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Valid only for eutectoid steels, and constant temperature transformations
Two solid lines: start and end of transformationDashed line: 50% transformationEutectoid temperature a horizontal line (above which is austenite)Transformation rate increases with decreasing temperature
Isothermal Transformation
Diagrams
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Rapid cooling from austenite along ABIsothermal heat treatment along BCDTransformation from austenite to pearlite begins at C, and is complete at D
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Isothermal Heat Treatment Example
Isothermal Transformation
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981
Isothermal Transformation
Austenite
19.2 min.
24.2 min.
5.8 min.
22.0 min.
66.7 min.
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981
Isothermal Transformation
Diagram of Eutectoid
Carbon Steel
More practical than isothermal transformation (which require constant temp.)With continuous cooling, time required for reaction to begin and end is delayedStart and end of transformation depends on cooling curveExperimentally determined phase diagram with parameter of time introduced
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Continuous Cooling Transformation Diagrams
Microstructure of Eutectoid Steel
Pearlite heated just below eutectoid temperature and held
Cementite lamellae transform to spherical shaps (reduced stress concentration)
Increasing cementite fraction results in harder, stronger material
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
•Slow cooling, coarse pearlite formed with thick ferrite andcementite layers•Increased cooling rate, lamellar thickness decreases•Increased cooling rate still allows formation of bainite•Faster cooling rates allow formation of martensite
Austenite Transformations
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
• fine pearlite if rapidly cooled (short diffusion distance)• coarse pearlite if slower cool
The faster the cooling rate, the finer the layers
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Thickness of ferrite and cementite phases in pearlite depend on heat treatment
Isothermal transformation diagram for an eutectoid steel, indicating the cooling path for the formation of bainite.
•Plain carbon eutectoid steel quenched from austenitic region to intermediate temperature (250° C to 550° C) then isothermally transformed (higher cooling rates than for Pearlite)•Mixture of ferrite and cementite phases•Very fine microstructure•Stronger and more ductile than Pearlitic steel of the same hardness
Transformation of Austenite to Bainite
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981
Formed from 350° C to 550° CCementite in form of rods
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981
Upper Bainite
Formed from 250° C to 350° CLow diffusion rates produceiron carbide precipitates withinferrite plates -- predominantly along single orientation (55° tolongitudinal axis of ferrite)
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981
Lower Bainite
Transformation of Austenite to MartensitePlain carbon eutectoid steel (Fe - 0.8% C)
cooled rapidly from austenitic region (miss nose of IT curve)martensite formed below 220° Csupersaturated solid solution of carbon in alpha ferrite
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981
1. Various microstructures occur depending on Carbon content of steel~0.2 wt% C well-defined laths of martensite~0.6 wt% C plates of martensite form, mixed with laths~1.2 wt% C well-defined plates of martensite
2. Martensitic transformation is diffusionless (no time for atoms to intermix)
3. No compositional change to parent phase (relative position of carbon atoms with respect to iron atoms identical to austenite parent)
4. Crystal structure changes from BCC to body centered tetragonal as carbon content increases (solid solubility difference of C in FCC austenite and BCC ferrite)
5. Martensitic transformation begins at definite temperature
6. In higher carbon steels, martensitic plates are formed by displacive shearliketransformation which causes a shape deformation on a flat surface
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981
Characteristics of Martensitic Transformation in Plain Carbon Steels
Martensitic Transformation Temperature
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981
Microstructure of Martensitic Transformation in Plain Carbon Steels
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981
~0.6%C ~0.2%C ~1.2%C
Microstructure of Martensite
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Diffusionless, and independent of time
Critical cooling rate is minimum rate of quench to produce 100%martensite
Alloy elements (chromium, nickel, molybdenum, manganese, silicon and tungsten) reduce critical cooling rate by retarding formation of pearlite
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
MartensiticTransformation
Microstructure Determination Example
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Microstructure Determination Example
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
• As quenched, martensite is brittle with significant internal stresses• Ductility and toughness ofmartensite enhanced by tempering (heating to temperature beloweutectoid for a specified time)• Produces extremely small, uniformly dispersed cementite in uniform ferrite(similar tospheroidite, but much smaller)• Nearly as strong as martensite, with much better ductility and toughness• Increased cementite particle size results in softer more ductile material (larger particles produced at higher tempering temperature)
TemperedMartensite
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Hardness of tempered martensite decreases, corresponding to the growth and coalescence of cementite particles
As temperature approaches eutectoid (700° C) and after several hours, microstructure will be spheroiditic with large cementite spheroids. Thusovertempering will produce soft and ductile materialWilliam D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Hardness Varies With Tempering
Time
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981
Microstructure of Tempered Martensite
Material exposed to elevated temperature for extended period of time, and then slowly cooled.
