Question

Explain the concept of the TTT diagram, e.g why is there a “nose”?

How are cooling rates controlled to produce coarse and fine pearlite/bainite?

Applications of low, medium, and high-carbon steels, cast irons, stainless steels

Answer 1

1.Concept of TTT diagram

Time-temperature-transformation diagrams are plots of temparature vs time on logarithmic scale..It is also known as isothermal transformation diagram.They are useful for observing the transfoemation of alloy steel when it is cooled isothermally.It graphicallt describes the cooling rate required for the transformation of austenite to pearlite ,bainte and martensite. and it also gives the temparature at which these transformation takes place.

curves on TTT diagram have a characterictic 'C' shape that can be understood using phase transformation concept..AT THE NOSE TEMPERATURE fine parlite and upper bainite form simultaneously but the mechanism of their formation are different,NOSES are the result of two transformation and the transformation at which the transformation kinetics are FASTEST is called NOSE of TTT.

AT slightly lower than 727 degree celcius austenite is transformed to coarse pearlite,in this diffusion rate is very high.AS the temparature decreases to 500 degree celcius fine paerlite is formed and diffusion rate also decreases.

if we again lowers the temparature then bainite will be formed which is a altrnate layer of ferrite and cementite.

At further lower temparature martensite will be formed which have very low diffusion rate.it is formed below the nose of TTT diagram.

applications:-

low carbon steel - used for making cold headed fastners and bolts

medium carbon steel - railway wheels and tracks,gears and crank shafts

high carbon steel - used as tool and die steel

cast iron - cylinder heads,blocks,foundation of large machines,bridges

stainless steel - in making kitechen accessories,doors and window fittings,tubular products,electronic products