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

STAGES IN RECRYSTALLISATION 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 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 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