and the balance is a small set of residuals that are held down and not added. What makes the alloy useful is the solubility of the two main elements in each other: they are mutually soluble in all proportions, so the structure is single phase, with a face-centred cubic lattice and a lattice parameter of 3.534 Å. There is no second phase to precipitate, which is the reason there is no ageing treatment in this alloy and the reason the only hardening mechanism available is deformation.

Read that way, the properties of Monel 400 fall into two groups that behave completely differently. The ones fixed by the composition do not move at all: the density, the modulus, the melting range and the thermal properties are set by the chemistry and by the temperature, and no processing decision changes them. The ones the customer actually specifies move over a wide range, because they follow from how much cold work the part has seen and what thermal treatment came after it. The sections below keep the two groups apart, and then follow the alloy to both ends of its temperature range, which is where its behaviour is most distinctive.

The Composition, and What Each Element Is Doing

The sulfur line is the one that reaches into the shop instead of staying on the datasheet. Because sulfur damages this family of alloys at temperature, the published annealing practice requires a sulfur-free reducing atmosphere, and marking crayons, grease and some refractory materials have to be kept away from a part that is going into a furnace. That is a fabrication requirement that follows directly from a composition limit.

What the Working Does to the Properties

This is the table a buyer actually orders from, and the spread it contains is wider than most people expect from a single alloy. Every row is the same composition; what changes is the condition it was supplied in.

Two things in that table decide most orders. The first is that cold work is the whole of the mechanism, so the strength you get is the strength you asked for by specifying a temper. Going from annealed sheet to spring temper strip roughly doubles the yield strength and takes the elongation from 35 % down to a value that can be as low as 2 %. The second is that the published note attached to these ranges is explicit: they are composites for various product sizes, and they are not suitable for specification purposes. A purchase order therefore names the form, the condition and the standard, and takes its minimums from the standard instead of from a table like this one.

One further consequence of the single-phase structure is worth stating. Because there is no precipitation reaction, there is nothing to overage and no ageing treatment to schedule. The thermal treatments that exist for this alloy do a different job: annealing softens work-hardened material completely, and stress relieving reduces residual stress without recrystallising the grain structure.

Strength and Ductility as the Temperature Falls

This is the property that most clearly separates Monel 400 from the steels, and it runs in the opposite direction to what most engineers expect. As the test temperature falls, the alloy gets stronger and it keeps its ductility at the same time. There is no transition to brittle behaviour.

The forged row is the one to look at, because it runs the full span. Between room temperature and −423 °F the tensile strength rises from 92 ksi to 142 ksi, which is roughly half again as strong, and the elongation rises with it instead of falling. The published assessment of the alloy notes that this behaviour is maintained over a considerable range of temperatures, and the practical consequence is that a cryogenic duty does not require a change of material on account of toughness.

Two published caveats belong with the table. The values shown for the cold drawn and annealed conditions apply to sizes from 0.032 to 0.250 in diameter, and the source states that properties for other sizes may vary from these. And the cold drawn row tested at room temperature after being held at −110 °F for several hours gave a lower yield strength than the row tested at temperature, which is the kind of detail that matters only if a specification calls for a soak instead of a test.

Toughness at Temperature, Which Is the Same Story

Impact testing tells the same story more sharply, because a toughness test is where a steel with a transition temperature would fail. This alloy does not have one that can be reached.

The published assessment records that no evidence of brittle fractures was shown, and that weld metal tested in the same programme fractured in the weld and not in the plate. There is also a note on the torsion impact specimens: attempts to produce fractures in them by reducing the minimum area by 75 % were unsuccessful, on account of the toughness of the material.

The Fabrication Windows

Because the alloy hardens only by cold work, the thermal treatments do not build strength. They restore ductility and remove stress. They also set the grain size, and each one has a published window.

The distinction between stress relieving and stress equalizing is the one most often lost. They are published as separate treatments at separate temperatures for separate purposes, and a shop that substitutes one for the other will not get the result the drawing asked for.

