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;

Thengrow

gradually

Absorbs distorted

structure,

First slowly,

Thencompletely

These new crystals equiaxedRecrystallisation temp: ≈ 1/3 to ½ Tm

STAGES IN RECRYSTALLISATION

Recrystallisation temperature depends

on degree of cold work.

severe cold work- lower crystallisation temperature

GRAIN GROWTH

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

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

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

From FCC Austenite toBCC Tetragonal based cell on ABCD In 10-7 secondsBy DIFFUSIONLESS PHASE TRANSFORMATION

IRON ATOMS

CARBON ATOMS

AB = ao/√2

UPPER CRITICAL TEMPERATURE, A3

LOWER CRITICAL TEMPERATURE, A1