Time temperature transformation curves 2

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TIME TEMPERATURE TRANSFORMATION CURVES

Isothermal curves deals with the study of the change in phase at constant temperatureFrom these reaction curves, the time required to start the transformation and the time required to complete the transformation 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 an austenitic structure and the area to the right of these two curves represents a pearlitic structure.Between these two curves is a region containing pearlite and austenite with the relative ratios varying from all austenite to pearlite as on moves from left to right.The most important information that we get from TTT diagram is that very short time is required to form pearlite at temperature around 600 C (800K).

CONTD.

This TTT diagram corresponds only to the reaction of austenite to pearlite.It does not corresponds to the transformation of austenite which occur at temperature below about 823K (550C).For the complete study of TTT diagram, it is necessary to understand two types of austenitic reactions which are:Austenitic to martensitic reaction.Austenitic to bainitic reaction.

MARTENSITIC REACTION

A reaction which takes place in some metals on cooling with the formation of a needle like structure is called a martensitic reaction.

CHARACTERISTICS OF MARTENSITIC REACTION

Martensitic has a BCC-tetragonal crystal structure.It is assumed to be an intermediate structure between the normal phases of iron-FCC and BCC.Lattice shear and surface distortion results in the formation of a typical needle like martensitic structure.Martensitic transformation is a diffusionless transformation.

MICROSTRUCTURE OF MARTENSITE

BAINITIC REACTIONA product of austenite consisting of an aggregate of ferrite and carbide.

CHARATERISTICS OF BAINITIC REACTION

Bainite generally forms at temperature lower than those where very fine pearlite forms and higher than those where martensite begins to form on cooling.Bainite forms at temperature around 300-500C is termed as “Upper bainite”.Bainite forms at temperature around 200-300C is termed as “Lower bainite”.

CONTD.In iron-carbon steel, bainite transfornation tend to overlap the austenite to pearlite transformtion so it is difficult to distinguish between pearlitic transformation and bainitic transformation.The puzzling feature of bainite reaction is its dual nature i.e in a no. of respects it reavels properties similar to that of paerlitic reaction and at the same time it shows some properties similar to that of martensitic-transformation.

CONTD.Bainitic transformation involves compositional changes and requries the diffusion of carbon. In this respect it differs from a martensitic transformation.Another property that differs it from a martensitic transformation is that it is not an athermal transformation.

CONTD.Bainitic structure does not have alternative parallel layers of ferrite and cementite.In this respect it differs from a pearlitic structure.Due to unequal growth rates, bainite tends to develop in the form of plates or needle like structure which is a typical martensitic characteristic.

MICROSTRUCTURE OF BAINITE

The complete TTT-diagram for the eutectoid steel is shown in the figure.In this figure the curves corresponding to the start and finish of transformation are extended into the range where austenite transforms to bainite.

THE TTT OF A EUTECTOID STEEL

Time-sec

Tem

pera

ture

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.

References

Physical Metallurgy principleBy REED HILL

http://www.sv.vt.edu