TTT Diagram of steel

TTT Diagram of Steel

The TTT diagram of steel is considered an important transformation diagram for non-equilibrium transformation. There are various non-equilibrium products like Martensite, Bainite which can not be formed by continuous cooling and so can not be explained with phase transformation diagram explained in Martensitic transformation post and Widmanstatten transformation post. In those diagrams, continuous cooling takes place while this diagram explains constant temperature transformations. TTT diagram of steel has a very important application like austempering, martempering Patenting, and isothermal annealing which is commonly employed in industry for achieving specific properties in steel.

What Is the TTT diagram in Material Science?

TTT diagram of steel is called isothermal transformation diagram or time-temperature-transformation diagram. It presents a logarithmic plot between temperature and time. The common application of the TTT diagram of steel is to understand elevated transformation is alloy steel which follows non-equilibrium cooling and requires important microstructural features. 

For history, it should be noted that Pearlitic and Bainitic bay in the TTT diagram of steel are identified and explained by Davenport and Bay. Martensitic start and end transformation lines were later added by Cohen. Follow isothermal transformation in steel on Wikipedia for complete history.

Important points to note for TTT diagram of steel are;

  1. TTT diagram of steel is valid only for one single composition. With varying compositions, plots, and curve also vary.
  2. This diagram is only understood if steel immediately cools down from austenitizing temperature to transformation temperature and held constant during the completion of transformation.
  3. TTT diagram of steel is used to explain various concepts related to kinetic equilibrium and non-equilibrium changes in steel.

TTT diagram for Eutectoid Steel

TTT diagram for Eutectoid Steel
TTT diagram for Eutectoid Steel

Presented above is the TTT-diagram for eutectoid steel. For each composition, there will be a slight variation in diagram curves. TTT diagram of steel indicates the time-temperature and transformation curve. This means transformation is dependent upon time, temperature, and cooling mechanism. 

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Difference between phase diagram and TTT diagram of steel

For a clear understanding of this diagram, it is better to understand the phase transformation of the Fe-Fe3C curve. The phase transformation diagram and TTT diagram has a major difference that you must understand. Phase transformation just describe continuous and equilibrium cooling product. All remaining products or transformation which are the result of isothermal cooling, fast cooling like bainite and martensite are only understood by the TTT diagram. Follow the quenching procedure and martensitic formation for understanding the Phase transformation diagram.

Why TTT diagram is c shaped?

We have mentioned before, TTT diagram is time and temperature transformation diagram. Pearlitic transformation is a diffusion-based transformation which involves solid-state conversion into alpha iron and iron carbide colonies. Without the required temperature and time, diffusion is not possible, and structure will, then, have martensite or bainite transformations. The edge point on the nose is called a critical point. If the cooling or temperature drop is high enough that the line does not touch the nose, then it generates diffusionless transformations. This critical point in the nose also indicates the point above which diffusion is possible. Picture of nose for clear understanding is shown below;

Why TTT diagram is c shaped?
c shape in TTTT diagram

In this picture, four important concepts are being explained. 

  • Critical Cooling Point: This point indicates if the cooling line does not touch this point or stay on the left side of this line, transformation does not start in steel. In this case, transformation is diffusionless. This diffusion less or partial diffusionless transformation results in martensite or bainite respectively. 
  • Start and End of Transformation: This nose consists of two lines; one line indicates the start of transformation and the second line indicates the end of the transformation. What we are doing here is cooling the material so fast to such a temperature that the nucleus of solid becomes stable. At high temperature, diffusion is very high and nucleus forms and breaks again and again before proper solidification takes place. At high temperatures, the required size for the nucleus needed to be higher for stabilization. The nuclei stabilization follows Gibbs free energy concept, which is not a topic of discussion here. I am going to show you the Gibbs free energy formula, which shows critical size for nucleus as follows;
Critical Radius of Nucleus Formations
Critical Radius of Nucleus Formations

In this figure, we have shown that with a decrease in transformation temperature up to the nose of the TTT diagram, the size of critical radius decreases i.e. high transformation temperature, lower delta T and higher critical radius. 

