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Unformatted text preview: PHASE TRANSFORMATIONS Iron – Iron Carbide: Review Defining characteris:cs: Three phase of pure iron- α - Ferrite (BCC) γ- Austenite (FCC) δ- Ferrite (BCC) Rela:vely low concentra:on of soluble carbon Intermediate compound - Fe3C (iron carbide, cemen.te) Greatest solubility of carbon in austenite Presence of eutec:c and eutectoid phase transforma:ons Iron – Iron Carbide IRON C < 0.008 wt% STEEL 0.008 wt% < C <2.14 wt% CAST IRON 2.14 wt% < C < 6.7 WT% Iron – Iron Carbide: Review - Microstructure Product of the eutectoid reac:on is called pearlite. Iron – Iron Carbide: Review Phase transforma:ons occurring for %C below the eutectoid invariant point are said to be hypoeutectoid. The ferrite precipitated before the phase transforma:on is said to be proeutectoid ferrite. Iron – Iron Carbide: Review Phase transforma:ons occurring for %C above the eutectoid invariant point are said to be hypereutectoid. The ferrite precipitated before the phase transforma:on is said to be proeutectoid cemen:te. Iron – Iron Carbide Phase transforma:ons are formally treated as equilibrium events. Time and temperature changes within actual processing steps produce a range of structures. Bainite, formed at lower temperatures than pearlite, consists of finer microstructure Coarse pearlite Fine Pearlite Bainite Iron – Iron Carbide Pearlite If pearlite or bainite structures are held at elevated temperatures for sufficient :me, the cemen:te phase will change shape, morphing the overall lamellar structure to that of a precipitate structure (spheroidite). Spheroidite Bainite Iron – Iron Carbide When austenite is rapidly cooled (quenched), a metastable structure is formed (martensite) in which the BCC structure is transformed to a body- centered tetragonal structure (BCT) (a≠c). Note that no diffusion of C is involved in this transforma:on. The quenching temperature and precise %C influences the resul:ng microstructure and mechanical proper:es. Supersaturated solid solu:on– all C atoms are in inters::al spaces Iron – Iron Carbide A number of mechanical proper:es of pearlite are enhanced through increased C% (in form the form of cemen:te). Iron – Iron Carbide Microstructure within the iron- iron carbide compound further influences mechanical proper:es. Iron – Iron Carbide Iron – Iron Carbide When martensite is annealed, significant changes in the microstructure and related mechanical proper:es are observed. Annealed martensite is referred to as tempered martensite. The tempering temperature plays a central role in establishing specific mechanical proper:es. The tempering process results in the following solid transforma:on. BCT martensite BCC ferrite + Fe3C Increasing T Iron – Iron Carbide The increase in hardness of martensite over pearlite is desirable, yet bribleness ocen accompanies this change. Tempering is used to produce a desired matrix of proper:es. Summary • Phase diagrams are useful tools to determine: - - the number and types of phases present, - - the composi:on of each phase, - - and the weight frac:on of each phase given the temperature and composi:on of the system. • The microstructure of an alloy depends on - - its composi:on, and - - whether or not cooling rate allows for maintenance of equilibrium. • Important phase diagram phase transforma:ons include eutec:c, eutectoid, and peritec:c. ...
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