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TIME TEMPERATURETRANSFORMATION CURVES Isothermal curves deals with the study ofthe change in phase at constant temperatureFrom these reaction curves, the timerequired to start the transformation andthe time required to complete thetransformation can be obtained.The partial isothermal diagram for the iron-carbon steel is shown.
CONTD.The area to the left of the C-shaped curve represents anaustenitic structure and the area to the right of these twocurves represents a pearlitic structure.Between these two curves is a region containing pearliteand austenite with the relative ratios varying from allaustenite to pearlite as on moves from left to right.The most important information that we get from TTTdiagram is that very short time is required to formpearlite at temperature around 600 C (800K).
CONTD. This TTT diagram corresponds only to the reaction ofaustenite to pearlite.It does not corresponds to thetransformation of austenite which occur attemperature below about 823K (550C).For the complete study of TTT diagram, it isnecessary to understand two types of austeniticreactions which are:Austenitic to martensitic reaction.Austenitic to bainitic reaction.
MARTENSITIC REACTIONA reaction which takes placein some metals on cooling withthe formation of a needle likestructure is called a martensiticreaction.
CHARACTERISTICS OF MARTENSITIC REACTION Martensitic has a BCC-tetragonal crystal structure.It is assumed to be an intermediate structure between thenormal phases of iron-FCC and BCC.Lattice shear and surface distortion results in theformation of a typical needle like martensitic structure.Martensitic transformation is a diffusionlesstransformation.
BAINITIC REACTIONA product of austenite consistingof an aggregate of ferrite andcarbide.
CHARATERISTICS OF BAINITIC REACTION Bainite generally forms at temperature lowerthan those where very fine pearlite forms andhigher than those where martensite begins to formon cooling.Bainite forms at temperature around 300-500Cis termed as “Upper bainite”.Bainite forms at temperature around 200-300Cis termed as “Lower bainite”.
CONTD. In iron-carbon steel, bainite transfornation tendto overlap the austenite to pearlite transformtionso it is difficult to distinguish between pearlitictransformation and bainitic transformation.The puzzling feature of bainite reaction is itsdual nature i.e in a no. of respects it reavelsproperties similar to that of paerlitic reaction andat the same time it shows some properties similarto that of martensitic-transformation.
CONTD.Bainitic transformation involvescompositional changes and requries thediffusion of carbon. In this respect it differsfrom a martensitic transformation.Another property that differs it from amartensitic transformation is that it is not anathermal transformation.
CONTD. Bainitic structure does not have alternativeparallel layers of ferrite and cementite.In thisrespect it differs from a pearlitic structure.Due to unequal growth rates, bainite tendsto develop in the form of plates or needlelike structure which is a typical martensiticcharacteristic.
THE TTT OF A EUTECTOID STEEL The complete TTT-diagram for theeutectoid steel is shownin the figure. TemperatureIn this figure thecurves corresponding tothe start and finish oftransformation areextended into the rangewhere austenitetransforms to bainite. Time-sec
Cont’d Consider some arbitrary time- temperature paths along which it is assumed that austenitized specimen are carried to room temperature. These paths are shown in the figure given as:
PATH 1 The specimen is cooled rapidly to 433k and left there for 20 min, the steel remains in the austenitic phase until the martensitic temp is passed where martensite is begins to form athermally @ 433k (160C) half of the austenite transforms to martensite. PATH 2 . The specimen is held at 523K and left there for 100 sec, so the second quench from 523K to room temperature forms a martensitic structure
PATH 3 An isothermal hold at 573K for 500 sec produces half bainite and half austenite. cooling quickly would result in final structure of martensite and bainite. PATH 4 Austenite completely to fine pearlite after 8 sec at 873K.this phase is stable and will not be changed on holding for 100,000 sec at 873 sec the final structure, when cooled, is fine pearlite.
Conclusion Fine pearlite is harder and stronger than coarse pearlite. Bainite is harder and stronger than pearlite. Martensite is the hardest, strongest and the most brittle.