TTT refers to Time Temperature Transformation
=> Depending upon the temperature of transformation, Austinite may be transferred to Pearlite, Martensite (or) Bainite.
=>This TTT diagram indicates the phase existing in STEELS at various temperature & time.
This diagrams are also useful in Heat Treatment of steels of Steels.
=>With the help of these diagrams, one can choose a proper cooling cycle to obtain the desired transformation & required properties in the component.
Determination of TTT Diagram:
=> For a given steel TTT diagrams can be
experimentally determined as below
Step 1: Heat large no. of steel pieces of suitable size in Austinite region. ThroughOut the experiment,Austinite temperature must be kept constant.
During Heating Oxidation & De-carbonation should be
avoided by suitable measures such as SALT
Bath. Step 1: Heat large no. of steel pieces of suitable size in Austinite region. ThroughOut the experiment,Austinite temperature must be kept constant.
Step 2: Soak these samples to for sufficient time so as to obtain Homogeneous Austinite. The time of soaking should be kept constant throughout the experiment.
Step 3: Transfer all these samples into SALT Bath kept in another furnace at some constant temperature.
Step 4: Remove the samples One-by-One at fixed intervals of time. Now Quench them in Brine or Cold Water. Due to this Un-transformed Austinite is transformed to Martensite.
Step 5: Study these samples metalographycally & find out the time of start of particular transformation & end of particular transformation.
Step 6: Similar studies are conducted at different temperature & time of start amd time of end of transformation.
Step 7: To obtain time at different isothermal transformation, Temperature are plotted on Temp vs Time graph & smooth curves are drawn on these curves.
.'.The resultant diagrams are called TTT diagrams. These diagrams are also called as IT diagrams (Iso Thermal diagrams) because it is plotted from the steady of transformation at constant temperature.
=> The curves are usually in the shape of S or C. Hence it is called as S-curve or C-curve.
=> Austinite dosen't transform to Pearlite but, transform to Martensite.
=> A different transformation will occur if Austinite is cooled isothermally at some temperature below lower critical temperature. That is called Binate Transformation.
For Example, study is done for Eutectoid steels at 600°C temperature. First few samples may show Martensite, indicating that pearlite transformation has not started.
Next few samples will show Pearlite & Martensite transformation is in progress.
The rest of the samples will show only Pearlite indicating that Pearlitic transformation is completed.
In above example, we have to note down the Time of start & end of Pearlitic transformation.
=> In Hypo-Eutectoid steel., the pro-eutectoid Ferrite will first separate out from Austinite. Hence we have to note down the time of start of pro-eutectoid Ferrite along with time of start-of-Pearlite & time of end-of-Pearlitic transformation.
=> In Hyper Eutectoid steels., the pro-eutectoid Cementite will first separate out from Austinite. Hence we have to note down the time of start of pro-eutectoid Cementite, along with time of start-of-Pearlite & time of end-of-Pearlitic transformation.
=> For showing Martensite transformation, two straight lines are drawn,one at point Ms another at point Mf. Which indicate the time-of-start of Martensite & time-of-finish of Martensite.
In the above TTT diagram, curves have the following meaning
Fs= Start of Pro-eutectoid Ferrite formation.
Cs= Start of Pro-eutectoid Cementite formation.
Ps= start of Pearlite formation.
Pf= Finish of Pearlite formation.
Bs= start of Bainite formation.
Bf= Finish of Pearlite formation.
Ms= start of Martensite formation.
Mf= Finish of Martensite Formation.
=> The transformation product b/w Ac1 & Nose temperature is Pearlite & pearlite increases as the transformation temp. decreases, becoming unsoluble fine near nose temperature.
=> The transformation product b/w Nose temperature & Ms is Binate. Below nose temperature, Binate is feathery in nature(soft) & above Ms it is acicular of sharp.
=> Martensite transformation occurs between Ms & Mf as the linear function of temperature.
=> The hardness value of the product for eutectoid steels are marked on TTD Diagram.
=> Pearlite is relatively soft, Binate is medium hard & Martensite is totally hard.
=> This clearly shows that, as transformation temperature decreases, the hardness of steel increases.
This helps in the suitable adjustment of heat cycle so as to obtain the desired properties in the component.