1. Recrystallisation and Grain Growth
Cold working: for malleable in cold and weak and brittle when heated materials –
often in finishing stages of production
to get clean smooth finish;
to straighten (in some);
to get required degree of hardness
Hot working: Increase in temperature increases interatomic spacing; decreases bond strength. Dislocations moved more easily through crystal. Becomes softer more malleable, less energy for deformation.
Carried at temperatures above RECRYSTALLISATION

2. Relief of stresses: At low temperatures- atoms move to positions nearer to equilibrium
Small movements- reduces local strain& stress, without change in shape
Hardness & tensile strength, as in cold worked, are high

3. When annealing temperature increased, at a point of temperature,
new crystals form-
at high P.E. positions,
grain boundaries.

4. First, small;
Then
grow gradually

5. Absorbs distorted structure,
First slowly,

6. Then
completely

7. These new crystals equiaxed
Recrystallisation temp: ≈ 1/3 to ½ Tm

8. STAGES IN RECRYSTALLISATION
Recrystallisation temperature depends
on degree of cold work.
severe cold work- lower crystallisation temperature
STAGES IN RECRYSTALLISATION

9. When annealing temperature above recrystallisation, newly formed crystals continue to grow
By absorbing each other (in a cannibal fashion)
Final structure - coarse grained.
Crystal boundary moves towards centre of curvature
GRAIN GROWTH

10. IMPURITIES IN STEEL SULPHUR, PHOSPHEROUS, SILICON, MANGANESE
PROPERTIES DEPEND ON THE WAY BY WHICH THESE IMPURITIES ARE DISTRIBUTED
EVEN DISTRIBUTION PREFERRED TO CORED
CORING CONCENTRATES IMPURITIES

11. Sulphur & Phosphorus segregate and precipitate at grain boundaries----- coring.
Silicon & Manganese evenly distributed (Even 0.3% effect is minimum)
Si- imparts fluidity, upto 0.3%- [In HCS, kept lower (decomposes to graphite)]
Mn- soluble in Austenite and Ferrite Mn3C. Increases ‘depth’ of hardening, improves strength & toughness, max 0.3%

12. Sulphur- Forms brittle FeS
Solubility 0.03%, Precipitates at boundaries; Iron Sulphide brittle and makes steel not suitable for cold working. Difficult to reduce below 0.05%. Excess Mn as MnS nullifies the effect.
MnS globules
Isolated . Insoluble and mostly removed as slag during processing
P – max 1% -hardening effect. (0.05% general). [More- brittle phosphide forms]

13. Nitrogen-forms nitrides during manufacture.
Makes steel not suitable for cold working. (Fe4N brittle).
Possible to bring to very low (0.002%), with good processes.

14. HARDENING- From above A3 when cooled RAPIDLY,
HARDENS.
Degree of hardening –
on initial quenching temperature, size, constitution, properties
and temperature of quenching medium
IRON ATOMS
CARBON ATOMS
From FCC Austenite to
BCC Tetragonal based cell on ABCD
In 10-7 seconds By
DIFFUSIONLESS PHASE TRANSFORMATION
AB = ao/√2

15. UPPER CRITICAL TEMPERATURE, A3
LOWER CRITICAL TEMPERATURE, A1