ME 330 Engineering Materials Lecture 12 Phase Diagrams, Solidification, Phase transformations Solidification Solidification microstructures Iron-Carbon alloys Phase transformations Read Chapter 10

Kinetics of phase transformations 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 10

Development of microstructure in single or two phase alloys usually involves phase transformations – alteration in the number and/or character of phases.

Phase transformations At least one new phase is formed that has different physical/chemical characteristics and/or different structure that the parent phase They do not occur instantaneously Transformation rate (dependence of reaction progress on time) Phase transformations have two stages Nucleation (appearance of very small particles of new phase which are capable of growing) Growth

Phase transformations Simple diffusion-dependent transformation (no change in either the number or composition of phases present) Ex: solidification of pure metal, recrystallization and grain growth Diffusion dependent transformation (some alteration in phase compositions and often in number of phases present) Ex: eutectoid reaction Diffusionless trasformation (metastable phase is produced) Ex: martensitic transformation

Nucleation Homogeneous phase Heterogeneous phase Nuclei of new phase form uniformly throughout the parent phase Heterogeneous phase Nuclei form preferentially at structural inhomogeneities (contain surfaces, grain boundaries, dislocations, etc)

Homogeneous Nucleation Free Energy Change Solid-liquid interface Solid Find critical radius for maximum free energy change Activation free energy required for the formation of stable nucleus

heat given up during solidification is zero at Tm heat given up during solidification These two quantities decrease as temperature decreases. Lowering of temperature at temperatures below the equilibrium solidification temperature, nucleation occurs more easily.

So far we considered temperature dependence. f10_03_pg316.jpg So far we considered temperature dependence. Next, we consider time dependence.

General Solid State Reaction Kinetics Time dependence of rate Phase transformations generally time & temperature dependent Time split between nucleation & growth Measure transformation % versus time for various temperatures Transformation rate (Avrami equation) Typical kinetics behavior 1.0 Fraction of transformation,y 0.5 t0.5 t0.5 Fraction of transformation Growth Nucleation log of heating time, t

f10_11_pg323.jpg Fig. 10.11 in Callister

Iron-Iron Carbide Phase Diagram Recall: 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,  Cementite Hard, strong, brittle , Ferrite (BCC) , Austenite (FCC) , Ferrite (BCC) Cementite (Fe3C)

Eutectoid Solidification Recall: Eutectoid Solidification 1 % 1000 800 600 Temperature °C g a + g g + Fe3C a + Fe3C a 1076 Steel 6.70% Fe3C 727 Composition (wt % C) 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 Fe3C Properties intermediate between constituents 

Eutectoid Composition Recall: Eutectoid Composition 1076 Steel 1000 g Temperature °C g + Fe3C 800 a + g 727 a 0.022 0.76 Fe3C 600 6.70% a + Fe3C 1 % Pearlite 0.76 % C Ferrite (white) Cementite (black) Composition (wt % C)

Time-Temperature-Transformation (TTT) 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 End Begin transformed Percent  50 100 650 °C 675 °C Austenite (stable) Austenite (unstable) Pearlite Te time (s) Temperature °C 1 10 102 103 104 105 400 500 600 700 50 % completion 100 % completion 0 % completion

f10_14_pg327.jpg Fig. 10.14 in Callister

Completing the Plots 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 time (s) 1 10 102 103 104 105 10-1 Temperature °C 400 500 600 700 300 200 800 Te  +C (Hypereutectoid) (Hypoeutectoid) +   +P Pearlite, P  +B Bainite, B  M (start) M (50%) 0% 50% 100% M (90%) Martensite, M

Pearlite, Bainite, Martensite Formed by diffusion Ferrite and cementite Lamellar structure Stronger than ferrite Bainite Not as much diffusion 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

Bainite & Martensite Upper bainite Martensite Lower bainite From: Callister

Pearlite Formation Course pearlite Fine pearlite 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 time (s) 1 10 102 103 104 105 10-1 Temperature °C 400 500 600 700 300 200 800 Te  (Coarse pearlite)   +P Pearlite, P (Fine pearlite)   +B Bainite, B M (start) M (50%) M (90%) Martensite, M

