Chapter Lecture 11 Phase Diagrams, Solidification, Phase transformations ME 330 Engineering Materials Solidification Solidification microstructures Iron-Carbon alloys Phase transformations Read Chapters 9 and 10
Chapter Phase diagrams are useful tools to determine: --the number and types of phases, --the wt% of each phase, --and the composition of each phase for a given T and composition of the system. Alloying to produce a solid solution usually --increases the tensile strength (TS) --decreases the ductility. Binary eutectics and binary eutectoids allow for a range of microstructures. Summary
Chapter Binary Systems Two component systems Isomorphous systems - –Solid and liquid are completely soluble over all compositions Eutectic systems - –Limited solubility in solid state –Phase diagram has invariant point where a single phase liquid solidifies to a two phase solid Peritectic systems - Invariant point where a liquid and solid phase solidify into a different solid phase Eutectoid systems - Invariant point where a solid phase solidifies into two different solid phases Compound forming systems - Compounds form rather than solid solutions
Chapter Iron-Carbon (Fe-C) Phase Diagram 2 important points -Eutectoid (B): + Fe 3 C -Eutectic (A): L + Fe 3 C Adapted from Fig. 9.24,Callister 7e. Fe 3 C (cementite) L (austenite) +L+L +Fe 3 C + L+Fe 3 C (Fe) C o, wt% C 1148°C T(°C) 727°C = T eutectoid A SR 4.30 Result: Pearlite = alternating layers of and Fe 3 C phases 120 m (Adapted from Fig. 9.27, Callister 7e.) RS 0.76 C eutectoid B Fe 3 C (cementite-hard) (ferrite-soft)
Chapter Hypoeutectoid Steel Adapted from Figs and 9.29,Callister 7e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in- Chief), ASM International, Materials Park, OH, 1990.) Fe 3 C (cementite) L (austenite) +L+L + Fe 3 C L+Fe 3 C (Fe) C o, wt% C 1148°C T(°C) 727°C (Fe-C System) C0C Adapted from Fig. 9.30,Callister 7e. proeutectoid ferrite pearlite 100 m Hypoeutectoid steel R S w =S/(R+S) w Fe 3 C =(1-w ) w pearlite =w rs w =s/(r+s) w =(1-w )
Chapter Hypereutectoid Steel Fe 3 C (cementite) L (austenite) +L+L +Fe 3 C L+Fe 3 C (Fe) C o, wt%C 1148°C T(°C) Adapted from Figs and 9.32,Callister 7e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in- Chief), ASM International, Materials Park, OH, 1990.) (Fe-C System) 0.76 CoCo Adapted from Fig. 9.33,Callister 7e. proeutectoid Fe 3 C 60 m Hypereutectoid steel pearlite RS w =S/(R+S) w Fe 3 C =(1-w ) w pearlite =w s r w Fe 3 C =r/(r+s) w =(1-w Fe 3 C )
Chapter Iron-Iron Carbide Phase Diagram Not Iron-Carbon phase diagram –Only look at iron rich portion Cementite –Iron carbide (Fe 3 C) –Intermediate compound –Metastable phase will transform to & carbon at high temperatures over time –Considered a component of diagram Metastable phase diagram (Pure Iron, %C < 0.008) Steels –%C < 2.14 –Rarely exceed 1% Cast Iron –%C > –Usually 3-5% Steels Cast Irons Cementite (Fe 3 C)
Chapter Steel Designations From: Callister, p
Chapter Iron-Iron Carbide Phase Diagram Ferrite, –BCC structure –Carbon dissolved in iron (0.02% max.) –Soft, weak, ductile Austenite, –High temperature phase, above 727°C –FCC crystal structure –Carbon dissolved in iron (2.14% max.) Ferrite, –BCC structure Cementite –Hard, strong, brittle , Ferrite (BCC) , Austenite (FCC) , Ferrite (BCC) Cementite (Fe 3 C)
Chapter % Temperature °C + Fe 3 C + Fe 3 C 1076 Steel 6.70% Fe 3 C 727 Composition (wt % C) Eutectoid Solidification Start with pure austenite, Eutectoid Reaction – –0.76 % Carbon at 727 °C Develop pearlite microstructure –Similar to eutectic microstructure from last time –Alternating lamellae of and Fe 3 C –Properties intermediate between constituents