High critical radius, more time is required for stabilization. This is the very reason, nose shifts towards the right as we move towards high temperature. At high temperature, critical size is very large which requires more time for stabilization. But, as soon as nuclei forms, the end of transformation takes very little time due to a higher diffusion rate about temperature. At lower temperatures closer to the nose, the temperature is low and the diffusion rate is, also, low. This low diffusion rate causes slow growth and requires more time for complete transformation. 

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If we summarize this concept, we can say, large nuclei are formed at high temperatures, and transformation takes less time for completion but, closer to the nose, small nuclei are formed and transformation takes more time for completion. This is clear in the TTT diagram, End transformation line is closer to start the transformation line at high temperature. Coarse and fine grain structure: Since, closer to the nose, small nuclei are formed that’s why several nuclei will be more in the same space giving finer structure compared to high-temperature nuclei which are larger.

  • Diffusion and Diffusion less transformation: As we have explained, with a decrease in temperature, the diffusion process slows down so nuclei size tends to decrease. Below, nose, diffusion becomes so slow that all type of diffusion-based transformations stops and structures which can be formed without the support of the diffusion process come into existence. 

How to read the TTT diagram of steel?

As we have already explained above, the TTT diagram is time and temperature-dependent diagram and it is a result of non-equilibrium cooling. For a clear understanding, we showed a few red lines denoted by S1 to S5 in the figure. We also explained above that austenitization temperature is essential for all transformation. Below austenitizing temperature, all structure is either bainite, martensite, pearlite and ferrite. Heating below austenitizing temperature does not bring considerable changes within the microstructure. That’s why steel with certain composition is heated to an austenitizing temperature and, after homogenizing, it is cooled. Now, for TTT diagram understanding, we are going to cool from austenitizing temperature by following red lines shown in a curve.

Cooling – Pearlitic Region

How to read TTT diagram of steel
Pearlitic Region – Cooling Curve

You can see in the above picture which shows the Pearlitic transformation region of the TTT diagram. This region indicates a region above the nose of the TTT diagram. Above the nose, diffusion-based transformation results in Pearlitic transformation. In the case of S1, we swiftly cool temperature and reaches the nose, that’s why several nuclei will be large in number and small in size, while in S2 line, transformation temperature achieves at high temperature, that’s why nuclei will be small in numbers and large in size. S1 generates finer Pearlitic structure while S2 generates a coarse Pearlitic structure.

Cooling – Bainitic Region

Below 600o C, diffusion processes become very sluggish and it becomes difficult for nuclei to transform into pearlite without any driving force. So, in-between 150o C and 550o C, bainitic transformation takes place which is partially diffusion dependent and partially diffusionless. The bainitic structure consists of cementite in needle-like ferrite with a large amount of dislocations. 

The bainitic region in TTT diagram is shown below;

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Bainitic tranformation in steel
Bainitic tranformation in steel

It can be seen in figure lathe-like bainite will be formed here. Bainite will be formed a certain temperature region in between pearlite and martensite. The cooling rate here is not sufficient for material to cross martensite start transformation line and diffusion is not high enough to start Pearlitic transformation. Bainite is considered partial diffusionless transformation. This is best understood by the concept of lower and upper bainite. In upper bainite, the temperature is enough for carbon to come out of ferrite to form cementite white in lower bainite, carbon stays in retained austenite which will later convert into martensite after it gets cooled down. 

That’s why two types of bainite structure exits;

  1. Upper Bainite (high dislocation needle-like ferrite and cementite)
  2. Lower Bainite (high dislocation needle-like ferrite and retained austenite which is convertible into martensite)

Cooling – Martensitic Region

Martensite is considered a deformed BCC structure. The structure of martensite is named as BCT or Body-centered tetragonal structure. Austenite is an FCC structure which has capacity for 2% carbon in interstitial positions. As the temperature is lowered, carbon diffuses out and FCC converts into BCC alpha iron which can absorb only 0.125% carbon. If the cooling rate is very high, carbon does not get chances to diffuse out of FCC structure and when FCC converts into BCC structure, BCC gets deformed generating strain field. This strain field stops a moment of dislocation giving high strength.

