Transcript Slide 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
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
When annealing
temperature increased,
at a point of
temperature,
new crystals form-
at high P.E. positions,
grain boundaries.
First,
small;
Then
grow
gradually
Absorbs
distorted
structure,
First slowly,
Then
completely
These new crystals equiaxed
Recrystallisation temp: ≈ 1/3 to ½ Tm
Recrystallisation temperature depends
on degree of cold work.
severe cold work- lower crystallisation temperature
STAGES IN RECRYSTALLISATION
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
IMPURITIES IN STEEL
• SULPHUR, PHOSPHEROUS, SILICON,
MANGANESE
• PROPERTIES DEPEND ON THE WAY BY WHICH
THESE IMOURITIES ARE DISTRIBUTED
• EVEN DISTRIBUTION PREFERRED TO CORED
• CORING CONCENTRATES IMPURITIES
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%
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]
• 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.
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
UPPER CRITICAL TEMPERATURE, A3
LOWER CRITICAL TEMPERATURE, A1