Monel K500 Nickel-Based Alloy Welding Rod

Monel K500 can be welded, and the joint will not be as strong as the alloy it joins. That is not a defect in the procedure. It is a property of the only filler metal recommended for the job, and it is the first thing a fabricator has to know before the drawing is fixed.

The reason sits in a single published sentence. Filler metal deposited by the grade used for this alloy is not age hardenable, and when it is used to join Monel K500 it has lower strength than the base metal. Monel K500 gets its strength from a heat treatment that precipitates particles through the material. The matching filler has a similar chemistry but no usable ageing response, so a post-weld heat treatment hardens the base metal around the weld and leaves the weld metal where it was.

One filler is recommended for joining this alloy, and it is designated in every classification system a buyer is likely to order against. The same wire answers to all of them, which is why an enquiry quoting one system and a mill certificate quoting another can describe the same product.

Two of those numbers are easy to transpose and worth separating once. The filler is 2.4377. The base metal is 2.4375. They differ in the last digit, and they are different materials.

The two figures below come from two different documents, and putting them side by side is the whole reason this page exists. One is the minimum the filler metal has to deliver. The other is the minimum the base metal has to deliver in the condition most parts are bought in.

The deposit is guaranteed at half the tensile strength of the age-hardened bar it is joining, and the gap is not closed by anything done after welding. The published description of the deposit is that its properties are similar to those of Monel alloy 400, which is the right way to think about it: a K500 weldment contains a region of alloy 400 level material running through it.

Two consequences follow for the drawing. A welded joint in K500 has to be placed where the design allows a lower-strength region, which usually means away from the highest-stress section. And where the full aged strength has to cross a joint, the joint has to be made by a method that does not rely on the filler, or the part has to be redesigned to avoid the joint.

The published alloy guidance sits alongside that and does not contradict it, though the two read as though they might. The alloy bulletin states that welds can be produced with the strength of age-hardened base metal if the ageing treatment is applied after welding, which is a statement about the assembly. The filler datasheet states that the weld metal is not age hardenable, which is a statement about the deposit. Both hold at once: the base metal around a weld recovers its aged strength when the welded assembly is aged, and the deposit does not.

This is the part of the procedure that is most often compressed, and compressing it produces a part that fails inspection or fails early in service. The published sequence for a welded K500 component has an anneal between the welding and the ageing, which means three thermal operations in total.

The annealing step has a second consequence that reaches beyond the weld. Every part of the assembly goes through 850 to 900 °C and then through the ageing cycle, so anything else joined into the same fabrication has to survive both. A gasket, a seal, a bolted fitting or an insert that cannot take those temperatures has to be fitted after the heat treatment, which is a routing decision and not a welding one and is worth settling at the drawing stage.

The published guidance also warns about the other direction. Overaged material cannot be recovered by a further ageing treatment: the part has to be solution annealed at 1,800 to 1,900 °F to redissolve the precipitate and then aged again, and the benefit of any cold work is lost in that anneal. A welded assembly that sees too long at temperature therefore has no route back to its designed condition.

The covered electrodes are the other half of the range, and they are designated with the prefix EN and not ER. The two prefixes are not interchangeable and they are not for the same process: the ER grades are bare wire and rod for gas shielded work, and the EN grades are covered electrodes for manual metal arc. The table below pairs each covered electrode with the base material it is for.

Three corrections are worth carrying forward from that table, because published pages get each of them wrong. Monel 400 is N04400; N02200 is Nickel 200, which is a different alloy and a different family. Alloy 825 is Incoloy 825 and not Inconel, since the Incoloy grades are the nickel-iron-chromium family. And the covered electrode for 254SMO is ENiCrMo-3, which is the same designation used for Inconel 625, because the filler is chosen for the chemistry of the joint and not for the name of the base metal.

One of the reasons Monel K500 is specified is that it is non-magnetic, and a welded component can lose that property at the seam. The published description of the alloy is that it is non-magnetic down to −100 °C in the age hardened condition, with a relative magnetic permeability at room temperature of 1.0015 or less. The weld metal has a lower quality than that in this respect, and the reason is the sentence already quoted: the deposit has properties similar to those of Monel alloy 400.

Alloy 400 is the one nickel-copper grade whose Curie temperature falls inside the ambient range. Its published range is 21 to 49 °C, with the accompanying note that the value is affected by composition and that some heats will therefore be magnetic at room temperature while others will not. Following the published statement that the deposit behaves like alloy 400, a K500 weld made with this filler carries a region whose magnetic behaviour at room temperature is less certain than that of the base metal around it.

For a component whose whole purpose is to avoid disturbing a magnetic sensor, that has to be handled deliberately and not assumed. The practical options are to keep the seam away from the sensor, to specify a magnetic permeability check on the finished component and not on the base metal alone, or to avoid welding in the sensitive region altogether. A purchase order that specifies the base metal and stays silent on the seam has not specified the property that matters.

The published fabrication guidance for this family is unusually prescriptive about the shop, and the reason is that nickel alloys are intolerant of contamination that carbon steel work tolerates without consequence.

ERNiCu-7 for gas tungsten arc, gas metal arc and submerged arc work, also designated AWS A5.14 ERNiCu-7 and UNS N04060, with the trade name Monel Filler Metal 60. The covered electrode equivalent for manual metal arc is ENiCu-7. The German designation is SG-NiCu30MnTi at material number 2.4377, and the international designation is SNi4060, or NiCu30Mn3Ti.