The Weld Deposit at Temperature

The alloy is joined with a matching filler, and the published all-weld-metal data shows how the deposit behaves as it gets hotter. It is worth having separate from the base metal figures, because a joint at temperature is limited by its weakest constituent.

The last row is the one that matters, because 1000 °F, or 538 °C, is the temperature at which the alloy reaches the end of its useful range in air. The deposit still carries 386 MPa in tension at that point, which is a useful figure to have, and the reduction of area has fallen to 24.5 %, which is the number that limits how much a joint at that temperature can be expected to accommodate.

A full-sized butt joint behaves differently from all-weld metal, and the published data records that the transverse tests across a weld all broke in the weld and not in the plate, which is the expected result when the deposit is weaker than the parent metal. A heat treatment of 1150 °F for 10 hours followed by air cooling did not change the room temperature weld properties to any useful extent, which is consistent with the deposit having no ageing response to develop.

The Study That the Published Body Was Taken From, and Why Its Numbers Cannot Be Used

The version of this page that has been published carries the method and results of a hot tensile study, and the study is described accurately enough to identify: a Gleeble-1500D thermal simulation machine, a specimen 6 mm in diameter and 120 mm long, heated at 10 °C per second to 1100 °C and held for three minutes, then cooled at 3 °C per second to test temperatures of 600, 700, 800, 900, 1000 and 1100 °C, strained at 10⁻³ per second in vacuum, with the fracture surfaces examined under a scanning electron microscope.

Two things about it are worth keeping. The ductility finding is consistent with what the published working ranges would lead you to expect: the study reports the high temperature plasticity rising with temperature to a peak around 900 °C and then falling again, and 900 °C sits inside the published hot working window of 927 to 1177 °C while 1100 °C sits near its top. The second is that a specimen held at 1100 °C for three minutes before testing has been effectively annealed, so the figures describe annealed material and not the condition the alloy is usually supplied in.

The strength figures cannot be republished as properties of this alloy. The study reports a tensile strength of 106.49 MPa at 600 °C and 22.41 MPa at 1100 °C. The published short-time data for the same alloy records 55.95 ksi, which is 386 MPa, for all-weld metal at 1000 °F, or 538 °C, and weld metal is normally the weaker constituent of a joint. A wrought figure of 106 MPa at a temperature above that is therefore roughly a factor of three to four below what the published data implies, and the discrepancy is large enough that the number should be treated as specific to that one test configuration. It is not a strength this alloy can be specified to.

The premise of the study also has to be read against the published assessment of the alloy. The study was undertaken because of a hot cracking tendency attributed in part to the low thermal conductivity and low expansion coefficient, but the published description of Monel 400 records that it is readily joined by conventional processes, that it has good weldability, and that it can be hot formed into almost any shape. Those two positions are not incompatible, in that a study can usefully investigate a problem inside a material that is generally workable, and they should not be read as a general warning against welding the alloy. The distinction is worth keeping, since the same low thermal conductivity that concentrates heat in a weld pool is also why the alloy holds its strength as the temperature rises.

Questions Buyers Ask

What is the chemical composition of Monel 400?

Nickel at 63.0 % minimum and copper between 28.0 % and 34.0 %, with the balance nickel and the residuals held down: carbon at 0.3 % maximum, iron at 2.5 %, manganese at 2.0 %, silicon at 0.5 % and sulfur at 0.024 %. The nickel figure is a minimum and not a range. It is determined by difference. Published data sheets sometimes give the nickel range as 63.0 to 70.0 %.

How is Monel 400 strengthened?

Only by cold working. Nickel and copper are mutually soluble in all proportions, so the alloy is single phase with a face-centred cubic lattice and there is no second phase to precipitate. There is therefore no ageing treatment for this alloy, and the strength of a delivered part is a function of how much cold work it has seen. That is the opposite of the position with a precipitation hardening grade such as Monel K500.

What is the strength range available from one composition?

