Martensitic transformation in steel

Martensitic transformation (and not only in steel) occurs during rapid cooling of the high-temperature phase. This process is often referred to as hardening., although for different metals and their alloys the term "hardening" can have completely different meanings, as, eg, "Steel hardening" and "hardening of aluminum alloy».

To the most important characteristics of martensitic transformation in steel, which distinguish it from other types of transformations, in particular, from pearlite transformation, include the following.

Martensitic transformation – diffusionless

Martensitic transformation is realized upon rapid cooling of steel from a temperature above A1, eg, in water. In this case, the diffusion decomposition of austenite into a mixture of two phases - ferrite and cementite - is completely suppressed.. The concentration of carbon in martensite is the same, as in austenite. The main difference between martensitic and pearlite transformations is that, that the first is completely diffusion-free.

Temperature range transformation - the same

The transformation of austenite to martensite begins at the initial temperature of the martensitic transformation Mn.The temperature of the onset of pearlite transformation decreases with an increase in the cooling rate, and the temperature of the onset of martensitic transformation is almost independent of the cooling rate. Martensite is formed in a certain temperature range - always in the same temperature range for a given alloy composition, eg, carbon content

No cooling - no transformation

When cooling is stopped at a temperature within the Mn - Mk interval, the martensitic transformation also stops.. This property radically distinguishes martensitic transformation from pearlitic transformation.. Pearlite transformation continues to the end at a constant temperature below point A1. The end result of pearlite transformation is the complete disappearance of austenite. During martensitic transformation, a certain amount of retained austenite always remains.

No incubation period

Unlike pearlite transformation, martensitic transformation has no incubation period. When cooled below the temperature Mn, a certain amount of martensite is formed instantly.

No growth – immediately "adult" martensite

When cooled below the temperature Mn the amount of martensite increases very quickly due to the rapid formation of new plates. Instantly formed plates no longer grow. This property distinguishes martensite from transfusion.. During the pearlite transformation, new colonies are constantly being born, and the old colonies continue to grow.

Martensit remembers austenite

Martensite plates are ordered in relation to the austenite lattice. At pearlite transformation of the phase lattice, which are included in the eutectoid mixture have a random orientation with respect to the original austenite lattice.

Crystal lattice of martensite

The crystal lattice of martensite is not cubic, and the tetragonal.

More carbon – lower temperature of the onset of martensitic transformation

For a given steel, the martensitic transformation always begins at the same temperature, regardless of the cooling rate. The value of this temperature depends on the composition of the alloy and sharply decreases with an increase in the carbon content in the steel. (cm. picture). Part of carbon is included in carbides, which coexist with austenite. If the quenching temperature rises, then part of the carbides dissolves in the austenite and, hence, the carbon content in austenite increases and point Mn goes down.

martensitnyy-intervalPicture – Effect of carbon content in steel
on the temperature of the beginning and end of the martensitic transformation

Martensitic transformation – phase shift

The formation of martensite is characterized by a shear mechanism of rearrangement of the austenite lattice.. Martensitic (shear) the mechanism of phase transformation is characterized by an ordered interconnected motion of atoms over distances, which are shorter than the interatomic distances, that is, atoms are not swapped.