Monel 400 or K-500: when the heat treatment is worth the extra strength

The two nickel-copper grades share a base chemistry and almost nothing else. K-500 is 400 plus aluminium and titanium, aged to roughly double the yield strength — with real penalties in ductility, welding and magnetic behaviour.

Monel 400 is a solid-solution alloy: about two thirds nickel, one third copper, and whatever strength the mill's anneal leaves in it. Monel K-500 uses the same base and adds 2.30–3.15% aluminium and 0.35–0.85% titanium, then ages the part to precipitate a nickel-aluminium-titanium phase. The result is a grade that behaves like 400 in corrosion service and like a different material in the workshop.

	Monel 400	Monel K-500
UNS	N04400	N05500
Nickel	≥ 63.0%	≥ 63.0%
Copper	28.0 – 34.0%	27.0 – 33.0%
Aluminium + titanium	—	2.65 – 4.00% combined
Condition	annealed	aged (or annealed for machining)
Strength	moderate, very ductile	roughly double the yield strength
Magnetic	non-magnetic	slightly magnetic when aged
Welding	straightforward	possible, properties reduced at the weld

Monel 400: the default for seawater and hydrofluoric acid

400 is the grade to reach for when the medium is seawater, brine, hydrofluoric acid or a fluoride-bearing process stream, and the part is a vessel, pipe, valve body or heat exchanger tube that will be welded. It is non-magnetic, holds toughness down to cryogenic temperature, and machines cleanly at moderate speeds. Almost every Monel enquiry we see starts here, and most of them should stay here.

Monel K-500: shafts, impellers, bolting

K-500 exists because pump shafts, impellers, valve stems and subsea fasteners need yield strength that annealed 400 cannot provide. Ageing roughly doubles it while keeping the seawater corrosion resistance that made the base alloy attractive. The costs are real:

• Ductility and toughness drop. Not a problem for a shaft in torsion, a problem for a part that sees impact or thermal shock.
• Welding is a compromise. The weld and heat-affected zone do not recover full aged properties, so K-500 is usually selected for parts that are not welded after ageing.
• It is slightly magnetic. If the drawing says non-magnetic, K-500 is the wrong answer — specify 400.
• It can be stress-corrosion sensitive in some sour and high-temperature chloride duties where 400 is more forgiving.

How we quote the pair

For K-500 we always ask two questions before pricing: is the part welded after ageing, and does the drawing require a magnetic permeability limit? If the answer to either is yes, we usually quote 400 and explain why. For a shaft or a fastener that is machined from bar and never welded, K-500 is straightforward and we hold aged bar in the common diameters.

Values quoted here follow the ASTM / EN edition named in each paragraph. For a specific heat, the governing numbers are those printed on the EN 10204 certificate that ships with the material. Written by the technical desk at Shanghai Hangbo Alloy Group.