Chapter % Temperature °C + Fe 3 C + Fe 3 C 1076 Steel 727 Composition (wt % C) Eutectoid Composition Pearlite 0.76 % C Ferrite (white) Cementite (black) 6.70% Fe 3 C
Chapter Hypoeutectoid Solidification Hypoeutectoid - compositions before eutectoid (%C < 0.76%) Start with pure austenite, nucleates on grain boundaries –Proeutectoid ferrite Upon crossing eutectoid isotherm –Remaining austenite all turns to pearlite. Eutectoid ferrite Cementite –Proeutectoid ferrite remains unchanged Further cooling will result in only minor microstructural changes 01 % Temperature °C + Fe 3 C + Fe 3 C 1045 Steel 6.70% Fe 3 C 727 Composition (wt % C) Fe 3 C
Chapter % Temperature °C + Fe 3 C + Fe 3 C 1045 Steel 727 Composition (wt % C) Hypoeutectoid Composition 6.70% Fe 3 C
Chapter Hypereutectoid Solidification Hypereutectoid - compositions after eutectoid (%C > 0.76, %C < 2.14) Start with pure austenite, Fe 3 C nucleates on grain boundaries –Proeutectoid cementite Upon crossing eutectoid isotherm –Remaining austenite all turns to pearlite. Eutectoid cementite Ferrite –Proeutectoid cementite remains unchanged Further cooling will result in only minor microstructural changes 01 % Temperature °C + Fe 3 C + Fe 3 C Steel 6.70% Fe 3 C 727 Composition (wt % C) Fe 3 C
Chapter % Temperature °C + Fe 3 C + Fe 3 C Steel 727 Composition (wt % C) Hypereutectoid Composition 6.70% Fe 3 C % Fe 3 C= % total = % Fe 3 C proe = % pearlite= % Fe 3 C eut =
Chapter % Carbon Strength, MPa Yield Strength Tensile Strength Hot rolled bars Mechanical Properties & Carbon % EL or % RA Elongation Reduction in Area
Chapter Heat Treating Steels Forces steel to change phase (BCC FCC), then transform back to room temperature under control Variety of annealing processes –Relieve stresses –Increase softness, ductility –Produce specific microstructure Stages –Heat to desired temperature –Hold for desired time –Control the cooling rate to room temperature Quenching greatly increases strength, decreases ductility Tempering reverses much of this process –Processes combined much like cold work & annealing
Chapter Temperature °C 0 1 % + Fe 3 C + Fe 3 C % Carbon Heat Treating Steel Full Annealing – Heat to °C above phase transition line, hold for diffusion, and slow cool (usually in a furnace, often takes number of hours). – Returns microstructure to coarse pearlite. –Often performed on steels to be machined or formed to increase ductility
Chapter Temperature °C 0 1 % + Fe 3 C + Fe 3 C % Carbon Heat Treating Steel Normalizing –Heat to the fully austenite region and air cool. –Fine pearlite microstructure –Refine grain size and distribution (often done after a rolling operation)
Chapter Heat Treating Steel Spheroidizing – Heat a medium-high carbon steel below the eutectoid line and hold for several hours – Forms small Fe 3 C spherical particles in matrix. –For higher carbon steels, even pearlite is often brittle and difficult to deform –Spheroidizing minimizes hardness and is highly machinable Temperature °C 0 1 % + Fe 3 C + Fe 3 C % Carbon
Chapter Heat Treating Steel Quenching –Heat to fully austenitize steel –Rapidly cool (quench in water, oil, etc) –No time for carbon to diffuse –Produces a non-equilibrium microstructure called martensite Very hard, strong, brittle with large internal stresses Temperature °C 0 1 % + Fe 3 C + Fe 3 C % Carbon
Chapter Large number of atoms undergo cooperative motion FCC (austenite) polymorphically transforms to BCC (martensite) Carbon atoms trapped in interstitials –Supersaturated with carbon –Energetically unstable Rapidly transforms to other structures upon heating Martensitic Transformation
