Characteristics of Pearlite, Bainite and Martensite

Characteristics of Pearlite, Bainite and Martensite

1. Characteristics of Pearlite, Bainite and Martensite Presenter: Syed Ali Afzal Roll No.: MM-02/2017-18 Subject: Heat Treatment Presented to: Dr. Shahid Abro 1

2. Table of contents Phase Transformation Types of Phase T ransformation Possible Transformations in Steel Pearlite Bainite Martensite 2

3. Phase Transformation Phase: A homogeneous portion of a system that has uniform physical and chemical characteristics Nucleation: The appearance of very small nuclie of a new phase Growth: The nuclie increase in size  disappearance of parent phase 3

4. Types of Phase Transformation Diffusion-independent: No change in number or composition of the phases present E.g. S olidification of pure metal, allotropic transformations etc Diffusion-dependent: Changes in composition or number of phases. E.g. Eutectoid transformation Diffusionless : A metastable phase is produced by small displacements of atoms in structure E.g. Martensitic transformation 4

5. Possible Transformations in Steel 5

6. Pearlite 6

7. Introduction Diffusion-dependent  Eutectoid mixture  0.76%C @ 727°C on very slow cooling Very fine platelike or lamellar mixture of ferrite and cementite. White ferritic background /matrix contains thin plates of cementite (dark). Properties: Tensile strength = 120,000 psi Elongation = 20% in 2 in Hardness = Rockwell C 20, Rockwell B 95-100, or BHN 250-300 7

8. Isothermal Transformation Diagram 8 Also known as Time-Temperature-Transformation (T-T-T) curves Only valid if temperature is held constant

9. Nucleation and Growth of Pearlite 9

10. Coarse and Fine Pearlite 10 Thickness ration of ferrite and cementite is 8:1, depends on temperature. At high T, thick layers of ferrite and Fe 3 C  Coarse Pearlite. B/c diffusion rate are very high  C atoms can diffuse relatively long distances  thick lamellae At low T, thin layers of ferrite and Fe 3 C  Fine Pearlite. B/c C diffusion rate dec

11. Mechanical Behavior of Pearlite 11 Inc. the fraction of Fe 3 C  harder and stronger  dec. in ductility and toughness

12. Mechanical Behavior of Pearlite 12

13. Bainite 13

14. Introduction 14 Named after E. C. Bain Ferrite ( α ) + Cementite ( Fe 3 C )  d iffusional transformation Needles or plates  depends on T Microstructure  very fine  e - microscopy Grain of bainite  α (matrix) + elongated particles of Fe 3 C

15. Isothermal Transformation Diagram 15 At upper T  resembles pearlite  Upper or feathery bainite At low T  black needle like structure resembling martensite  lower or acicular bainite Growth of bainite : n ucleation by ferrite crystal c arbide rejected as discontinuous small platelets

16. Upper and Lower Bainite 16

17. Upper Bainite 17 T iny platelets of carbides oriented with ferrite needles

18. Lower Bainite 18 Ferrite needles become thinner  carbide platelets become smaller, oriented at an angle of about 60°

19. Austempering 19 HT process  100% bainite Steel directly from austenite to bainite Greater ductility and toughness along with high hardness Less distortion and danger of quenching cracks

20. Martensite 20

21. Diffusionless Transformation 21 Occurs without the long-range diffusion , but rather by cooperative movement of many atoms Movements are small ( less than 1 interatomic distance ) Also called “ Displacive or Military Transformation”

22. Diffusionless Transformation 22 https://www.doitpoms.ac.uk/tlplib/superelasticity/displacive.php

23. Martensite 23 Non-equilibrium single-phase structure results from the diffusionless transformation of austenite Supersaturated interstitial solid-sol of C in α -iron (BCC) Quenching rate should be rapid enough to prevent C diffusion  n o activation: Athermal FCC Austenite  BCT (body centered tetragonal) Martensite Metastable  persists indefinitely at RT, transforms to equilibrium phases at elevated T

24. Martensite 24 All the C remain as interstitial Occurs instantaneously  nucleate and grow at very rapid rate  time independent Grains  platelike or needle like + retained austenite (white)

25. Model of Transformation 25

26. Tempered Martensite 26 Martensite  v ery hard + brittle + internal stresses  cannot be used in most applications Ductility + toughness + stress relieving  Tempering Temp between 250°C and 450°C  Tempered Martensite Microstructure  extremely fine-grained, well dispersed cementite grains in ferrite matrix

27. Tempered Martensite 27 Mech props depend upon cementite particle size: fewer larger particle means less boundary area softer more ductile material Particle size inc. with higher tempering temp and/or longer time (more C diffusion)

28. Effect of Alloying Elements on Ms 28 Most alloying elements lower Ms except Co and Al

29. 29 Thank you. Questions?