Calorising

Calorising is a high-temperature diffusion process used to impart a protective surface layer, typically aluminium, onto a metal substrate, most commonly steel. The process involves heating the steel part in a mixture of aluminium powder, aluminium oxide, and ammonium chloride in a sealed retort. At elevated temperatures, the aluminium diffuses into the surface of the steel, forming a series of aluminium-iron alloys. This diffusion coating provides excellent resistance to high-temperature oxidation and scaling, making calorised components suitable for use in aggressive environments.

The primary benefit of calorising is the enhanced corrosion and heat resistance it offers. The aluminium-rich surface layer acts as a barrier, preventing the substrate metal from reacting with oxygen at high temperatures, thereby significantly extending the service life of the component. This makes it particularly useful for applications such as heat exchangers, furnace parts, exhaust systems, and other components exposed to extreme heat and corrosive atmospheres. The process is also effective in preventing the embrittlement that can occur in steels at high temperatures due to sulfur or other contaminants.

Calorising is typically carried out in batch furnaces. The temperature and duration of the process are carefully controlled to achieve the desired coating thickness and alloy composition. After the diffusion process, the parts are usually cleaned and may undergo further finishing treatments depending on the specific application requirements. While primarily applied to steel, variations of the calorising process can also be used for other metals.