Alloy 400 vs. Alloy K500

Alloy 400: a stable nickel-copper alloy

Developed by the International Nickel Company in the early 1900s, Monel 400 was the first in a family of nickel-copper alloys, based on the nickel-to-copper ratio found in natural ores from Canadian mines. Because an ore is normally a very stable composition, the resulting alloy was equally stable and corrosion resistant. The Monel trademark, now owned by Special Metals Corporation, has become synonymous with this family even though the alloys are now produced by a number of mills.

The Alloy 400 composition is almost entirely nickel (more than 63%) and copper (28-34%), with only small amounts of iron, manganese and trace elements. Nickel and copper are mutually soluble, so the microstructure is single-phase. It can be strengthened by cold working, but not by heat treatment. Compared with other copper alloys it has relatively high strength, with impact properties maintained from sub-zero up to 480degC.

Because it is almost entirely nickel and copper, Alloy 400 resists seawater corrosion well, forming a complex but thin, adherent protective film quickly in clean seawater. That film protects the alloy in most conditions - the exceptions being very slow flow rates that allow fouling and then pitting, or particular contaminants that damage the film.

Alloy 400 also resists corrosion in other chloride solutions and a range of acids, including sulphuric acid. It resists hydrofluoric and hydrochloric acid provided the solutions have minimum oxygen content (reducing conditions). It is used across marine and chemical engineering applications that exploit its corrosion resistance.

Alloy K500: the same corrosion resistance, far more strength

The strength of Alloy 400 limits it to more structural applications, so Alloy K500 was developed to overcome that. It has effectively the same composition as Alloy 400, with small additions of aluminium and titanium (3-4% in total). These combine to form microscopic precipitates that are controlled by heat treatment.

After forging or rolling, the bars are aged - held at an elevated temperature for several hours and allowed to cool slowly in air. This lets as much of the aluminium and titanium as possible precipitate out of solution into a widely dispersed fine network. The result is a significant uplift in mechanical properties: yield strength up to four times greater, and tensile strength up to twice as great.

The higher strength of Alloy K500 suits it to more challenging components such as marine shafts, valve stems, fasteners, mixer and agitator shafts, doctor blades, drill collars and heat exchanger components.

Because the composition of Alloy K500 is practically identical to Alloy 400, the fundamental chemistry and corrosion resistance are unchanged. Alloy K500 is sometimes seen as more susceptible to stress corrosion cracking in limited circumstances, but this is most likely because it is used in more highly stressed applications, given its higher strength.

Why we stock Alloy K500

With no obvious downside to Alloy K500 over Alloy 400, and a significant strength uplift opening up many new applications, we concentrate our stock range on Alloy K500. We stock a wide range of sizes in solid round bar from 5/8" up to 10", available for immediate delivery.

Our history is rooted in alloy development and production. Many of our earliest inventions were copper alloys, and for many years we cast our own ingots for conversion into Alloy K500. The alloy has been part of our range for many years, allowing us to build up a comprehensive stock range and deep product knowledge by working with end users.

Designations

Alloy 400: BS 3076 NA13, Werkstoff No. 2.4360, ASTM A164, UNS N04400, N04405, QQ-N-281, Monel 400.

Alloy K500: UNS N05500, Werkstoff No. 2.4375, ASTM B865, AMS 4676, Monel K500.