MARTENSITIC STAINLESS STEEL

Martensitic stainless steels constitute a family that, while sharing certain similarities with ferritic steels—such as a body-centered cubic (BCC) crystal structure in their initial state and ferromagnetism—are primarily distinguished by their high carbon content. This high carbon content allows for the induction of a transformation to the martensitic phase through heat treatment. This structure, also known as the β-phase of iron, is obtained by heating the steel to reach the γ-phase (austenite), which allows for greater carbon solubility, and then rapidly cooling it, either in water or mineral oil. The result is a dislocation in the crystal lattice that adopts a distorted tetragonal or trigonal form, generating an extremely hard and rigid matrix. 

The typical chromium content in these steels ranges between 16% and 18%, while carbon is around 1–1.2%. This combination provides the material with outstanding mechanical strength, low malleability, and poor ductility, making it ideal for applications requiring high hardness, wear resistance, and some tolerance to elevated temperatures. Unlike ferritic steels, which are valued for their chemical resistance in moderate environments, martensitic steels are deliberately employed in contexts demanding superior mechanical performance without sacrificing corrosion protection. 

Interestingly, these steels were the last to be developed within the stainless steel universe. While the brand “Nirosta”, associated with austenitic steels like AISI 302, had already been patented in Germany, martensitic steel was named in honor of the German Adolf Martens, discoverer of the structure that bears his name. It was the American metallurgist Elwood Haynes who named this family and also patented the first cobalt-chromium alloys, commercially known as Stellite. 

The great advantage of martensitic stainless steels lies in their ability to combine the mechanical strength of high-carbon steels with chemical protection that the latter do not offer. To achieve this, an alloy with at least 0.8% carbon is used, which, after heat treatment, forms martensite and chromium carbides (Cr₂₃C₆) within the metallic matrix. These carbides reinforce the material's hardness, while chromium acts as a stabilizer of the martensitic phase, allowing the steel to maintain its structure even at higher temperatures than non-stainless steels like AISI 52100 or AISI 1090. 

However, this family presents certain limitations. At sub-zero temperatures, martensitic steels tend to become brittle due to internal crystallization, which can lead to fractures. Furthermore, their flexibility is reduced, limiting their use in applications requiring plastic deformation. Despite these drawbacks, their versatility, durability, and relatively low cost make them a very attractive option for cutting tools, demanding mechanical components, and parts subjected to abrasion. 

The most well-known representative of this family is AISI 440, especially its 440C variant, which offers an exceptional combination of hardness, wear resistance, and anticorrosive behavior. This steel is widely used in professional cutlery, bearings, surgical instruments, and industrial components requiring precision and reliability.