Bainite Formation ~300 - 540 °C Below knee form bainite Upper bainite ~300 - 540 °C Ferrite grows first, then Fe3C drops out Needles of ferrite separated by elongated cementite particles Lower bainite ~200 - 300 °C Thin plates of ferrite containing fine blades of cementite Cannot transform pearlite to bainite Can coexist with each other time (s) 1 10 102 103 104 105 10-1 Temperature °C 400 500 600 700 300 200 800 Te    +P Pearlite, P (upper Bainite)   +B Bainite, B (lower Bainite) M (start) M (50%) M (90%) Martensite, M

Martensite Formation Happens quickly (velocity of sound) 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 time (s) 1 10 102 103 104 105 10-1 Temperature °C 400 500 600 700 300 200 800 Te    +P Pearlite, P   +B Bainite, B M (start) M (50%) Martensite, M M (90%)

Representative TTT Diagrams Presence of other alloying elements 1021 Steel 1045 Steel 1095 Steel 4140 Steel Mn 0.77 Cr 0.98 Mo 0.21 4340 Steel Mn 0.78 Cr 0.80 Mo 0.33 Ni 1.79

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

Continuous Cooling Transformation (CCT) 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 time (s) 1 10 102 103 104 105 10-1 Temperature °C 400 500 600 700 300 200 800 Te   +P  Pearlite, P  Bainite, B  +B M (start) M (50%) Martensite, M M (90%)

Example: 4340 Steel 1. Martensite 2. Martensite Banite 3. Martensite Bainite Pearlite 1. Martensite 1 2. Martensite Banite 2 4. Ferrite Pearlite 4 5. Pearlite 5 3. Martensite Ferrite Banite 3

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

Mechanical Properties (Pearlite & Bainite) Cementite harder & more brittle than ferrite Spheroidite: spherical Fe3C particles in a matrix, very soft & ductile Pearlite: Fine is harder & more brittle than coarse Bainite: Stronger & harder than pearlite, good ductility 120 240 280 160 200 80 60 40 20 Brinell Hardness %RA wt% C 0 0.2 0.4 0.6 0.8 1.0 Bainite Pearlite 600 300 500 400 100 200 700 Transformation temperature (°C) Brinell Hardness Spheroidite Fine Pearlite Coarse Pearlite

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… 700 600 500 400 300 200 100 Brinell Hardness 0.2 0.4 0.6 0.8 1.0 0.0 Carbon, % Martensite Pearlite Tempered martensite

Mechanical Properties (Tempered Martensite) Tempering: Reheating martensite up to a sub-eutectoid temperature for long time Trade strength for ductility 1800 1600 1400 1200 1000 800 Strength, MPa 60 50 40 30 % RA Tensile Yield 200 300 400 500 600 Tempering temperature, °C 4340 steel

Austenitic Transformations

Objectives for Chapter 11 Have basic understanding of types of steels, relative C content, strength, and applications. Know fundamental differences between the types of cast iron. Describe the purpose of and procedures for process annealing, stress relief annealing, normalizing, full annealing, and spheroidizing. Define hardenability and list the class steels that have low hardenability. Know how to use the Jominy End Quench Test - see lab on hardenability. Explain the procedure to precipitation harden an alloy and what characteristics on the phase diagram allow the alloy to be hardened by this mechanism. Explain the shape of the strength vs time curve for precipitation hardened alloys in terms of the mechanism of hardening in each stage. Distinguish between the four types of forming operations. Distinguish between the four types of casting techniques.

