Monel K500 vs Monel 400: What the Extra Strength Costs

Monel K500 and Monel 400 share the same base: roughly two parts nickel to one part copper, with the same corrosion character and the same face-centred cubic matrix. Neither alloy is a precipitation hardened superalloy, and neither is chosen for the top of a temperature range. The difference between them is two alloying additions and the heat treatment those additions make possible, and almost every property difference in this article comes back to that.

One consequence is worth stating at the top, because it is where the comparison is most often mishandled. The strength figures published for these two alloys are given per form and per heat treatment condition, and the gap between them changes enormously depending on which conditions are being compared. A version of this comparison that states no condition cannot be used to specify anything, and the numbers below are therefore given with the condition attached.

Alloy 400 is a solid solution alloy. Nickel and copper are soluble in each other in all proportions, so the material is single phase, and the only way to harden it is to cold work it. Alloy K500 takes the same base and adds aluminium and titanium, which allows the alloy to be age hardened: a controlled heating cycle precipitates submicroscopic particles of Ni₃(Ti, Al) throughout the matrix. The published description puts it plainly, that K500 combines the corrosion resistance of 400 with the added advantages of greater strength and hardness, and that the increase comes from those two additions and the ageing treatment.

The compositions differ in exactly those two elements plus small shifts in the residuals. Every other figure is close, which is why the two alloys behave so similarly in service.

The sulfur limit is the quiet line in that table. It is roughly two and a half times tighter in K500 than in 400, and the reason is the same property that makes the alloy strong: once the precipitate is in place there is less ductility available to absorb the damage that a sulfur inclusion does along a grain boundary.

The published comparison states that K500 has approximately three times the yield strength and two times the tensile strength of 400. That is a real figure, and it describes one pairing of conditions. The table below holds the two alloys against each other in the conditions each is actually supplied in, on rod and bar.

Read down that table and the claim resolves. K500 in the annealed condition is stronger than 400 in the annealed condition, by about 1.2 times on tensile strength and about 1.4 times on yield, which is a margin and not a factor. Hot finished, the two overlap almost completely: the yield ranges are 276 to 690 MPa for 400 and 276 to 758 MPa for K500, so a hot finished bar of 400 can be as strong as a hot finished bar of K500.

What the published three and two figures actually compare is K500 that has been age hardened against 400 in the annealed condition. That is a legitimate comparison, because it is how the two alloys are most often bought against each other, but it is a comparison between two conditions instead of between two alloys.

The comparison is only half finished when the strength has been established. Four costs come with it, and each one is a line in a specification and not a footnote.

Alloy 400 has none of these costs. It is a single phase solid solution that hardens only by cold work, it has no ageing step to overshoot, no contraction to allow for and no loss of weldability in the condition it is supplied in. Where its strength is sufficient, it is the simpler material to specify.

Two of the physical differences between these alloys are counter-intuitive, and one of them is regularly stated the wrong way round in supplier literature.

The Curie temperature row deserves the space. Alloy 400 has a Curie temperature inside the ambient range, and the published note that accompanies it is explicit: the value is affected by variations in chemical composition, the range shown is what normal production can be expected to give, and therefore some heats will be magnetic at room temperature and others will not. The same source adds that if there is a strong requirement for non-magnetic characteristics, other alloys in the family should be considered.

K500 has no such ambiguity. Its Curie temperature lies between about −90 and −134 °C depending on condition, which puts room temperature well above it, and the published description is that the alloy is virtually nonmagnetic even at quite low temperatures. So on a non-magnetic requirement, the age hardenable grade is the safer of the two. That is the opposite of the assumption most buyers arrive with, since K500 is the stronger alloy and strength is usually assumed to cost something.

One processing caveat belongs with that reading. The published source notes that heating can leave a magnetic film on the outside of the piece, because aluminium and copper may oxidise selectively, and that the film can be taken off in acid. A magnetic requirement should therefore name the surface condition on the purchase order alongside the alloy.

The published position is that the corrosion resistance of K500 is substantially equivalent to that of 400, with one exception. In the age-hardened condition, K500 has a greater tendency toward stress corrosion cracking in some environments. The word substantially is doing real work in that sentence, and the exception is the one thing a buyer has to check before choosing the aged condition for a wet service.

Where the two are together, they are together in the media that matter. Both are widely used in marine service, and both show very low corrosion rates in flowing seawater. Both are subject to the same limitation in that medium: stagnant or slow moving seawater induces crevice and pitting attack, and the published description of K500 notes that fouling may occur followed by pitting, which slows after a fairly rapid initial attack. Alloy 400 is also resistant to stress corrosion cracking and pitting in most fresh and industrial waters, and offers exceptional resistance to hydrofluoric acid in all concentrations up to the boiling point, which the same source describes as perhaps the most resistant behaviour of any commonly used engineering alloy. It is also resistant to a number of sulfuric and hydrochloric acid duties under reducing conditions.

