Kinetics - Heat Treatment

Transcription

1 Kinetics - Heat Treatment Nonequilibrium Cooling All of the discussion up till now has been for slow cooling Many times, this is TOO slow, and unnecessary Nonequilibrium effects Phase changes at T other than predicted The existence of nonequilibrium phases at room temperature MSE 271 Unit 7 1

2 Time, the third dimension Phase diagrams only represent what should happen in equilibrium (e.g. slow cooling) Most materials are not processed under such conditions Time constraints Desirable characteristics of nonequilibrium microstructures Heat treatment Time - temperature history required to generate a certain microstructure Kinetics - the science of time-dependent phase transformations Time - temperature - transformation (TTT) diagrams are used to indicate the microstructure MSE 271 Unit 7 2

3 Solid State Reactions Most transformations do not take place instantaneously e.g. to change crystal structures, atoms must diffuse - Which takes time Energy is required to form phase boundaries between parent and product phases Phase Transformations Metallic Materials are extremely versatile They possess a wide range of mechanical properties Microstructure development occurs by phase transformations Properties can be tailored by changing microstructure MSE 271 Unit 7 3

4 Stages of Solid State Reactions Nucleation The formation of very small particles of the new phase Often begins at imperfection sites - especially grain boundaries* Growth The nuclei increase in size Some or all of the parent phase disappears Complete when system reaches equilibrium (may never be complete) Rate of Transformation The fraction of reaction that has occurred is measured as a function of time Usually at a constant T Progress is usually determined by microscopy or other physical property Data is plotted as fraction transformed vs. log time MSE 271 Unit 7 4

5 1.0 Plot of solid state reactions G=Cexp(-Q/RT) 0.5 At constant T r F a c t o i 0 Nucleation t 0.5 Growth Log of Heating Time, t n Multiphase Transformations Phase transformations occur when Temperature changes or Composition changes or External pressure changes Temperature is most common method to induce phase transformations Phase boundaries are crossed during heating or cooling MSE 271 Unit 7 5

6 Phase diagrams When a phase boundary is crossed, the alloy proceeds towards equilibrium according to the phase diagram Most phase transformations require a finite time Phase diagrams cannot indicate how long it takes to achieve equilibrium Many times the preferred microstructure is metastable Property changes in Fe-C alloys Examples of kinetic principles can be found in the Fe-C system Pearlite transformation Martensitic transformation MSE 271 Unit 7 6

7 Pearlite transformation Consider the eutectoid reaction γ(0.77 wt% C) α(0.22% C) + Fe 3 C(6.70% C) Austenite transforms to ferrite and cementite - changing iron content Carbon diffuses away from ferrite to cementite Temperature affects the rate: Construct isothermal transformation diagrams from % transformation diagrams Pearlite transformation Austenite Grain Boundary Austenite (γ) Austenite (γ) Ferrite, α Growth of Direction of Pearlite Cementite Fe 3 C MSE 271 Unit 7 7

8 Pearlite transformation Interpretation of Isothermal Diagrams Eutectoid T is a horizontal line The start and finish curves are nearly parallel Austenite exists to the left (not stable) Pearlite exists to the right MSE 271 Unit 7 8

9 Isothermal Diagrams 727 C, Eutectoid Temperature- Austenite (stable) 700 Temperature, T, C Austenite Pearlite Transformation Fine Pearlite Coarse Pearlite Time, t, s Validity of Isothermal Diagrams Only valid for a particular composition for a particular system Other compositions will have different curves Only valid when the temperature is constant throughout the transformation MSE 271 Unit 7 9

10 Martensite formation Other microstructures form if there is a temperature profile other than isothermal Martensite (fast quenching - prevents C diffusion - plate or needle like) - Since it is diffusionless - it is almost instantaneous Only dependent on T to which it is quenched Martensite Tempering of Steel MSE 271 Unit 7 10

11 Mechanical Behavior of Alloys Pearlite Cementite is harder but more brittle than ferrite Thus, the more Fe 3 C in an alloy the stronger and harder the material However, also makes it less ductile and not as tough (low impact resistance) Layer thickness also has an effect Fine pearlite is harder and stronger than coarse Martensite Mechanical Behavior Strongest, hardest, and most brittle Hardness is dependent on C content Martensite is not as dense - therefore when it transforms it causes stress Tempering (heat treatment) of martensite relieves stress - makes it tougher and more ductile MSE 271 Unit 7 11

12 Hardenability The ability of a steel alloy to transform to martensite is its hardenability Dependent on: Composition Quenching medium There is a relationship between mechanical properties and cooling rate Hardenability refers to the degree to which is transforms to martensite and to the depth an alloy may be hardened Hardenability Jominy End-Quench Cylindrical specimen is cooled from the end by a spray or water Specimen size, shape is specified Water spray and time is specified The hardness is measured with respect to the distance from the quenched end Flat is ground along length Rockwell hardness measured MSE 271 Unit 7 12

13 Jominy End-Quench Results The quenched end is cooled most rapidly and has highest hardness 100% martensite Cooling rate decreases away from end and the hardness decreases A hardenable steel retains large hardness values for long distances Each steel has unique hardenability curves Cold Working A ductile material can become harder and stronger as it is plastically deformed Strain hardening = work hardening = cold working (meaning the strain is imposed at low temperatures) Most metals strain harden at room temperature. MSE 271 Unit 7 13

14 Cold Working The degree of plastic deformation is expressed as % cold worked: %CW = A o A d A o x 100 A and A o d are the cross sectional areas before and after plastic deformation occurs Cold Working Why does this occur? Dislocation-dislocation strain field interactions Dislocation density increases with cold working - so the average separation between dislocations decreases Strain hardening may be removed by annealing (heating to higher T to allow dislocations to move) MSE 271 Unit 7 14

15 Recovery, Recrystallization Plastic deformation results in changes in microstructure and properties Grain shape Strain hardening Increased dislocation density Original properties can be regained by appropriate heat treatment Recovery,recrystallization, grain growth Recovery Some of the stored strain energy is relieved by movement of dislocations at high T Number of dislocations is reduced Configuration of dislocation is altered Properties return to their pre-cold-worked state MSE 271 Unit 7 15

16 Recrystallization Even after recovery, grains are still in a high energy state (they have been deformed) Recrystallization is the formation of a new set of strain-free equiaxed grains. New grains form by nucleation and growth Short range diffusion Recrystallization Recrystallization depends on Time Temperature Recrystallization temperature Defined as the temperature at which recrystallization reaches completion in 1 hr. MSE 271 Unit 7 16

17 Stages of Recrystallization Cold Worked Initial Stage Intermediate Stage Complete Recrystallization Grain Growth Grain Growth, Higher Temperature Grain Growth After recrystallization, grains will continue to grow Occurs in all crystalline materials - why? Energy is associated with grain boundaries - As grain size increases, total boundary area decreases All grains can t grow Large ones grow at the expense of small ones MSE 271 Unit 7 17

18 Grain Growth Fine grained materials usually have superior properties to coarse-grained materials If an alloy is coarser than desired, refinement may be accomplished by physically deforming and then recrystallizing. Summary TTT diagrams are not used for nonmetallic materials Grain growth is especially important in ceramics Heat treatment to bond powder particles and reduce porosity is called sintering for ceramics MSE 271 Unit 7 18