Monel grades

Chemical composition

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

The roughly two-thirds nickel and one-third copper balance gives Monel its corrosion resistance. In K-500, the added aluminium and titanium form fine particles during age hardening, which raise strength and hardness while keeping corrosion resistance close to that of alloy 400.

Monel products and specifications

Flanges are made to ASME B16.5 and B16.47, butt-weld fittings to ASME B16.9 and forged fittings to ASME B16.11. Pipe dimensions follow ASME B36.19M or B36.10M as ordered. Round bars in alloy 400 are also listed on our Monel 400 round bars page.

Where Monel is used

Monel 400 is used in marine engineering – seawater piping, pump shafts and impellers, valve trim, propeller shafts and sheathing – because it resists seawater and brackish water, including under fast-flowing conditions, and is not prone to chloride stress corrosion cracking. In chemical and hydrocarbon processing it is used for hydrofluoric acid alkylation units, crude oil distillation overhead systems, heat exchangers, brine heaters and caustic soda equipment. It also resists dilute sulphuric and hydrochloric acids when they are free of air.

Monel K-500 is used where alloy 400's corrosion resistance must be combined with higher strength and hardness: pump and propeller shafts, oil-well tools and drill collars, valve stems, springs, fasteners and doctor blades. Because K-500 is non-magnetic down to low temperatures, it is also used for instrumentation and some electronic components.

Mechanical properties and condition

Monel 400 cannot be hardened by heat treatment; it is supplied annealed, or cold worked for higher strength in bar and wire. Annealed alloy 400 has a minimum tensile strength of about 480 MPa (70 ksi) and a minimum yield strength of about 195 MPa (28 ksi), with high elongation, and keeps good toughness at sub-zero temperatures. Monel K-500 is supplied annealed or hot finished, and is age hardened to reach a tensile strength of roughly 900 to 1100 MPa or more, depending on the product form and cold work before ageing. The exact property requirements for each form and condition are given in the relevant ASTM specification and are reported on the mill test certificate.

Working with Monel

Monel 400 is readily formed hot or cold and welded by the common arc processes using nickel-copper filler metals. As with other nickel alloys, joints must be clean and free of sulphur, lead and other low-melting contaminants, which can cause cracking. Monel work-hardens during machining, so rigid set-ups, sharp tools and positive cutting are needed. K-500 is usually machined in the annealed or hot-finished condition and then age hardened, or machined in the aged condition with the allowance that the higher hardness requires. Welding of K-500 is possible but, for most applications, parts are machined from bar or forgings rather than fabricated.

Monel compared with other alloys

In seawater, Monel 400 performs well in flowing conditions but can pit in stagnant water, where super duplex stainless steel or titanium may be preferred. In hydrofluoric acid service, Monel 400 is one of the standard materials. For strongly oxidising acids such as nitric acid, Monel is not suitable, and stainless steel or nickel-chromium alloys are used instead. Compared with copper-nickel alloys, Monel has higher strength and better resistance to erosion. Other nickel alloys are covered on our materials page.

Documents and inspection

Monel material is supplied with mill test certificates (MTC) showing heat number, chemical composition, mechanical properties and condition. Third-party inspection is available on request, and material can be supplied to NACE MR0175 / ISO 15156 on request for sour service. Because Monel looks like other nickel alloys and stainless steels, positive material identification during receiving inspection is recommended.