About a factor of two between the extremes, and the extremes are not close together. Annealed sheet is published at 482 to 586 MPa tensile with 35 % or more elongation. Spring temper strip of the same alloy reaches 690 to 965 MPa with elongation as low as 2 %. A purchase order therefore has to name the temper as well as the alloy, because the condition is what carries the strength.

Does Monel 400 get brittle at low temperature?

No, and it does the opposite. Forged material tested at −423 °F, which is −253 °C, shows a tensile strength of 142 ksi against 92 ksi at room temperature, with the elongation rising from 31 % to 38.5 %. Charpy V-notch values at the same temperature run from 121 to 219 ft·lb depending on orientation, and the published assessment records no evidence of brittle fractures. This is a reason to choose the alloy for cryogenic service and not a limitation of it.

What are the hot working and annealing temperatures?

Hot working runs from 1700 °F to 2150 °F, or 927 °C to 1177 °C, with an optimum around 2000 °F, and the alloy must never be heated above 2150 °F because permanent damage may result. Annealing is published as 1600 to 1800 °F for 2 to 10 minutes in the open heating method, or 1400 to 1500 °F for 1 to 3 hours in box annealing, with a general range of 1300 to 1800 °F. The low end of the range is used with longer times to keep the grain size fine. The annealing atmosphere has to be a sulfur-free reducing one.

What is the difference between stress relieving and stress equalizing?

They are separate published treatments with separate purposes. Stress relieving is done at 1000 to 1050 °F for 1 to 2 hours and reduces residual stress without recrystallising the grain structure. Stress equalizing is done by holding for about 3 hours at 575 °F and raises the yield strength at 0.00 % offset without marked effects on the other properties. Substituting one for the other does not produce the result the drawing asked for.

Summary

Monel 400 is a single phase nickel-copper solid solution, UNS N04400 and W.Nr. 2.4360 or 2.4361, that can be hardened only by cold working. The composition is nickel at 63.0 % minimum with copper between 28.0 % and 34.0 %, and because the two elements are mutually soluble there is no precipitation reaction and no ageing treatment. The properties therefore divide into those fixed by the composition, such as density, modulus and melting range, and those fixed by the condition the material is supplied in, where the published ranges run from 482 MPa tensile in annealed sheet to 965 MPa in spring temper strip. At falling temperature the alloy gains strength while keeping its ductility, reaching 142 ksi in forged material at −423 °F with no evidence of brittle fracture. At rising temperature the useful limit in air is 1000 °F, the hot working window is 1700 to 2150 °F, and the weld deposit still carries 386 MPa at 1000 °F.

Monel 400 is supplied from Changzhou, Jiangsu in sheet, plate, strip, rod, bar, wire, forgings, seamless tube and pipe, and condenser tubing, in the annealed, stress relieved, half-hard, full-hard and spring temper conditions, from a plant that holds ISO 9001 certification and is audited by SGS. Send the form, the temper and the service temperature, and the properties follow from those three answers.

References

1. Special Metals technical bulletin SMC-053, MONEL nickel-copper alloy 400. Source for the composition limits, the density of 8.80 g/cm³ and the melting range, the modulus of elasticity in tension, compression and torsion with the Poisson ratio, the Curie temperature and its note, the description of the alloy as a binary solid solution with a face-centred cubic lattice parameter of 3.534 Å, the mechanical property ranges by form and condition, the low temperature tensile and impact tables, the note that the published ranges are composites which are not suitable for specification purposes, the all-weld-metal short-time properties at temperature, the transverse weld test result, and the temper hardness table for sheet and strip.
2. The same bulletin, on thermal treatment and fabrication. Source for the open heating and box annealing ranges, the statement that the alloy forms an adherent oxide film when allowed to cool in air after heating, the statement that cooling rate has no significant effect, the stress equalizing treatment at 575 °F and its effect on the 0.00 % offset yield strength, the statement that grain growth occurs when material is heated to elevated temperature, the 2150 °F hot working ceiling with the warning that permanent damage may result, and the guidance that in no case should the alloy be heated above that figure.
3. Published technical data for alloy 400 as circulated by