Chapter Heat Treating Steel Tempering –After quenching to form martensite –Reheat below eutectoid temperature –Hold for several hours to precipitate carbides and relieve residual stresses –Forms tempered martensite Relatively hard, strong, much more ductile Often ideal microstructure... Temperature °C 0 1 % + Fe 3 C + Fe 3 C % Carbon
Chapter Strength, MPa % RA Tensile Yield Tempering temperature, °C 4340 steel Mechanical Properties (Tempered Martensite) Supersaturated carbon transforms to stable ferrite and cementite Extremely small particles Trade strength for ductility –But, can be nearly as strong as martensite, but much more ductile
Chapter Heat Treated Microstructures Spheroidite 1150x Pearlite 1150x Martensite 1170x Tempered Martensite 17200x
Chapter Equilibrium Iron-Carbon Diagram Fe 3 C is a metastable compound –Can dissociate under certain conditions – Formation of graphite is –regulated by composition and rate of cooling –promoted by silicon (if concentration is >1%) and slow cooling
Chapter Cast Iron Formation P, pearlite , ferrite G f, flake graphite G r, graphite rosettes G n, graphite nodules
Chapter Cast Iron Microstructure Gray IronNodular (ductile) Iron White IronMalleable Iron -ferrite graphite flakes -ferrite graphite nodules pearlite cementite -ferrite graphite rosettes 1% Silicon added to promote formation of graphite 0.05 % Magnesium added to produce spheroidal graphite
Chapter New Concepts Calculate weight percent of total ferrite, cementite, pearlite, and proeutectoid vs. eutectoid phases Understand solidification difference for eutectoid, hyper-, and hypo-eutectoid compositions Heat treatments –Methods –Effect on mechanical properties
Chapter Next... Hardenability –Reaction kinetics … what happens when time is introduced to solidification –TTT curves –CCT curve –Hardenability and Jominy end quench test –Surface hardening treatments Please read chapter 10Please read chapter 10
Chapter Effects of Nonequilibrium Cooling Much more realistic –Equilibrium cooling rates often impractically slow –Often do not have time for complete diffusion Often desirable –Achieve phase transformations at higher temperatures than predicted –Trap non equilibrium phases not present on phase diagram
Chapter Phase Transformations (Ch.10) Objectives Understand the difference between equilibrium and non equilibrium transformations, and when to use a phase diagram, or a TTT diagram, or a CCT diagram. Understand the parameters (T, t combinations) and the kinetics processes (e.g. nucleation and growth) that lead to the distinct shape of the TTT. Understand and be able to describe the difference between isothermal and non isothermal (or continuous cooling) transformations. Be able to determine the end microstructure given an Fe-C alloy, the heating/cooling treatment and either an isothermal (TTT) or continuous cooling (CCT) diagram. Describe and sketch the following microstructures for steel alloys: fine and coarse pearlite, spheroidite, bainite, martensite, and tempered martensite, and be able to determine the relative phase fractions and phase compositions (the latter two are obtained from the equilibrium phase diagram). Designate a heat treatment that will produce the microstructures listed above starting with austenite or with a different microstructure (take bainite and produce martensite, for example). Cite relative mechanical characteristics for fine and coarse pearlite, spheroidite, bainite, martensite, and tempered martensite and relate these relative characteristics to the microstructures. Understand how a coarse versus a fine microstructure changes the mechanical properties of the alloy and how to make a fine or coarse microstructure using the CCT or TTT diagrams. Define supercooling and superheating and how these phenomena affect the transformation relative to the equilibrium phase diagram.
