I think this video lecture is the ultimate explanation of TTT diagram in my opinion! So detailed, elabroated, and thorough lecture. Thank you so much sir!
Hats off. Mind blowing professor. Never understood the reason of finer and coarser grains. Also never paid heed to how TTT diagram is created. Amazing.... Thank you.
16:15 In Fast Cooling temperature range is lower than Slow Cooling And for Fast Cooling due to low temperature range we will get Fine Grains and for Slow Cooling due to High temp range we will get Coarse Grains 20:00 In case of Quenching 22:20 we will get Glass or Amorphous Solid
Thanks for the video. Can anyone help? I would like to find out how to find out what quenching time is needed to achieve bainite transformation (bainite hardening) in chromoly alloy steel 42CrMo4 (SAE 4140). For example when I use oil as quenching medium with constant 350°C and I then I dip chromoly workpiece with austenite temperature into it and I need to know for how long it should stay in the oil and if there is a max time that can not be exceeded.
Sir, I have a question. Why when you supercool the liquid, during the crystallization, does the temperature decrease? I think the temperature should be supposed to keep constant.
Sir, there can be a cooling curve in between nose of green curve and red curve, which cuts the green line but doesn't cut the red line. What would be out come of such curve?
If two identical materials are cooled with same cooling rate but one is cooled from say 820 and the other from 790, transformation temp of former being higher ,will it have coarser grains?
Sir we've already seen that for an alloy, transformation occur at a range of temp.(except for eutectic composition)...but in TTT diagram phase transformation start and finished at the same temp...HOW IS THIS POSSIBLE??
Sir since at temperature below and above the nose the transformation rate is small so shouldn't it be that the gap between start and finish curve along isotherm should be larger as compared to nose because larger the transformation rate less time it will take to transform
Sir you said that for faster cooling,the temperature range for which transformation occurs is lower. Does this mean that the difference of upper and lower temperature values is lower than that for slow cooling? Because by observing the TTT diagram,it seems the other way around.
It means that the start temperature for faster cooling is lower than that for the slow cooling. similar the end temperature for faster cooling is lower than that for slower cooling.
Sir, will you please explain under what conditions it is called fast cooling and slow cooling? And if we do water quenched aluminum composite what will be the effects on grains, will it developed finer grains?
Lets see it for iron...Water quenching of austenite is fast. So it will give martensite directly(fast cooling). Oil quench will give very fine pearalite,air cooling will give fine pearalite. Nd furnace cooling will give coarse pearalite.
16:20 You said that "On faster cooling, the temperature range over which the transformation happens is lower than in slower cooling."; but instead the temperature range increases in faster cooling. Like in the cooling lines you have drawn, the slower cooling line has a small temperature range(the vertical height) when compared to faster cooling. I am confused here. Please help!!
Sir, thank you for your very nice explanation of TTT diagram, but i have a quesiton.. in this video, your diagram looks like CCT diagram, not TTT diagram because of the continuous cooling rate you draw... so.. isn't it a CCT diagram??
You have asked a serious question. And that points to subtle point regarding the difference between TTT and CCT. The diagram I have drawn is a TTT diagram. The CCT diagram does not have the lower part of C curve. So strictly speaking I should not draw a continuous cooling curve on these diagrams. I avoided discussion on CCT diagrams in these videos. So I did not go into these details and loosely drew continuous cooling curve on a TTT diagram, which in a strict sense is incorrect.
Sir here at 16:19 the two cooling curve u took was it for water or alloy?because if its liquid water then its pure substance and pure substance solidifies at fixed temperature,if its alloy then how we are able to superimposed an alloy curve to an pure substance's TTT diagram?
This example is of a pure liquid. When we say a pure liquid freezes at a fixed temperature, we mean the temperature at which solid and liquid are in thermodynamic equilibrium. Freezing will happen at this temperature only if the cooling rate is extremely slow to allow for equilibrium to be attained. At normal or faster cooling rates undercooling is possible, i.e., the liquid will remain as a metastable liquid even below the equilibrium freezing temperature. This liquid can thus solidify at a temperature below the equilibrium freezing temperature.
The distinction between crystal and glass is their atomic arrangement. In crystals the atoms are periodically arranged. Atomic arrangement in glass is disordered. The atomic arrangement of liquid is also disordered and in this sense, glass is compared to a liquid.
Think of casting. Sand casting gives relatively slower cooling rate than die casting. And then there are techniques called rapid quenching where one cools so fast that one gets amorphous or glassy phase.
