Alloy 400 vs. Alloy K500

Alloy 400 vs. Alloy K500

Abstract:

Alloy K500 differs only slightly from Alloy 400 in composition, so their corrosion resistance is comparable. However, Alloy K500 has several times higher yield and tensile strengths, making it more suitable for heavy-duty applications. Gnee Steel offers solid round bars in a variety of sizes, ranging from 5/8-inch to 10-inch diameters.

Discussion:

Alloy 400 consists almost entirely of nickel (>63%) and copper (28-34%), with only small amounts of iron, manganese, and other trace elements. Nickel and copper are miscible, so its microstructure is single-phase. It can be strengthened by cold working, but not by heat treatment. Alloy 400 has relatively high strength compared to other copper alloys, and maintains good impact properties at temperatures from subzero to 480 degrees Celsius.

Being composed almost entirely of nickel and copper, Alloy 400 is particularly resistant to seawater corrosion because it quickly forms a complex but thin, adherent protective film when in contact with clean seawater. This protective film protects the alloy in most situations, with the only exception being very slow-moving seawater, which can cause scale formation that can lead to pitting or certain contaminants that destroy the protective film.

In addition to seawater, 400 alloy is also resistant to corrosion from other chloride solutions and a variety of acids, including sulfuric acid. It is resistant to hydrofluoric acid and hydrochloric acid as long as the oxygen content in the solution is very low, that is, under reducing conditions. 400 alloy is widely used in various marine engineering and chemical engineering fields, which mainly take advantage of its excellent corrosion resistance.

However, the strength of 400 alloy limits its use to structural applications. Therefore, K500 alloy was created to overcome this limitation. It has basically the same composition as 400 alloy, but with small amounts of aluminum and titanium added (3-4% total). Aluminum and titanium combine to form microscopic precipitates, which can be controlled by heat treatment. After forging or rolling, the round bars are subjected to an aging heat treatment, which is kept at high temperature for several hours and then slowly cooled in air. This process ensures that as much aluminum and titanium as possible precipitates out of solution and forms a widely dispersed fine network structure. These precipitates significantly improve the material's mechanical properties - yield strength can be increased by up to 4 times and tensile strength can be increased by up to 2 times.

The increased strength of K500 alloy means that it can be effectively used to manufacture more challenging components such as ship shafts, valve stems, fasteners, stirrer and agitator shafts, scrapers, drill collars and heat exchanger components.

Since K500 alloy has almost the same composition as 400 alloy, its basic chemical and corrosion resistance remains unchanged. It has been suggested that in limited circumstances, K500 alloy may be more susceptible to stress corrosion cracking, but this is most likely due to its higher strength and use in higher stress applications.

Since K500 alloy's specifications are not significantly inferior to 400 alloy, and the significant increase in strength also opens up a number of new applications, Langley Alloys decided to focus its inventory on K500 alloy. We stock solid round bar in a variety of sizes, from 5/8" to 10", for immediate delivery.