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STABLE AUSTENITE

STABLE AUSTENITE. Near A 1. Sluggish transformation Au. to Coarse Pearlite Greater time for diffusion. 550. Bianite in feather shaped patches Degree of under cooling high. 550-220. Slow rate of diffusion of C atoms retards increased tendency of Au. transformation ,.

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STABLE AUSTENITE

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  1. STABLE AUSTENITE Near A1 Sluggish transformation Au. to Coarse Pearlite Greater time for diffusion 550 Bianite in feather shaped patches Degree of under cooling high 550-220 Slow rate of diffusion of C atoms retards increased tendency of Au. transformation,

  2. 4000C -6000C range Temperature 1 sec 1000 sec Time (on logarithmic Scale)

  3. Transformation ends Transformation starts/begins Stable Austenite Coarse Pearlite Unstable Austenite Fine Pearlite Feathery Bainite Unstable Austenite Acicular Bainite Ms Austenite + Martensite Mf Martensite Time-Temperature Transformation Curves

  4. STABLE AUSTENITE Near A1 Sluggish transformation Austenite to Coarse Pearlite Greater time for diffusion 550 Bianite in feather shaped patches Degree of under cooling high 550-220 Slow rate of diffusion of Carbon atoms retards increased tendency of Austenite transformation,

  5. Transformation V/s logarithm of Time plot Isothermal Transformation diagram plot

  6. Transformation ends Transformation starts/begins Stable Austenite Coarse Pearlite Unstable Austenite Fine Pearlite 4000C -6000C range Feathery Bainite UnstableAustenite Acicular Bainite Ms ≈ 2500C M 90% at 1100C Austenite + Martensite Mf ≈-500C Martensite Time-Temperature Transformation Curves

  7. Ferrite nucleates first, over which layer of Cementite (Iron Carbide)- Growth is quick.

  8. Complete Isothermal Transformation diagram for EUTECTOID composition A- Austenite B- Bainte M- Martensite P- Pearlite

  9. Isothermal Transformation Diagram for alloy steel (with 0.40%C, Ni, Cr & Mo small %) A- Austenite B- Bainite P- Peralite M- Martensite F- proeutectoid ferrite

  10. Complete Isothermal Transformation diagram for EUTECTOID composition With Continuous Cooling Transformation diagram- CCT superimposed

  11. Transformation starts/begins Transformation ends A- fine Pearlite Continuous cooling Transformation EFFECT OF DIFFERENT COOLING RATES A - 50C/sec(normalising) B - Rapid 4000C/sec (water quench) C - 1400C/sec- Martensitic D - 500C/sec- P + M + Au B- M + little Au D C Continuous Cooling Transformation diagram

  12. Moderately rapid [1] and slow cooling [2] curves superimposed on continuous cooling transformation diagram for EUTECTOID [2] Full Annealing [1] Normalising

  13. CCT Dependence of final microstructure on transformations occurring during cooling Martensite for quenching rates greater than the critical

  14. Quenching Medium Media, arranged In order of quenching speed: • 5% Caustic Soda - highest • 5-20% Brine • Cold Water • Warm Water • Mineral Oil • Animal Oil • Vegetable Oil - least

  15. LARGE STEEL PART WHENCOOLED COOLING CURVES CORE WITH FINE PEARLITE, SURFACE MARTENSITIC- CALLED MASS EFFECT OF HEAT TREATMENT BAINITE SURFACE CORE MARTENSITE

  16. QUENCH CRACKS MAY FORM, EVEN IF COOLING RATE INCREASED

  17. Quenched in bath of temperature E, kept for long, removed and cooled – martensite forms Also called MARQUENCHING E Air Cool Transformation Complete MARTEMPERING

  18. Transformation Complete at G FINE BAINITE FORMS, AIR COOLED STRUCTURE AS OF TEMPERED MARTENSITE GOOD FOR INTRICATE SHAPES. BY REPLACING QUENCHING AND TEMPERING, CRACKS AVOIDED G Air Cool AUSTEMPERING

  19. Classification scheme for various ferrous alloys

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