Chromium Carbide

Chemical formula: Three modifications: Cr3C2, Cr7C3, and Cr23C6.

Melting point: 1895°C

Density: 6.68 g/cm3

The study of chromium carbide solidified in parallel with the development of alloy steels in the 20th century, when this compound was recognized as the main factor responsible for the hardness and corrosion resistance of chromium steels. Its production is not usually carried out as an independent material, unlike tungsten carbide which is sintered and commercialized in pure form. Instead, chromium carbide is spontaneously generated during the casting process of high-carbon steels, as chromium has a much greater chemical affinity for carbon than iron. Consequently, carbon atoms preferentially combine with chromium, forming intergranular carbides that are dispersed within the metallic matrix. This transformation not only hardens the steel but also refines its grain size and acts as a deoxidizer, improving the metallurgical quality of the final product. 

Chromium carbide stands out for its high hardness and low cost, making it one of the most widely used compounds in metallurgy. It is abundant, easy to generate, and represents the most widespread metallic carbide in the production of special steels. It provides wear resistance and some protection against corrosion, although its excessive presence can reduce the steel's ability to form protective passive layers. In this regard, the carbon-to-chromium ratio is fundamental: for stable carbides to form, the steel must contain at least 0.9 % by mass of carbon. In steels with higher carbon content, the necessary amount of chromium must be proportionally increased, which directly affects corrosion resistance. For example, AISI 420 steel contains about 12 % chromium and 0.30 % carbon, while AISI 440 steel reaches 18 % chromium and 1.25 % carbon. Although the latter has more chromium, the higher proportion of carbon reduces the metal's passivation capacity, which is why both grades exhibit similar corrosion resistance. This phenomenon is explained because chromium atoms bonded to carbon are not available to react with surface oxygen and generate the passive layer characteristic of stainless steels. 

Chromium carbide is an essential constituent in High Speed Steels (HSS), where its presence guarantees high hardness and durability against intensive mechanical stresses. It is also fundamental in chromium steels, which should not be confused with stainless steels, and in numerous cobalt-based and nickel-based alloys. In steels, the carbon proportion should not exceed 1 %, as an excess of this element would lead to the simultaneous formation of ferrite with iron and chromium carbides, increasing hardness but reducing the toughness of the alloy. In contrast, in cobalt-chromium alloys, the carbon limit is higher, potentially reaching up to 4 %, because cobalt does not form carbides and acts as a ductile matrix that retains mechanical strength. In these types of alloys, when abrasion resistance is sought, the carbon content is high, while in applications like jewelry, it is significantly reduced to avoid excessive brittleness.

In nickel alloys, such as nichrome, the situation is different. The presence of carbon is minimal or even non-existent, as in this case, the goal is not hardness but thermal resistance and ductility. Chromium is fundamentally added to increase oxidation resistance at high temperatures, an indispensable characteristic in electrical resistors, furnaces, and components subjected to continuous thermal cycles.

Chromium carbide is probably the most influential of all metallic carbides in materials science. Its relevance is not limited to the mechanical properties it confers but also to its role in the microstructure of steels, where it largely determines the relationship between hardness, toughness, and corrosion resistance. Due to its low cost, abundance, and ease of formation, it has become a pillar of modern metallurgy and an essential subject of study for understanding the fundamentals of steels and their multiple industrial applications.