Chapter Phase Transformations Induce phase changes by changing temperature –Must cross a phase boundary To achieve equilibrium phase, finite time required –For most metals, equilibrium solidification rate too slow Rarely achieve equilibrium structures Phase Transformations –Nucleation –Growth
Chapter Phase transformations 1.Simple diffusion-dependent transformation (no change in either the number or composition of phases present) Ex: solidification of pure metal, recrystallization and grain growth 2.Diffusion dependent transformation (some alteration in phase compositions and often in number of phases present) Ex: eutectoid reaction 3.Diffusionless trasformation (metastable phase is produced) Ex: martensitic transformation
Chapter Homogeneous Nucleation Solid-liquid interface Solid Free Energy Change Find critical radius for maximum free energy change
Chapter heat given up during solidification Lowering of temperature at temperatures below the equilibrium solidification temperature, nucleation occurs more easily. These two quantities decrease as temperature decreases.
Chapter f10_03_pg316 T 2 < T 1
Chapter n* - have radii greater than r* Nucleation rate Fig in Callister Note: solidification begins after the temperature is lowered below T m. Supercooling (undercooling)
Chapter Phase transformations generally time & temperature dependent Time split between nucleation & growth Measure transformation % versus time for various temperatures Transformation rate (Avrami equation) t 0. 5 Growth Nucleation log of heating time, t Fraction of transformation,y General Solid State Reaction Kinetics t 0.5 Time dependence of rate Typical kinetics behavior Fraction of transformation
Chapter Fig in Callister
Chapter Eutectoid composition –0.76 % C – Transformation rate has a strong temperature dependence –Rapidly cool to given temperature –Hold to solidification More convenient to plot time versus temperature –Plot initiation and completion lines –Eutectoid plotted as horizontal line Valid only for –Given composition –Isothermal transformation TeTe time (s) Temperature °C % completion 100 % completion 0 % completion 675 °C Time-Temperature-Transformation (TTT) End Begin Percent transformed °C Austenite (stable) Austenite (unstable) Pearlite
Chapter Other than eutectoid compositions have proeutectoid phases –Cementite (%C > 0.76) – , Ferrite (%C < 0.76) –At low enough temperatures, this phase is suppressed Bainite –Forms below “knee” of curve –Not really a new phase –Ferrite and cementite phases –No longer lamellar structure Martensite –Quench fast enough to avoid other transformations –Forms at very low temperatures –Nonequilibrium and diffusionless Completing the Plots 0%50%100% +P Pearlite, P Bainite, B +B time (s) Temperature °C TeTe +C (Hypereutectoid) (Hypoeutectoid) + Martensite, M M (start) M (50%) M (90%)
Chapter Pearlite, Bainite, Martensite Pearlite –Formed by diffusion –Ferrite and cementite –Lamellar structure –Stronger than ferrite Bainite –Not as much diffusion –Ferrite and cementite –Not lamellar structure –Harder that pearlite Martensite –Diffusionless transformation –Speed of sound –BCT Structure (body- centered tetragon) with carbon interstitials –Strong and brittle Austenite FCC Structure Above 725 C Transforms to other phases Ferrite Iron + C in solid solution Max. C is 0.022% Ductile Cementite Compound, Fe3C Hard and Brittle Contains 6.7% C
Chapter Bainite & Martensite Upper bainite Lower bainite Martensite From: Callister
Chapter Back to eutectoid composition Above knee form pearlite as described in last lecture Thickness of lamellae depends on isotherm –Course pearlite Higher temperatures Diffusion rate high Carbon travels larger distances –Fine pearlite Close to 540 °C Diffusion suppressed Pearlite Formation +P Pearlite, P Bainite, B +B (Coarse pearlite) (Fine pearlite) time (s) Temperature °C TeTe Martensite, M M (start) M (50%) M (90%)