K500 carries one piece of corrosion data of its own that is worth knowing, because it applies to a service where the strength is the reason for the choice. It is described as resistant to a sour gas environment: after six days of continuous immersion in saturated hydrogen sulfide solutions at 3,500 ppm, across acidic and basic pH values from 1.0 to 11.0, U-bend specimens of the age-hardened sheet showed no cracking. A tight black scale formed and the specimens measured 28 to 40 HRC. That is the qualification behind the alloy in oilfield and sour service, and it is a qualification for the aged condition.

Only for one pairing of conditions, and that pairing has to be stated. K500 that has been age hardened has about three times the yield strength and about twice the tensile strength of 400 in the annealed condition, which is the comparison the published figures describe. Comparing like conditions gives a much smaller gap: annealed against annealed is about 1.2 times on tensile strength, and in the hot finished condition the two ranges overlap.

Two alloying additions and the heat treatment they allow. Alloy 400 is a solid solution of nickel and copper that is single phase and can only be hardened by cold work. Alloy K500 adds between 2.30 % and 3.15 % aluminium and between 0.35 % and 0.85 % titanium, which makes a controlled ageing treatment precipitate submicroscopic particles of Ni₃(Ti, Al) through the matrix. Everything else follows from that.

Substantially, with one exception that the published data states clearly. In the age-hardened condition, K500 has a greater tendency toward stress corrosion cracking in some environments. Where the medium carries that risk, 400 is the safer choice unless the strength is essential, and either way the medium has to be checked against published data before the condition is fixed.

K500, and for a reason that is easy to get backwards. The Curie temperature of 400 lies between 21 and 49 °C, which straddles ambient, and the published note is that some heats will be magnetic at room temperature and others will not. K500 has a Curie temperature between about −90 and −134 °C, well below ambient, and is described as virtually nonmagnetic even at quite low temperatures.

Not in the age-hardened condition, and the published guidance is explicit that welding of age-hardened material should be avoided because of greatly reduced ductility. Where a joint has to reach the aged strength, the sequence is to weld annealed material and then perform the ageing treatment. Alloy 400 can be joined by the conventional processes in the condition it is supplied in.

The properties are lost and cannot be recovered by ageing again. Overaged material has lower mechanical properties than properly aged material, and to strengthen it the part has to be solution annealed at 1,800 to 1,900 °F to redissolve the precipitate and then reaged. All the benefit of any cold work is lost in that anneal, so the best result available afterwards is the annealed and aged figure.

Monel K500 is Monel 400 with aluminium and titanium added and an ageing treatment applied. That gives it about three times the yield strength and twice the tensile strength of annealed 400, and it takes four things away: ductility, some resistance to stress corrosion cracking in the aged condition, weldability in that condition, and the simplicity of a material with no irreversible process step. Between those two positions the physical differences are small, except in two places: 400 conducts heat better by about 28 %, and K500 is the more reliably non-magnetic at room temperature because its Curie temperature sits far below ambient while that of 400 sits inside it.

Both alloys are supplied from Changzhou, Jiangsu, in bar, wire, strip, plate and tube, in the forms and conditions the published ranges above describe. The plant holds ISO 9001 certification and is audited by SGS. Send the service medium, the strength required and the fabrication sequence, and the choice between the two follows from those three answers.

1. Special Metals technical bulletin SMC-053, MONEL nickel-copper alloy 400. Source for the composition limits, the density and melting range, the modulus and Poisson ratio, the Curie temperature and the note that it straddles ambient so that some heats are magnetic at room temperature, the mechanical property ranges by form and condition, the statement that the alloy is a single phase solid solution hardenable only by cold working, the 1,000 °F limit in oxidising atmospheres, and the corrosion behaviour in seawater, fresh water and hydrofluoric acid.
2. Special Metals technical bulletin SMC-062, MONEL alloy K-500. Source for the aluminium and titanium ranges, the tighter residual limits, the description of the alloy as combining the corrosion resistance of 400 with greater strength and hardness through precipitation of Ni₃(Ti, Al), the mechanical property ranges by form and condition, the density, the magnetic characteristics and Curie temperatures by condition, the magnetic surface film and its removal, the corrosion equivalence and the stress corrosion exception in the aged condition, the sour gas data, the seawater velocity note, and the ageing contraction.
3. The same bulletin, on joining. Source for the statement that welding of age-hardened material should be avoided because of greatly reduced ductility, and that ageing treatments are performed after welding annealed material where the joint has to reach the aged strength.
4. The same bulletin, on heat treatment and machining. Source for the overageing behaviour, the requirement to solution anneal at 1,800 to 1,900 °F and reage, the loss of all cold work benefit in that anneal, the annealed and aged figure being the highest available afterwards, and the practice of machining oversize, age hardening and finishing to size.
5. The same bulletin, on the published nominal property ranges. Source for the caution that the ranges are composites for various product sizes and are not suitable for specification purposes.
6. UNS N04400 and N05500 as published for these two alloys. Source for the identifiers used in the composition table.
7. Delivery records held for DLX Alloy covering both grades. Source for the product forms and the conditions in which they are offered.

Every figure above is a published value for the standard product, and several of them are composites that span a range of sizes and tempers. They are not suitable for specification purposes. The condition is part of the specification on both alloys, so a purchase order that names the alloy without naming the temper does not describe the material being bought. Acceptance rests on the mill test certificate for the heat supplied.