Metallurgy basics (Iron phase diagram)
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- 1. PIYUSH VERMA 3rd year undergraduate, Department of Metallurgical and Materials Engineering Indian Institute of Technology, Kharagpur
2. Topic for today Fe-Fe3C Phase Diagram (For a plain C steel) (plain because no element presence other than C in Iron) (A base for understanding how different phases of iron evolves) 3. Why steel is important It is the Gold-standard against which emerging structural materials are compared. Moving standard: exciting discoveries being made in the context of iron and alloys, now and then. Most successful and cost-effective structural material. Overwhelming dominance of steel : Due to variety of microstructures and properties generated by the solid-state transformation and processing. Steels-Microstructure and Properties By Honeycombe & Bhadeshia) 4. Some facts Presence of carbon in iron has such a great effect that even 0.1-0.2 wt% (approx 0.5- 1.0 at % i.e. 5-10 C atoms in 1000 Fe atoms) produces strengthening effect on ferritic iron, fact, known to smiths for over 2500 years ago, since iron heated in a charcoal fire can easily absorb carbon through solid-state diffusion. Carbon nano-tubes are 100 times more stronger than steels but cannot be used in engineering scale because of inevitable defects which arise as these tubes are grown. Inner core of earth may consist of pure phase of iron (apart from ,,,) in shape of double HCP. 5. Pure Fe Phase Diagram fcc bcc hcp bcc 6. Iron-Iron Carbide Phase Diagram 7. What is a phase? A homogeneous portion of the system that has a uniform chemical and physical characteristics. Every pure material is a phase : elements, compounds. Example solid sugar is a solid phase and sugar-syrup solution is a liquid phase Both have different chemical composition: one is pure sugar (C12H22O11) and the other is a solution of sugar and water (H2O) Every teaspoon of the sugar-syrup is always of same composition 8. Pro-eutectic Austenite + Ledeburite Cementite + Ledeburite Liquid steel Austenite + Liquid Austenite Ferrite Liquid+Cementite Pro-eutectoid Ferrite+Pearlite Pearlite+ 9. It is a Binary Alloy phase diagram. Binary because here only two elements, Fe and C are considered. The phase-diagram is only applicable for the equilibrium cooling, meaning a very slow cooling. (By equilibrium we mean very minute change in the system-parameters/compositions etc for a long period of time) Some features of Fe-Fe3C Phase Diagram 10. Each Vertical Green Line represents a different Carbon wt% Fe-C alloy 11. Each C wt% on the X-Axis is a different alloy system 12. Phases of iron in Fe-Fe3C Phase Diagram -Ferritic (alpha phase)-BCC crystal lattice -Austenite (gamma phase)-FCC crystal lattice - (delta phase)-BCC crystal lattice Fe3C Cementite phase - Orthorhombic 13. Carbon goes into the interstitial sites 14. Tetrahedral site in BCC Octahedral sites in FCC Octahedral site in BCC 15. Carbon goes into the octahedral sites (both in BCC and FCC) to avoid strain due to surrounding Fe atoms in Tetrahedral sites. (Strain due to C in octahedral site is less than tetrahedral sites due to its large size) Solubility of C in Austenite is larger than Ferrite because Octahedral site is larger in FCC than BCC. 16. Maximum solubility of C in -Ferritic phase is 0.022 wt%, in -Austenite phase is 2.1 wt%, in -phase 0.015 wt%. 17. It means that by heating Iron we can add more C in it and get a homogenous solution, as the solubility of Austenite (which is stable at higher temperature) is more than the ferrite phase( which is stable at room temperature) If we cool the iron( when it is in austenite phase) then due to less solubility at lower temperature, C atoms will come out of the Fe Matrix in the form of precipitates , so that the solution will be no more homogeneous. 18. Ferrite is softest whereas Cementite is the most hardest and brittle phase in the entire phase diagram Because presence of C enhances strength and the maximum solubility of C in ferrite is only 0.02 wt% whereas in Cementite it is 6.67 wt% (& which is constant) Cementite is only a metastable phase, which will decompose into Ferrite and graphite if heated between 650-700 for several years. (A metastable phase may persist indefinitely and often, experiencing only slight and imperceptible changes as time progresses) 19. Hypoeutectoid Steel 20. Eutectoid Steel 21. Hypereutectoid Steel 22. Determination of phase amounts at different temperatures LEVER RULE 23. MECHANISM OF PHASE TRANSFORMATION 24. Two types of transformation processes in - phase transition (Ferrite-Austenite-Ferrite) RECONSTRUCTIVE TRANSFORMATION DISPLACIVE OR SHEAR TRANSFORMATION 1. Involves diffusion of atoms (both solute-Fe and the solvent) 1.No diffusion 2.No resultant stress in the microstructure 2. Results in combination of elastic and plastic strain in the matrix(microstructure) 3.No resultant shape change in the sample 3. Resultant shape change of the bulk sample 4.Stable above 835C (in steels) 4.Stable below 835C (in steels) 25. Austenite to Pearlite transformation 26. Pearlite : A grain containing alternate lamellae of ferrite and Cementite. In pearlite the width ratio of to Cementite lamellae (Fe3C) is generally 8: 1 . Pearlite forms through diffusion of C atoms out from austenite making left out Fe atoms in austenite to rearrange and transform their crystal structure from FCC to BCC (Ferrite). C diffusing out forms Cementite(Fe3C 6.67 wt% C ) . 27. Dependence of Eutectoid Composition 28. References Steels-Microstructure and Properties Honeycombe & Bhadeshia (Introduction and the Crystal Structure part) Materials Science and Engineering-William Callister (Fe-Fe3C Phase diagram part) Pictures-Google 29. Thank You 30. Common Doubts Yes, the Fe-Fe3C is just a part of the Fe-C phase diagram, the entire Fe-C phase diagram extends to the right hand side upto 100 wt% C, which is pure Carbon phase called Graphite. (Fe-C phase diagram has got not so significant use in steel industry, thats why its generally not shown). Ferrite is a phase, Austenite is a phase, Cementite is a phase but pearlite is not a phase, pearlite is just a colony of 2 phases-ferrite & Cementite in a grain. We discussed the solution of Fe & C and definitely not a mixture. Solution is homogenous in its composition and properties everywhere whereas a mixture is not. (Sugar-Syrup is a solution and clay in water is a mixture). The width of the precipitate at the grain boundary varies as a grain boundary can be a high-angle grain boundary or a low-angle grain boundary, High angle grain boundaries are highly preferential sites for nucleation of precipitates due to their high incoherency.(Different orientation of grains) Different Heat-Treatment processes changes the microstructure differently for the same alloy composition. We discussed the phase diagram and microstructures for a Slow cooling process. Quenching (Oil and water) is a fast cooling process and it produces martensite (another phase of iron) along with other phases depending on the processing parameters.