Within the Monel family of nickel-copper alloys there are two particular alloys, Alloy 400 and Alloy K500, that appear outwardly to be very similar. An in-depth examination reveals there are differences between the two. Further explanation of these differences can prove of benefit in issues of material selection. In this post we will look at the two alloys and compare and contrast their metallurgical properties.
Alloy 400 and Alloy K500 are both nickel-copper alloys containing approximately 65% nickel and 30% copper. The chemical composition of Alloy K500, however, includes 3% aluminium and 0.5% titanium.
Chemical Composition
| Ni | Cu | Al | Fe | Mn | Ti | |
|---|---|---|---|---|---|---|
| Alloy 400 | ||||||
| Alloy K500 |
The two alloys also have different strengthening methods. Alloy 400 is solid solution strengthened and K500 is precipitation hardened. It is these different strengthening mechanisms that produce the mechanical differences between Alloy 400 and K500. The solid solution strengthening of Alloy 400 gives a lower tensile strength than that of K500 but makes it tougher than K500.
Mechanical Properties (specification minima)
| Alloy 400 40mm cold worked, stress relieved | Alloy K500 <25mm cold worked, precipitation hardened | |
|---|---|---|
| UTS (MPa) | ||
| 0.2% Proof Strength (MPa) | ||
| Elongation (%) |
The different alloying additions impact the cost of each material. The extra alloying additions for K500 have a commensurate effect on the price of the alloy. Alloy 400 is available at a lower price point for a given size while still providing a high tensile strength and maintaining the excellent corrosion resistant properties of K500.
The corrosion behaviour of Alloy 400 and Alloy K500 often finds them gainful employment, particularly in the chemical processing industries. Both alloys provides excellent corrosion resistance, although in some environments Alloy K500 can be susceptible to stress corrosion cracking (for example, hot hydrofluoric acid vapour at tensions close to the alloy’s yield strength). This is not an issue with Alloy 400, which makes it an excellent choice for use in equipment in contact with fluorine, hydrofluoric acid and hydrogen fluoride or their compounds.
Typical Physical Properties
| Alloy 400 | Alloy K500 precipitation hardened | |
|---|---|---|
| Density (g/cm3) | ||
| Melting Range (oC) | ||
| Young’s Modulus (GPa) | ||
| Thermal Conductivity (W/moC) | ||
| Coefficient of Thermal Expansion (μm/moK) | ||
| Electrical Conductivity (IACS) |
The final difference between Alloy 400 and Alloy K500 is the processing route, which can differ depending on size. Columbia Metals stocks Alloy 400 in the cold worked condition up to 55mm diameter, whereas Alloy K500 is cold worked up to 63.5mm diameter.
The choice between these two alloys ultimately depends on a few key factors, summarised below:
- Cost – the lower level of alloying elements in Alloy 400 provides, size for size, a cheaper alloy
- Mechanical properties – if the need for greater toughness is of prime consideration, then Alloy 400 would be the choice. Where greater tensile strength is required, Alloy K500 would be preferred.
- Corrosion performance – while both are good performers, Alloy 400 has the edge in fluorine or hydrofluoric environment