No. The published statement is that the weld metal is not age hardenable and has lower strength than the base metal when used to join K500. The numbers make the size of the gap concrete: the filler is specified at a minimum tensile strength of 70,000 psi, or 483 MPa, while age-hardened K500 bar is specified at a minimum of 140 ksi, or 965 MPa. The deposit is guaranteed at half the strength of the bar it joins, and no post-weld treatment closes that gap.

It strengthens the base metal around the weld and not the weld metal. The alloy guidance that welds can be produced with the strength of age-hardened base metal if the ageing treatment is applied after welding describes the assembly. The filler datasheet statement that the weld metal is not age hardenable describes the deposit. Both are correct, and reading the first as a statement about the weld metal is the mistake this distinction exists to prevent.

Weld the part with the base metal in the solution annealed condition, then anneal the welded component between 850 and 900 °C with cooling as quickly as possible, then age harden it. The published ageing range is 580 to 610 °C, with 4 to 16 hours for rod and forgings, furnace cooling at roughly 12 °C per hour down to 480 °C, and then air cooling. The anneal between welding and ageing is the step most often left out.

It should not be. The published guidance is to avoid welding age-hardened material because of greatly reduced ductility. The sequence that works is to weld in the annealed condition and apply the ageing treatment afterwards. Where a part has been aged and then needs to be joined, the options are to strip the ageing by solution annealing first, or to change the joint design.

ENiCu-7, which is the covered electrode for the nickel-copper family and shares the designation number with the bare wire used for K500. A published page on this subject gives Monel 400 as N02200; the correct UNS number is N04400. N02200 is Nickel 200, a commercially pure nickel and a different material.

Monel K500 is welded with a nickel-copper filler, designated ERNiCu-7 as bare wire and ENiCu-7 as a covered electrode, with material number 2.4377 as against 2.4375 for the base metal. That filler is not age hardenable, so the deposit is guaranteed at 70,000 psi, or 483 MPa, against the 140 ksi, or 965 MPa, minimum of the age-hardened bar it joins. The joint is therefore the weakest region in the component by design and not by error, and it has to be placed accordingly. The correct sequence is three thermal steps and not two: weld in the annealed condition with the interpass temperature held below 120 °C, anneal between 850 and 900 °C, then age at 580 to 610 °C. The work area, the tooling and the cleaning regime all matter more than they would on carbon steel, because the alloy is less forgiving of contamination.

Nickel-copper welding consumables and the matching base metal are supplied from Changzhou, Jiangsu, in the bare wire and covered electrode forms described above, from a plant that holds ISO 9001 certification and is audited by SGS. Send the base materials, the process and whether the joint has to carry aged strength, and the filler and the sequence follow from those three answers.

1. Special Metals product datasheet for MONEL Filler Metal 60. Source for the fillers and base metals it is intended for, the statement that the weld metal is not age hardenable and has lower strength than K500 when used to join it, the similarity of the deposited metal to alloy 400, the minimum tensile strength of 70,000 psi and elongation of 30 %, the limiting chemical composition, the designation list across AWS, ASME, BS, DIN and ISO, the recommended barrier layer for certain gas metal arc conditions, the submerged arc flux, and the available diameters and straight lengths.
2. VDM Metals data sheet for Alloy K-500, Nicorros Al. Source for the welding section: the requirement that the material be in the solution annealed condition for welding, the maximum interpass temperature of 120 °C, the recommendation to anneal between 850 and 900 °C after welding and before age hardening, the age hardening range and times, the root protection requirement, the impulse technique, the cleaning and workplace requirements, the tool and machine restrictions, the heat tint removal and the pickling sequence, and the statement that the alloy is non-magnetic down to minus 100 °C in the age hardened condition.
3. The same data sheet, on the age hardening treatment and its limits. Source for the 580 to 610 °C range, the 4 to 16 hour retention for rod and forgings, the furnace cooling rate of approximately 12 °C per hour to 480 °C, and the shortened cycle at 640 °C.
4. ASTM B865, precipitation hardening nickel-copper-aluminium alloy bar, rod, wire and forgings. Source for the minimum tensile, yield and elongation values for age-hardened bar and rod used in the comparison of deposit against base metal.
5. Special Metals technical bulletin SMC-062, MONEL alloy K-500. Source for the guidance that welding of age-hardened material should be avoided because of greatly reduced ductility, that welds can be produced with the strength of age-hardened base metal if ageing is applied after welding, that overaged material has to be solution annealed before it can be reaged, and the Curie temperature range quoted for alloy 400.
6. Special Metals technical bulletin SMC-053, MONEL alloy 400. Source for the Curie temperature range of 21 to 49 °C, the note that it lies within the ambient range so that some heats are magnetic at room temperature, and the UNS number N04400 for that alloy.
7. Delivery records held for DLX Alloy covering nickel-copper welding consumables and base metal. Source for the forms supplied.

Everything above is a published value for the grade named, and none of it is an acceptance criterion for a particular fabrication. What a joint reaches depends on the condition of the base material, the design of the joint, the heat input and the heat treatment sequence taken together, which is why a procedure is qualified on the assembly and not on the wire. On the Curie temperature of the base material, two published sources differ by a small margin; the figures above are attributed to the source each one comes from.