Why Nickel-Copper Alloy 400 Stands Out
Alloy 400, designated UNS N04400 and Werkstoff 2.4360, is the standard wrought nickel-copper alloy: at least 63% nickel with 28.0-34.0% copper and a small iron addition. Its advantages follow directly from that chemistry. Copper supplies resistance to seawater, reducing acids and alkalis, while the high nickel level gives a stable austenitic structure, good toughness and outstanding resistance to chloride-induced stress corrosion cracking.
Unlike austenitic stainless steel, which relies on a passive chromium oxide film that chlorides can break down, Alloy 400 is inherently resistant in chloride-bearing water and keeps that resistance at both high and low temperature.
Advantages Compared with Austenitic Stainless Steel
Broad corrosion resistance in acids, alkalis, fresh water and salt water, with seawater performance well above that of 316 stainless steel.
Higher yield and tensile strength than 316 stainless steel, with typical annealed values of 550 MPa tensile and 240 MPa yield.
Lower coefficient of thermal expansion of about 13.9 µm/m·°C between 20 and 100 °C, against roughly 16-17 for austenitic stainless steels, which gives better dimensional stability when temperatures change.
Strong resistance to chloride stress corrosion cracking, a failure mode that regularly affects stainless steel in warm chloride service. Resistance improves as iron content is held low.
Non-magnetic in service because the Curie temperature lies close to ambient temperature.
Easy joining: the alloy can be welded, soldered and brazed, and formed hot or cold with conventional equipment.
Reliable performance from subzero temperatures up to about 480 °C without loss of strength.
Physical and Mechanical Data
| Property | Typical value for Alloy 400 |
|---|---|
| Density | 8.80 g/cm³ |
| Melting range | 1300-1350 °C |
| Tensile strength | 550 MPa |
| Yield strength (0.2% offset) | 240 MPa |
| Elongation | 40% |
| Modulus of elasticity | 179 GPa |
| Thermal expansion | 13.9 µm/m·°C (20-100 °C) |
| Thermal conductivity | about 21.8 W/m·K at 20 °C |
| Maximum service temperature | about 480 °C |
Figures describe annealed product at room temperature. Cold working raises strength and hardness while reducing elongation, and the alloy work hardens rapidly, so formed parts usually need an anneal before severe further deformation.
Thermal and Dimensional Behaviour
A low coefficient of thermal expansion is a real advantage in heat exchangers, condensers and precision assemblies, because differential movement between tubes, tube sheets and shells is reduced. The alloy also retains useful ductility at cryogenic temperature, so it is used for equipment that starts up cold and then operates hot, such as liquefied gas systems and steam tracing lines. Its thermal conductivity is higher than that of austenitic stainless steel, which helps when heat must be transferred through tube walls.
Working With Alloy 400: Welding and Machining
Gas tungsten arc, gas metal arc and shielded metal arc welding are all used with matching nickel-copper filler metal such as ERNiCu-7 or ENiCu-7. No preheat is required, and the interpass temperature should be kept low with clean, low-sulphur consumables. Machining is the slowest operation because the alloy is gummy and work hardens under the tool, so heavy feeds, moderate speeds, rigid setups and sharp carbide tooling are essential; cutting fluids should be free of sulphur and chlorine to avoid staining.
Where the Higher Cost Is Justified
Seawater piping, marine fixtures, pumps, valves, condensers and heat exchangers.
Desalination and shipbuilding systems exposed to brine and chloride attack.
Hydrofluoric acid alkylation and caustic soda handling in chemical plants.
Pump shafts, propeller shafts, valve trim, springs and fasteners in oil field and refinery equipment.
Petroleum, natural gas, power, metallurgy and construction pipeline connections.
Specialty products such as musical instruments, eyeglass frames and aerospace hardware.
Limitations to Weigh Before Selection
Alloy 400 is not suitable for strongly oxidising media such as nitric acid, and it is attacked by mercury, so contact with mercury in gauges, seals and thermowells must be avoided. Service temperature is limited to about 480 °C, and the alloy costs considerably more than carbon or stainless steel, so it is normally specified only where the corrosion duty justifies it. When it is joined to austenitic stainless steel in seawater, the fitting becomes anodic and corrodes preferentially, so insulated joints or anodes should be used.
Frequently Asked Questions
Q: Is Alloy 400 stronger than 316 stainless steel?
Yes. Typical annealed values of 550 MPa tensile and 240 MPa yield strength are higher than those of 316 stainless steel, and the alloy keeps its toughness down to cryogenic temperature.
Q: Why does Alloy 400 resist stress corrosion cracking so well?
Its high nickel content makes the alloy tolerant of chloride ions, and resistance rises further as iron content falls, which is why chemistry limits are held low in the specification.
Q: How hot can Alloy 400 be used?
Continuous service is normally restricted to about 480 °C; above that temperature strength drops and oxidation becomes significant.
Q: Is Alloy 400 magnetic?
Its Curie temperature is close to ambient, so it is essentially non-magnetic at room temperature and above, with only a weak response at low temperature.
Q: Can Alloy 400 be soldered and brazed as well as welded?
Yes. The alloy takes solder and brazing filler readily, which is useful for instrument tubing and leak-tight assemblies where welding heat is undesirable.
Q: What should be avoided when Alloy 400 is specified?
Avoid oxidising acids, mercury contact and galvanic coupling with stainless steel in seawater; each of these causes rapid, predictable failure.