Chapter Below knee form bainite Upper bainite –~ °C –Ferrite grows first, then Fe 3 C drops out –Needles of ferrite separated by elongated cementite particles Lower bainite –~ °C –Thin plates of ferrite containing fine blades of cementite Cannot transform pearlite to bainite Can coexist with each other Bainite Formation +P Pearlite, P Bainite, B +B (upper Bainite) (lower Bainite) time (s) Temperature °C TeTe Martensite, M M (start) M (50%) M (90%)
Chapter M (90%) Nonequilibrium (metastable) phase Due to FCC/BCT transition –Happens quickly (velocity of sound) –Little atomic motion –Diffusionless transformation –Time independent ––46 °C for complete transformation Lath: long thin plates (%C < 0.6) Lenticular: needlelike Characteristically brittle, strong, hard Martensite Formation +P Pearlite, P Bainite, B +B time (s) Temperature °C TeTe Martensite, M M (start) M (50%)
Chapter Representative TTT Diagrams 4140 Steel Mn 0.77 Cr 0.98 Mo Steel Mn 0.78 Cr 0.80 Mo 0.33 Ni Steel 1045 Steel 1095 Steel
Chapter Alloying Effects Higher carbon content –Shifts curve to right (slightly!) –Change proeutectoid phase from ferrite to cementite –If %C < 0.4, steel is not “hardenable” Necessary cooling rate would be far too quick to form martensite Book says 0.25%, but realistically very difficult below 0.4% Alloying other than carbon –Shift austenite nose to longer times –Formation of separate bainite nose What does this mean? –To form equilibrium products, cooling rate must be much slower –Easier to form martensite –Thicker parts will have more uniform hardness
Chapter M (90%) Isothermal heat treatment not common –Practically, want to cool steadily to room temperature Isothermal curves shifted to longer times and lower temperatures Bainite will not form for plain carbon steel Alloying agents shifts the pearlite transformation curve to the right –Now possible to obtain bainite –2 “knees” appear on curve Continuous Cooling Transformation (CCT) +P Pearlite, P Bainite, B +B time (s) Temperature °C TeTe Martensite, M M (start) M (50%)
Chapter Bainite Pearlite Example: 4340 Steel 1. Martensite 1 4. Ferrite Pearlite 4 3. Martensite Ferrite Banite 3 5. Pearlite 5 2. Martensite Banite 2
Chapter Linking Important Concepts The composition of an alloy determines the phase change kinetics The cooling rate determines the microstructure of an alloy The microstructure determines the mechanical properties
Chapter Mechanical Properties (Pearlite & Bainite) Cementite harder & more brittle than ferrite Spheroidite: spherical Fe 3 C particles in matrix, very soft & ductile Pearlite: Fine is harder & more brittle than coarse Bainite: Stronger & harder than pearlite, good ductility Brinell Hardness %RA wt% C Bainite Pearlite Transformation temperature (°C) Brinell Hardness Spheroidite Fine Pearlite Coarse Pearlite
Chapter Brinell Hardness Carbon, % Martensite Pearlite Tempered martensite Mechanical Properties (Martensite) Very strong and hard Brittle –C interstitials –Few slip systems Large internal stresses due to volume change from austenite phase To recover ductility, need to do special heat treatment called tempering…
Chapter Strength, MPa % RA Tensile Yield Tempering temperature, °C 4340 steel Mechanical Properties (Tempered Martensite) Tempering: Reheating martensite up to a sub-eutectoid temperature for long time Trade strength for ductility
Chapter New Concepts Calculate weight percent of total ferrite, cementite, pearlite, and proeutectoid vs. eutectoid phases Understand solidification difference for eutectoid, hyper-, and hypo-eutectoid compositions Heat treatments –Methods –Effect on mechanical properties