Chemical composition

Nickel includes cobalt. Limits as given in ASTM B127, B164 and B165 (alloy 400) and B865 (alloy K-500).

The table includes Monel K-500 for comparison. Alloy 400 contains no aluminium or titanium, so it cannot be age hardened.

Mechanical properties

Annealed condition. Hot-rolled, cold-worked and stress-relieved conditions have higher strength, with size limits as given in each standard.

Monel 400 is supplied annealed for forming, or cold worked and stress relieved for higher strength in bar, tube and wire.

Equivalent grades and physical properties

Equivalent designations are similar but not identical; order to the standard you need.

Typical values from recognised industry references, for guidance only; they are not specification limits.

See the full grade equivalent chart for other grades.

Corrosion resistance

Seawater: Monel 400 resists flowing seawater and brackish water very well, including at high velocities that would erode copper alloys, and is not susceptible to chloride stress corrosion cracking. In stagnant seawater, pitting and crevice corrosion can occur under deposits.

Hydrofluoric acid: it is one of the standard materials for hydrofluoric acid across a wide range of concentrations and temperatures, provided air is excluded.

Alkalis and salts: it resists caustic soda and other alkalis, and neutral and alkaline salts.

Acids: it resists dilute sulphuric and hydrochloric acids when air is excluded, but is not suitable for oxidising acids such as nitric acid or for oxidising salts such as ferric chloride.

In moist hydrofluoric acid vapour, highly stressed Monel can suffer stress corrosion cracking, so stress relief of fabricated parts may be specified.

Monel 400 products we supply

• Pipes and tubes – seamless to ASTM B165, welded to B725; heat-exchanger tubes to B163.
• Plates and sheets – ASTM B127.
• Round bars – ASTM B164; see Monel 400 round bars.
• Flanges and forged fittings – ASTM B564.
• Butt-weld fittings – ASTM B366 WPNC.
• Fasteners and wire – from B164 bar; welding wire ERNiCu-7.

All Monel alloys are on the Monel page.

Comparable alloys

Monel K-500 adds aluminium and titanium and is age hardened to about three times the yield strength of 400, with similar corrosion resistance; it is used for shafts and high-strength fasteners. Copper-nickel alloys (90/10 and 70/30) are lower-cost alternatives for seawater piping at lower velocities. Inconel 600 replaces copper with chromium and iron and is used for high-temperature oxidation and caustic service, where Monel is less suitable. Titanium and super duplex stainless steel are alternatives in stagnant or warm seawater.

Typical applications of Monel 400

Monel 400 is used for marine engineering – seawater piping, pump shafts and impellers, valves, propeller shafts and sheathing – HF alkylation units in refineries, hydrofluoric acid production and handling, crude oil distillation overhead condensers, brine heaters and evaporators in salt and desalination plants, caustic soda production, chemical processing equipment, boiler feed-water heaters and gasoline and fresh-water tanks.

Monel 400 in HF alkylation

In refinery HF alkylation units, Monel 400 is used for acid reboilers, acid coolers, isostripper overhead piping and other components where hydrofluoric acid and hydrocarbons are present with some water. Licensors and owners set requirements for hardness, weld quality and stress relief of Monel in this service, which should be listed in the enquiry.

Fabrication

Monel 400 forms readily cold and hot. It is welded by TIG, MIG and manual arc processes using ERNiCu-7 filler or matching covered electrodes, on clean joints free of sulphur and lead. Hot working is done in a sulphur-free atmosphere. Monel work-hardens during machining, so rigid set-ups, sharp tools and steady feeds are needed. No post-weld heat treatment is needed for general corrosion resistance.