Question

o What is easier: Homogeneous or heterogeneous nucleation? Why? o What does an Isothermal Transformation (IT) Diagram tell us? o How do nucleation rates and diffusion rates influence the shape of the IT Diagram? o What do the letters "A", "F", "P" "B", and "M" mean on these diagrams? o What are the similarities and differences between the following microstructures: Coarse Pearlite; Fine Pearlite; Bainite; Spheroidite; and Tempered Martensite? o How is Martensite (not the tempered kind) formed, and how is it completely different from the other microstructures listed above?

Answer 1

1) Homgeneous nucleation: It occur spontaneously and randomly but it require superheating or supercooling medium.

Heterogeneous nucleation : It begins on alien surface or particles or pre existing nuclei in the odd phase .Dust or atmospheric aerisis can serve as heterogeneous nucleation centers for water condenstation in the atmosphere.

hmogeneous nucleation compared to heterogeneous occurs with morre difficulty in the interior of uniform substance.

2) Isothermal transformation diagram : It is also know as time-temperature-tranformatiom diagramare plots of temprature versus time. They generated from percentage transformation Vs time.it is usefull to understanding the transformation of an alloy steel at elevated temprature.Tough useually used to represent transformation kinetics of steels and kinetics of crystallization of steels.

3) A-Austenite

F-Fearlite

P-Pearlite

B-Bainite

M-Martensite

4) Coarse pearlie : At heigher temperature diffusion allows for larger grain growth thus leading to coarse pearlite

Fine pearlite : it is two phased lamellar structure composed of alternating layers of ferrite and cementite that occurs in some steels and cast irons.

Bainite : It is the product of isothermal decomposition of austenite

Spheroidite : it consist of ferrite and cementite.

Tempered martensite : In tempering process unstable martensite decomposes into ferrite and unstble carbides and finally into stable cementite is called tempered martensite.

- bainite and spheroidite is a coarse phase that forms at temprature near to euectoid temprature

-pearlite,bainite,spheroidite all consist of ferrite and cementite phases.

-Bainite is a type of steel produced by faster cooling faster than pearlite but slower than martensite

5) Martensite : This is obtained by rapid cooling of austenite.IS is extremely hard and possess acicular needle like structure. it is magnetic and has carbon content up to 2%.it is extremely hard and brittle compare to other structures

.the decompostion of austenite below 320 degree starts formation of martensite

6) Only phase that can transform to into pearlite,bainite,or martensite is austenite.

7)Mulptiple heat treaments needed for formation of tempered martensite