Within the strain field of BCC structure, no other martensite forms as strain field is a region that acts as stress relaxant for BCT structure. In this region, austenite remains as it is and named as retained austenite. Retained austenite is a soft phase and can reduce the strength of a material. That’s why the reduction of retained austenite is preferable. Complete martensitic formation causes too much stress results in quench cracks in the steel. That’s why a certain percentage of retained austenite is essential. For understanding the relation between cooling rate and retained austenite formation, follow Quenching methods

Martensitic region of TTT diagram appears as below;

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What is TTT diagram in Material Science? - Martensite region
Martensite in TTTT Diagram

It can be seen in the figure, when we follow the S4 line in TTT diagram which can be shown in the above picture and top original TTT diagram, it is clear that cooling rate was so high that line does not touch the nose and, also, it crosses Ms line as well. If steel crosses Ms line than it indicates the start of martensitic transformation. This martensitic transformation is similar to pearlitic or bainitic transformation in one concept that it can be reversed until we reheat steel in austenitizing temperature. This region has two important lines i.e. Ms line for the start of martensitic transformation and Mf line for the end of martensitic transformation.

With a faster cooling rate, it is possible to go closer to the Mf line. As we got closer to the Mf line, the amount of retained austenite decreases, and the percentage of martensite in steel increases. Every cooling media have a certain cooling rate. Brine has a faster cooling rate than water that’s why the percentage of martensite will be higher in brine concerning that of water. A complete transformation is not possible as the structure will have too many stresses to accommodate generating quench cracks. The fastest cooling rate is possible with cryogenic treatment like cooling in liquid nitrogen which has a temperature -183o C and can generate a very severe cooling rate. 

Effect of Alloying elements in Steel?

Alloying elements added in Steel will shift the c-shaped curve towards the right delaying diffusion-based transformation of pearlite. Alloying elements added can be ferrite stabilizers or austenite stabilizers. Each has different characteristics which can be fully understood in the article, “Effect of alloying elements in steel”.

Applications of TTT diagram of steel

With an understanding of the TTT diagram, it is possible to optimize microstructure according to the user’s requirements. For example, fine pearlitic microstructure requires faster cooling which can be perfectly explained by the TTT curve. It is also discussed in the above sections that the degree of undercooling of fast cooling to a certain temperature prevents diffusion of atmos may generate various nonequilibrium products. The non-equilibrium cooling can also help suppress various products which are undesirable for certain applications. Based on the discussion, we are going to share certain applications of TTT diagram named as follow;

  1. Martempring
  2. Austempering
  3. Isothermal annealing
  4. Patenting

Martempring

As we have previously explained, a very fast cooling rate is required for achieving Ms temperature for martensitic transformation. This fast cooling rate in thick sections will create distortions or cracks. Thick sections will have a difference in cooling rate in center and surface. This difference in cooling rate generated thermal stresses. These stresses within a material are of a tensile type which can cause cracking within the material. To prevent quench cracking, the martempering process is used. 

A most common application of martempering is found on steel which is air hardenable or oil hardenable. The reason is that they have such an alloy configuration that their critical cooling rate is less and easily attainable due to composition. 

Austempering 

Austempering is a process normally employed for bainitic formation. The cycle described in S3 is normally employed in Austempering. In Austempring, steel form the Austenitizing temperature is immediately placed in a salt bath which is maintained at a temperature of 200-400o C. At the temperature, we have already discussed that, steel converts into bainite. 

Isothermal Annealing

Isothermal annealing is the process used for uniform pearlitic and austenitic microstructures. The reason for this is that the center and surface both have different cooling rates. Normally, this causes variation in grain size of the material as in center cooling rate is slow and pearlite will have large grain size while, on the surface of the material, the cooling rate is fast giving finer pearlite. Homogenous pearlitic transformation helps obtain uniform properties throughout the structure in comparison to conventional annealing. 

Patenting

Patenting is an application of isothermal annealing employed to coiled ropes to achieve better properties. Normally this type of treatment is employed to steel with carbon above 0.45%. This contains a fairly high amount of carbide in it. To prevent carbide formation, the material is held just at the nose, and then from thair air cooling is carried out. This air cooling results in partial bainitic and partial pearlitic microstructure. Details of the process will be discussed in upcoming articles.

References

  • Martensite in Steel: strength and structure by George Krauss (1999)
  • PowerPoint presentation by R. Manna Assistant Professor at Institute of Technology, Banaras Hindu University, titled “Temperature Transformation (TTT) Diagrams”

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