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Recrystallisation and Grain Growth

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

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Recrystallisation and Grain Growth

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  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

  9. 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

  10. 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

  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. 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.

  13. 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

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

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