Do you know Martensite Transformation ?

Martensite is a body-centered tetragonal form of iron in which some carbon is dissolved. Martensite forms during quenching, when the face centered cubic lattice of austenite is distored into the body centered tetragonal structure without the loss of its contained carbon atoms into cementite and ferrite. Instead, the carbon is retained in the iron crystal structure, which is stretched slightly so that it is no longer cubic. Martensite is more or less ferrite supersaturated with carbon. Compare the grain size in the micrograph with tempered martensite.

The term “martensite” usually refers to a form of steel with a distinctive atomic structure created through a process called martensitic transformation. Martensite is made from austenite, a solid solution of iron with a small amount of carbon in it.

Martensite, named after the German metallurgist Adolf Martens (1850–1914), most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or plate-shaped crystal grains. When viewed in cross-section, the lenticular (lens-shaped) crystal grains appear acicular (needle-shaped), which is how they are sometimes incorrectly described.

In the 1890s, Martens studied samples of different steels under a microscope, and found that the hardest steels had a regular crystalline structure. Martensitic structures have since been found in many other practical materials, including shape memory alloys and transformation-toughened ceramics.

The difference between austenite and martensite is, in some ways, quite small: while the unit cell of austenite is a perfect cube, in the transformation to martensite this cube is distorted so that it’s slightly longer than before in one dimension and shorter in the other two. Unlike cementite, which has bonding reminiscent of ceramic materials, the hardness of martensite is difficult to explain in chemical terms. The explanation hinges on the crystal’s subtle change in dimension, and the speed of the martensitic transformation.

Martensitic transformation occurs when the austenite is rapidly cooled in a process known as quenching. The rapid drop in temperature traps the carbon atoms inside the crystal structures of the iron atoms. This causes the crystals to change from FCC to body-centered tetragonal (BCT); the crystals are stretched so that they are square on each end but longer on the sides and the lattice points that were in the center of each face are now joined together at one point in the center of the crystal. This new structure is what greatly increases the hardness of the steel.

Austenite is transformed to martensite on quenching at approximately the speed of sound - too fast for the carbon atoms to come out of solution in the crystal lattice. The resulting distortion of the unit cell results in countless lattice dislocations in each crystal, which consists of millions of unit cells. These dislocations make the crystal structure extremely resistant to shear stress - which means, simply that it can’t be easily dented and scratched. Picture the difference between shearing a deck of cards (no dislocations, perfect layers of atoms) and shearing a brick wall (even without the mortar).

The martensitic transformation is the best-known example of displacive transformation, a type of phase change in which the atoms of a material move short distances in unison rather than diffusing individually over longer distances. A phase change occurs when a substance changes from one state, like a solid, to another, like a liquid. Because they are so well known as a type of displacive transformation, the terms “martensite” or “martensitic” are sometimes used in a broader sense to describe any material produced by displacive transformation.