Ferrous Alloys

Metal Fabrication

Heat Treating Steel Full Annealing Temperature °C 1 % 1000 800 600 g a + g g + Fe3C a + Fe3C a % Carbon Full Annealing Heat to 15-40 °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

Heat Treating Steel Normalizing Temperature °C 1 % 1000 800 600 g a + g g + Fe3C a + Fe3C a % Carbon Normalizing Heat to the fully austenite region and air cool. Fine pearlite microstructure Refine grain size and distribution (often done after a rolling operation)

Heat Treating Steel Spheroidizing Temperature °C 1 % 1000 800 600 g a + g g + Fe3C a + Fe3C a % Carbon Spheroidizing Heat a medium-high carbon steel below the eutectoid line and hold for several hours Forms small Fe3C spherical particles in  matrix. For higher carbon steels, even pearlite is often brittle and difficult to deform Spheroidizing minimizes hardness and is highly machinable

Heat Treating Steel Quenching Heat to fully austenitize steel Temperature °C 1 % 1000 800 600 g a + g g + Fe3C a + Fe3C a % Carbon 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

Heat Treating Steel Tempering After quenching to form martensite Temperature °C 1 % 1000 800 600 g a + g g + Fe3C a + Fe3C a % Carbon 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 ...

Hardenability Hardness vs. Hardenability Hardness – resistance to deformation Hardenability – ability to be form martensite (to be hardened) Formal definition: Hardenability – ability of an alloy to be hardened by formation of martensite due to given heat treatment High hardenability  martensite forms (i.e. hardens) far into specimen, not just at surface

Jominy End Quench Test Standard test for measuring hardness of any material Sample of fixed geometry suspended over water jet Geometric factors all constant Only effect on hardness is alloying content Hardness measured as function of distance from quenched end Cooling rate Distance Hardness, HRC Distance from quenched end

Distance from quenched end, mm Hardness Profiles 10 20 30 40 50 Distance from quenched end, mm Cooling rate at 700 °C (°C/s ) 170 70 18 9 5.6 3.9 2.8 2 60 Hardness, HRC 8660 8630 8640 8620 Cooling rate Highest at quenched end Smoothly decreases with distance Microstructure Martensite Bainite Fine Pearlite Course Pearlite Often plot cooling rate and distance Increasing carbon content Maximum possible hardness increases Higher overall hardness (more hardenable) Slower Cooling

Severity of Quench Rate of cooling! Depends on Medium- Fluid in which an austenetized part is plunged Most common: air, oil, water Agitation - Relative motion of quenching medium during cooling Agitation rate influences cooling rate Surface area to volume ratio - All heat removed at surface More surface area provides more opportunity to remove heat Irregular shapes with edges have large quenching surfaces For highly stressed part, want 80% martensite throughout part Usually from a “guideline” for specific company

Using Cooling Curves Normal quenching takes place on radial surface Can predict hardness across radius of a bar from Jominy tests.

Review: Heat Treatment of Steel Annealing- Heat, holding at temperature, gradual cool General term, but also used for specific processes Tempering -Heat martensite to get diffusional transformation: Still have ultra-fine microstructure, but more ductile Process annealing -Reverse effects of cold work; recovery, recrystallization, but little grain growth Stress relief - Lower temperature anneal to undo thermal stress or transformation mismatch stress Spheroidizing - Heat pearlite just below eutectoid temp to produce spheroidal structure; makes steel easier to machine

Surface Hardening Techniques Many applications, especially wear Strong, hard, wear-resistant surface Tough, fracture resistant inner core Two different heat treatments - through, then surface Methods Chose material with steep cooling curve Case Hardening - Change chemical composition of surface Carburizing Nitriding Carbonitriding Decremental Hardening – Very localized heat treatment Flame Hardening Induction Hardening

Alloys for Surface Hardness 10 20 30 40 50 Distance from quenched end, mm Cooling rate at 700 °C (°C/s ) 170 70 18 9 5.6 3.9 2.8 2 60 Hardness, HRC 100 80 Percent Martensite 4340 4140 5140 8640 1040 A B A B Prescribe: Minimum hardness at A Maximum hardness at B Must know the cooling rates at each point!

