Monel 400

What is Monel 400?

Monel 400 is a nickel-copper solid solution alloy designated UNS N04400 and material numbers 2.4360 and 2.4361. It contains 28.0 to 34.0 percent copper and at least 63.0 percent nickel plus cobalt. Cold work is its only hardening method. The alloy retains high strength and toughness across a wide temperature range. Its principal service environments include flowing seawater and hydrofluoric acid.

Monel 400 has exceptional resistance to hydrofluoric acid at all concentrations up to the boiling point. The alloy originator describes it as perhaps the most resistant commonly used engineering alloy for this service. No equivalent statement applies to any stainless steel.

The density is 8.80 grams per cubic centimetre. The melting range is 1300 to 1350 degrees Celsius. The remaining composition limits are 0.3 percent maximum carbon, 2.0 percent manganese, 2.5 percent iron, 0.5 percent silicon and 0.024 percent sulphur.

Laxcon Steels lists Monel 400 in its grades reference as a nickel alloy grade. The same steel is written Alloy 400, N04400 and 2.4360.

Limiting composition for UNS N04400. The nickel minimum includes cobalt and is determined by difference.

What is the chemical composition of Monel 400?

Composition limits in weight percent are carbon 0.3 maximum, manganese 2.0 maximum, sulphur 0.024 maximum, silicon 0.5 maximum, nickel 63.0 minimum, copper 28.0 to 34.0 and iron 2.5 maximum.

Corrosion behaviour

Monel 400 resists many reducing media. It also has greater general resistance to oxidising media than higher-copper alloys. These properties give it a broader service range than nickel or copper alone. It resists sulphuric and hydrochloric acids under reducing conditions. It also resists stress corrosion cracking and pitting in most fresh and industrial waters.

Monel 400 has very low corrosion rates in flowing seawater. Stagnant seawater can cause crevice corrosion and pitting. The alloy is therefore used for pump and propeller shafts, impellers, valve trim and piping where seawater remains in motion. Laid-up systems, deposits, crevices and spaces beneath gaskets require different treatment because they can retain stagnant water.

Monel 400 does not depend on the passive mechanism used by stainless steel. Nickel and copper in solid solution are thermodynamically resistant to attack under reducing conditions. The alloy can therefore resist environments that remove a chromium-bearing passive film. Strong oxidisers can attack Monel 400 even where they would preserve such a film.

Mechanical properties

Monel 400 has no hardening heat treatment. The properties of rod and bar therefore depend on the amount of cold work retained in the product.

Mechanical properties of rod and bar, from the alloy originator's technical bulletin.

Each condition covers a range of product sizes, which produces a wide property range. Cold work increases yield strength more than tensile strength and reduces elongation. This response is characteristic of a work-hardening alloy.

Thermal treatment and cold work

Annealing takes place at 760 to 816 degrees Celsius, equivalent to 1400 to 1500 degrees Fahrenheit, with a holding time of one to three hours at temperature. Cooling rate has no significant effect on the result. Heating and cooling in a reducing atmosphere, or quenching in an alcohol-water solution, retains a bright surface. Air cooling after heating produces an adherent oxide film that requires pickling.

Control of hot or cold work and selection of the corresponding thermal treatment determine the finished properties. Monel 400 is not hardened by quenching and tempering. Welding or local annealing produces a region softer than the surrounding worked material. Design must account for this difference.

Machining and related alloys

Monel 400 is gummy and work-hardens during machining. Monel R405 and Monel K500 modify the same alloy base for different requirements. Monel R405 is the free-machining version. Its sulphur content is increased to 0.025 to 0.06 percent. The resulting sulphide inclusions break chips and permit automatic screw machine processing. This modification reduces some corrosion performance and weldability.

Monel K500 is the higher-strength version. It contains 2.3 to 3.15 percent aluminium and 0.35 to 0.85 percent titanium. These additions permit age hardening. Its strength exceeds the levels attainable in Monel 400 through cold work, while it retains most of the corrosion resistance.

Monel R405 is used for parts turned in volume when the environment is not the most severe. Monel K500 is used when loaded components, including pump shafts and fasteners, require greater strength. Monel 400 applies where neither modification is required.

Designations and product forms

ASTM B164 and ASME SB164 cover rod, bar and wire. ASTM B564 covers forgings. Other specifications include BS 3076NA13, DIN 17752 and AMS 7233 for rivets. ASTM B366 and ASME SB366 cover fittings.

Applications

Applications include valves and pumps, pump and propeller shafts, marine fixtures and fasteners, and heat exchangers. Heat exchanger uses include boiler feedwater heaters and deaerating heaters. Other applications include chemical processing equipment, crude petroleum stills, process vessels and piping, gasoline and fresh water tanks, springs, and electrical and electronic components. Hydrofluoric acid service includes alkylation units and fluorine chemical plant. This application has no straightforward substitute for Monel 400.

Where Monel 400 is not the right choice

Monel 400 is unsuitable for oxidising conditions. Nitric acid, other strongly oxidising acids and oxidising salt solutions attack the alloy. This limitation corresponds to its resistance in reducing acids. Chromium-bearing alloys, stainless steels and nickel-chromium alloys are the appropriate families for oxidising environments.

Monel 400 is unsuitable for stagnant or oxygen-depleted seawater. These conditions can cause crevice corrosion and pitting. Vulnerable locations include laid-up systems, dead legs, gasketed joints and areas beneath deposits. Super duplex stainless steel, six percent molybdenum austenitic grades and titanium are alternatives where seawater remains stagnant.

Monel 400 is unsuitable for wet chlorine, chlorine dioxide and hypochlorite. Ammonia and ammoniacal solutions also attack it when oxygen is present. This vulnerability of copper-bearing alloys affects refrigeration and fertiliser plant applications.

Monel 400 is not a high-temperature alloy. Nickel-chromium alloys such as Inconel 600 or 601 apply to furnace and hot gas service. Cold work also provides limited strength compared with age hardening. Monel K500 is the age-hardenable member of the same family for loaded shafts and fasteners.

Other nickel alloy grades