Question

1. (a) Outline the principles and essential differences between Continuous Cooling Transformation and Isothermal Transformation Diagrams

   (b) Using the TTT diagram for a plain carbon steel with 0.53%wt C, define the nature of the final microstructure (in terms of micro-constituents present and approximate percentages) of small samples (5 x 5 x 5 mm) subjected to the following time-temperature heat treatments.In each case, assume that the specimen was austenitised at 860?C and held at this temperature long enough to achieve a homogeneous austenitic structure prior to cooling.

(i) Rapidly cool to 500^oC, hold for 7s, rapidly cool to 400oC, hold for 103s and water quench to room temperature.

(ii) Rapidly cool to 400^oC, hold for 7s and water quench to room temperature.

(iii) Rapid cooling to room temperature within 1s.

(iv) Rapidly cool to 650^oC, hold for 1h and water quench to room temperature.

(v) Rapidly cool to 400^oC, hold for 1s, followed by water quenching to room temperature.

Answer 1

In isothermal transformation austenite is rapidly brought to some specific temperature and to be maintained at elevated temperature from above eutectoid temperature.

But continuous cooling is more practical to apply. Most steels are heat treated with continuous cooling. Bainite won't form in a continuous cooling transformation. The nose of isothermal transformation diagram shifts to right in continuous cooling conditions.

These diagrams can be easily used to understand phase transformation in heat treatments

b)

i) When iti s cooled rapidly to 500 degree celsius and held for 7s, 50%of austenite transforms into Bainite. On further quenching to 400 degree celsius and holding there for 103 s, remaining austenite is transformed to bainite. So on further quenching no phase transformation occure.

ii) Here at the first situation, after 7s, it is still austenite only. So on quenching we get martensite with body centred tetragonal crystal structure

iii) On rapid cooling within 1s, the product is martensite. It is hardest, strongest phase of steel alloys

iv) On rapid cooling to 650 degree celsius and holding there for a 1h, pearlite with alternate layers of ferrite and cementite is formed. On water quenching this product, we get the grains of pearlite get refined.

v) martensite