In this article we will discuss about the crystallographic theory of martensitic transformation in steels.
In martensitic transformation in steels, the crystal lattice changes from FCC (austenite), to BCT (martensite). The parallel reference scratches (Fig. 3.45) on prepolished surface remain parallel, straight and continuous across the austenite-martensite interface after the transformation means homogeneous deformation must have taken place.
The continuity of these scratches indicates that the habit plane remains unrotated and undistorted. Additional deformation, the lattice invariant deformation involving deformation of crystal by twinning or slip keeps the habit plane unrotated, undistorted at least on a macroscopic scale.
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Wechsler, Lieberman and Read, and Bowles and Mackenzie have profounded theories to explain martensitic transformation.
The main points essentially are:
1. Homogeneous Deformation:
The Bain strain in steels ear; be taken to be the homogeneous deformation (Fig. 3.53). It converts austenite lattice to martensite lattice with a minimum of atomic displacements.
If lattice deformation (homogeneous) only is done (Fig. 3.63), the parent lattice may have transformed to martensite lattice but causes rotation of it, away from habit plane (the vertical dotted lines indicate the unrotated habit plane). If only lattice invariant deformation, for example by slip is done (Fig. 3.63 c), the habit plane is macroscopically unrotated but martensite lattice has not been produced. Thus, homogeneous lattice deformation combined with lattice invariant deformation by slip in (d), and by twining in (e) result in new martensite lattice with unrotated habit plane.
Fig. 3.63 (d) and 3.63 (e) illustrate the slip and twinning modes of lattice invariant deformation within martensite plates, which also keep the habit plane unrotated. When slip is the mode, not only ire dislocations introduced at austenite-martensite interface but a high density of dislocations remains in fine structure within plates. Twinning causes presence of high density of twins and relatively lower density of dislocations.
2. Hardness of Martensite:
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The most dramatic effect, produced by martensitic transformation obtained by fast quenching, is the rapid increase of hardness in steels. A martensitic microstructure is the hardest microstructure that can be produced in any carbon steel. It had been fully exploited in ancient times in hardening the steel swords and daggers, though the hissing sound, produced due to plunging of hot steel into water, was said to be the cause of hardening due to supernatural powers.
Even up to the beginning of 20th century, this hardening had been attributed to the conversion of carbon into diamond. The increase of hardness by similar martensitic shear transformations in many non-ferrous systems, and even in carbon-free steels is not that dramatic, and the martensite is not that brittle too.
To obtain maximum hardness, the steel should be fully martensitic and should have high carbon. Though a small amount of retained austenite starts appearing in steels with as low as 0.3% carbon, the most significant effect of the retained austenite on hardness occurs in steels containing more than 0.7% carbon and may even lead to decrease in hardness due to increased amount of retained austenite in high carbon steels (Fig. 3.64).
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Mild steels (containing carbon less than 0.25%) are not hardened as a rule in industry and thus, are called non-hardenable steels as the gain in hardness is not substantial, as well as, these steels are difficult to harden (need drastic quench with potentials of cracks etc.).
Carbon is the most important factor controlling the hardness and the strength of martensite (Fig. 3.64). At first, there is a rapid linear increase of hardness for carbon up to = 0.55%, but then rate of increase becomes less due to increased amount of soft retained austenite. Ductility and impact resistance decrease with carbon content, making martensite extremely brittle.
The question, why martensite is hard, has always baffled the metallurgists, and the origin of this cannot be explained by one single reason, however, the action of several mechanisms of dislocation trapping could probably be taken to be the cause of hardening effect in steels. And in almost all theories to explain martensitic hardening, carbon is said to be playing a decisive role.
The following strengthening mechanisms together could be the possible causes of high hardness of martensite:
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1. Interstitial solid solution strengthening by carbon
2. Substitutional solid solution strengthening
3. Imperfections in structure
4. Fine twins
5. Segregation of carbon atoms
6. Grain size of austenite
7. Precipitation of iron carbides.
Probably, the most important and central mechanism of strengthening the martensite is due to the role of carbon atoms trapped in octahedral interstitial sites with the displacement of iron atoms along C-axis in BCT lattice of martensite, i.e., carbon causes body centred tetragonal distortion of lattice, and also the volume expansion (Fig. 3.65). These two factors cause shear and hydrostatic stresses, which can lock screw as well as edge dislocations. This strong locking is considered to be a major cause of high hardness of martensite.
Substitutional solid solution strengthening by alloying elements of carbon martensite is relatively very small and in fact, in high carbon ranges the hardness may even decrease due to increased amount of retained austenite. Fig. 3.64 also illustrates the effect of increase of carbon on the hardness of austenite retained at room temperature by adding nickel in it, and the effect is very small. Carbon sitting in symmetrical octahedral sites in FCC-austenite does not cause tetragonality there, but just causes uniform expansion of the lattice.
Low carbon martensites have a dislocation density of 1015 m-2, which is of the order as in a heavily cold worked metal, and have some contribution to the strength of martensite of around 300 MNm-2, as the dislocation tangles serve as barrier to glissile dislocations. As there is no sudden increase of hardness, as transition from dislocated martensite to twinned martensite takes place (may be due to increase of carbon content of steels), the fine twins, Which too act as barriers, make similar contribution as the dislocations do.
Austenitic grain size controls the maximum size of martensitic plate in high carbon martensites. Even the size of the lath packets in low and’ medium carbon steels are related to austenitic grain size, i.e. decreases with grain size.
The yield strength of steels increases with decreasing martensitic lath size (Hall-Petch relationship exists between them). Lath boundaries, many of them, are really low angle sub-boundaries, which do act as barriers to dislocation motion and must be contributing to the strength of martensite.
Carbon atoms during quenching segregate to dislocations present. Carbon atoms also segregate to lath and packet boundaries, which make the dependency of strength on the packet size. Some contribution due to the carbon atom segregation to the strength of martensite is expected.
The carbon segregation at dislocations can lead to precipitation of iron carbide, which has been seen to increase the strength of aged martensite. Also, the steels which have higher Ms temperature, the martensite formed gets tempered during the quenching process (in temperature range of first stage of tempering ≈ 200°C), and may form iron carbide dispersion (process called auto-tempering). This may also contribute to some strength of martensite.