Carburizing Hardens steel by causing carbon to diffuse into the surface. Furnace heat to a temperature at which carbon will diffuse Hold until diffusion creates the proper case depth. Must be a carbon rich environment For steels with low carbon content (%C < 0.2) Used extensively on gears and shafts to harden them yet maintain the core toughness. Decarburization is opposite process for high carbon steels - softer case From: Callister p. 92, 224

Flame Hardening For steels of hardenable carbon content (%C  0.4) A high intensity oxy-acetylene flame is applied to the bring the region of interest to an austenite transformation. The interior never reaches high temperature. The heated region is quenched to achieve the desired hardness. The depth of hardening can be increased by increasing the heating time. In addition, large parts, which will not normally fit in a furnace, can be heat-treated From: Scheer p. 43

Precipitation Hardening Form extremely small, dispersed particles of a second phase Small second phase particles called precipitates Age hardening - strength develops as the alloy ages Time dependent There is a maximum attainable effect Different than tempering martensite Similar heat treatment procedure Different strengthening mechanism

Precipitation Hardening a Ti in b Ti matrix Primary strengthening mechanism for Aluminum Nickel based superalloys Titanium Examples Al/Cu Cu/Sn Mg/Al Ni3Al in Ni matrix From: Socie

Phase Diagrams & Treatment Precipitation hardenable alloys have: Appreciable maximum solubility Decreasing solubility with temperature Composition less than maximum solubility L L + a Temperature L + b a b Solution Treating: Heat into the single phase region and rapidly quench to room temperature to produce a supersaturated solid solution a + b Aging: Heat to a temperature below the phase transition to allow time for precipitates to form Composition

Why Precipitation Harden? Look at Aluminum rich side of Al/Cu system a is a substitutional solid solution of copper in aluminum q is an intermetallic compound CuAl2 One slow cool in is NOT helpful in strengthening Coarse q phase weakens the alloy q + a a °C 600 700 500 400 300 200 100 10 5 time 100% a (95.5% Al, 4.5% Cu) Temperature Coarse q precipitates At a grain boundaries Al wt % Cu

How to Precipitation Harden Two reheating treatments are needed: Solution treatment Age hardening Fine precipitates strengthen & harden material q + a a °C 600 700 500 400 300 200 100 10 5 wt % Cu 100% a solid solution Equilibrium Microstructure Temperature Fine precipitates in grains (retained after cooling) Al time

Overaging If precipitates get too big, strengthening/hardening is lost Process sped up by temperature Some alloys age at room temperature Why is strength lost? 100% a solid solution Temperature Fine precipitates in grains Coarse precipitates in grains taging time

Strength Development Fine precipitates (Guinier-Preston zones  q’’ phase) are coherent with lattice Deformation of crystal impedes dislocation motion At optimal aging time, precipitates are dispersed, small, and coherent. After optimal aging time, precipitates get too big, incoherent with matrix. From: Callister

Age vs. Material Properties Must age carefully to avoid overaging Too high temperature Too long age time Precipitation rate maximized at intermediate temperatures Near solvus  no driving force for nucleation Low temperatures  diffusion is slow t T Nucleation Diffusion From: Callister

Strengthening Mechanisms Dislocation Looping Dislocation cutting From: Hertzberg

What is Attractive About Aluminum? Upside Lightweight ( ) Ductile Heat treatable Castable Corrosion resistant Good conductor Downside Formability is lower Simpler shapes For same stiffness, need larger section Longitudinal stiffness(~EA/L) Bending stiffness (~EI) Rect. section, constant b b h

Aluminum Designations Tempering Designations -O: Annealed -F: As fabricated -H1: Strain Hardened -H2: Strain Hardened/ Partial Annealed -T: Thermally Processed -T2: Annealed (cast products) -T3: Solution treated, CW, naturally aged -T4: Solution treated, naturally aged -T5: Artificially aged, no solution treatment -T6: Solution treated, artificially aged -T7: Solution treated, over-aged -T8: Solution treated, CW, artificially aged -T9: Solution treated, artificially aged , CW Alloying Composition Aluminum 1xxx Aluminum Copper 2xxx Manganese 3xxx Silicon 4xxx Magnesium 5xxx Mg and Si 6xxx Zinc 7xxx Common alloys: 2024-T4 , 6061-T6, 7075-T6

New Concepts & Terms Applications of hardness profiles (Jominy curves) Precipitation hardening Heat treatment process Effect of overaging Microstructures Effect on strength Applications of aluminum Know tradeoffs, especially with steels Which alloys are precipitation hardenable?