Ttt diagram of eutectoid steel and martensitic transformation
Muhammad Umair Bukhari Engr.email@example.com www.bzuiam.webs.com 03136050151
T-T-T diagram is plotted for different length of times and temperature These curves gives us the information of transformation for different phases With the help of T-T-T diagram we can control the phase transformation by changing the conditions
The left side of C- shape curve is Austenitic region The right side of C- shape curve contain three regions • Pearlite • Pearlite+Bainite • Bainite
The left line of C-shape curve shows the start of phase transformation The right line shows the end of transformation The dotted line is theoretically determined line shows 50% transformation
The region below Ms is martensitic regionMs shows start and Mf shows end of Martensite transformation
Martensitic crystal structure is Body centered tetragonal (BCT) It is the intermediate structure of Fcc and Bcc It forms due to the realignment of atoms from austenitic structure to martensitic structure
Atoms arrange themselves to move towards stability from higher temperature to lower temperature Only realignment is occur but there is no diffusion There is no concentration change in parent and product phase
Due to the realignment of atoms the lattice smmetry is different from the parent phase
At 300oc the bainite and martensite are formed along curve 3 As temperature decreases the bainite formation rate is lowers At 160oC for 20min the 50% martensite and austenite are formed
Mf line which shows the end of martensite transformation But 100% martensite can’t be formed even theoretically at absolute zero At room temperature there is 3% retaine austenite
The reason is the retained autenite As the little amount of austanite is remain it is difficult to transform That’s why Mf lline is showed dotted
•Regular arrangements of atoms in acrystal is called order.•Order of atoms in crystals are mostlyat room temperature
Irregular arrangements of atoms in a crystal is called disorder . Disorder mostly occurs at elevated temperature.
In most substitutional solid solutions two kinds of atoms A and B are arranged more or less random on the atomic sites of the lattice. In solution of this type of a change in temperature is to Increase or decrease of a thermal vibrations. If the ordered solution is heated above Tc the atomic arrangement becomes more or less random again and the solution is said to be disordered.
The gold and copper atoms of AuCu3 above a critical temperature of about 390’C are arranged more or less random on the atomistic sites of FCC lattice as shown in following figure. Below the critical temperature the gold atoms in a perfectly ordered alloy occupy only the corner position of the unit cube and the copper atoms on the face centered cubic as shown in following figure.
When the periodic arrangement of A and B atoms persists over longer range distance in a crystal than it is called a long range order.
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