Martensite: how and why

The most remarkable property of steel is its ability to be hardened to a high level of strength by simple quenching.. Hardening of steel usually occurs when the heated metal is immersed in a coolant., like water, oil or liquid salt. To increase strength,, so that this heated steel contains austenite, and better - was completely austenitic. Then very rapid cooling will not give austenite the opportunity to turn into a thermodynamically "favorable" structure ferrite + cementite. Instead, a new structure is formed, which is called martensite. This martensitic phase gives the steel a very high level of strength..

Carbon: many in austenites - few in ferries

As you know, austenite has a face-centered cubic crystalline (Fcc) structure, ferrite - body-centered crystalline (OCK) structure. The phase diagram of steel shows, that the fcc structure - austenite - will dissolve much more carbon, than OCK structure – ferrite. At temperature A1 amount of carbon, which can dissolve in austenite 38,5 times (0,77/0,02 = 38,5) more, than in ferrite.

The thing is, that carbon atoms are much smaller, than iron atoms. Dissolved carbon atoms are located in the gaps between relatively large iron atoms. BCC structure is able to "absorb" more carbon atoms, since some gaps between atoms in this structure are much larger, than any gaps in the fcc structure.

Slow cooling of austenite - ferrite plus cementite

Picture 1 shows the transformation scheme of austenite steel with carbon content 0,60 % into ferrite. The vertical line represents the transformation front, which moves from left to right. After, How will this front move?, eg, Specifications of Android software 25 mm, in this area of ​​length 25 mm carbon content should drop from 0,6 % to 0,02 %. With slow cooling, carbon can have time to move ahead of the front of transformation in austenite along the direction, which is shown by the dashed arrow, due to the diffusion mechanism.

front-ferritePicture 1 – Scheme of advancement of the front of transformation of austenite into ferrite

Rapid cooling of austenite - martensite

but, if this transformation is forced to take place very rapidly by quenching, there will be no time for the diffusion movement of carbon atoms. Therefore, some of them - or all of them - will remain in the ferrite. This excessive carbon content in ferrite leads to a sharp distortion of its bcc structure - as a result, a martensitic structure appears..

atomic lattice: from ferrite to martensite

On the picture 2 the atomic cell of bcc ferrite and the distorted atomic cell of martensite are shown side by side. The atomic cell of martensite is similar to the bcc cell of ferrite in that, that it also has an atom in the center and an atom in each of the eight corners. However, this atomic cell is no longer a cube.. One of her sides, which is called the lattice period with or face with (cm. picture 2), longer, than the other two, which are called periods a or faces a. This crystal structure is called body-centered tetragonal. (OCT).

kristallicheskaya-reshetka-martensitaPicture 2 – Comparison of the crystal structures of ferrite and martensite

More carbon - more hardness

Picture 3 shows, how with an increase in dissolved carbon in martensite, its face c becomes larger and larger compared to face a. An increased carbon content in martensite is achieved by quenching austenite with a higher carbon content.. On the graph in the picture 3 it is seen, that with an increase in the carbon content, the distortion of the atomic lattice from the cubic one - the face c becomes more and more in comparison with the face a. This is due to the carbon atoms embedded in the bct martensite lattice..

size-martensitnoy-yacheykiPicture 3 – Dimensions of facets a and c of a body-centered martensite cell
(1 nm = 1000 μm)

The strength and hardness of martensite with an increase in its carbon content increases very strongly., as it can be seen from the figure 4.

uglerod-martensit-tverdostPicture 4 - Rockwell hardness of freshly hardened martensite
depending on carbon content

Understand, why does the strength of martensite increase with increasing carbon content, the following interpretation helps. It's convenient to imagine, that chemical bonds, that hold iron atoms together, are springs. As the carbon content increases, these springs stretch, to fit extra carbon atoms in the lattice. And in order to stretch these stretched springs further - to deform the martensite - more and more efforts are required..

Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007