Used to relieve stresses, increase softness, ductility and toughness, or to produce specific microstructure
Annealing:
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Used to refine grains and produce more uniform distribution in steels which have been plastically deformed (e.g. rolling) resulting in tougher steel.
Complete transformation to austenite, then air cooled to a fine pearlite
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Normalizing:
Transformation to austenite, then furnace cooled to coarse pearlite(relatively soft and ductile material)
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Full Annealing:
Material heated just below eutectoid, allowing cementite to form in spheroid particles (maximum softness and ductility)
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Spheroidizing:
A: Slow cooling in furnace -- lamellar coarse pearlite
B: Cooling in still air -- fine pearlite
C: Split transformation -- fine pearlite and martensite
D: Rapid cooling -- martensite
E: Critical cooling rate -- slowest rate to produce no
pearlite (full martensite)
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981
Microstructure Variations due to Cooling Rates
Jominy end quench test: measure hardenability, or depth to which hardness penetrates (material in standard shape and size austenized at given temperature for given time, then cooled to room temperature from one end only to produce differential cooling)
Hardenability: ability of alloy to form martensite
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Typical Hardenability Jominy Test Data
Hardness can be related to cooling rate, rather than to
location from quenched end in
Jominy test.
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Nonferrous alloys, stainless steels
Strength and hardness improved due to small particles of new phase formed within original phase
Solution Heat Treating: solute atoms dissolved to form single phase, then rapidly cooled to nonequilibrium position ofsupersaturation
Precipitation Heat Treating: intermediate temperature allows diffusion of supersaturated atoms, forming fine dispersed particles of second phase
Precipitation Hardening:
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Mechanism of Hardening
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Mechanism of Hardening
Typical Mechanical Properties of Precipitation Hardened Alloys
Increase in strength corresponds to reduced
ductility
Strengthening process accelerated as temperature
increases
William D. Callister, Jr. Materials Science and Engineering, An Introduction. John Wiley & Sons, Inc. 1985
Outline of Heat Treatment Processes for Surface Hardening
See Table 4.1 in text (Serope Kalpakjian. Manufacturing Engineering and Technology, 3rd Edition. Addison-Wesley Publishing Co. 1995)
Case Hardening: (alteration of surface properties)- heat treatment in alloying element rich environment
The outer layer of this steel gear was selectively hardened by a high temperature heat treatment during which carbon from the surrounding atmosphere diffused into the surface. The “case” appears as the dark outer rim of the sectioned segment.
(Surface Division Midland-Ross)
Case Hardening
Modified quench to minimize distortion of heat treated steel1) austenitize steel2) quench in hot oil or molten salt just above martensite start temperature3) hold in quenching medium until uniform temperature in steel (but beforeaustenite to bainite transformation begins)4) cool at moderate rate to prevent thermal gradients====> usually, parts are later tempered to toughen steel
Martempering (Marquenching)
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981
Reduced thermal gradients
Reduced residual stresses
1) Austenitize steel2) Quench in hot salt bath just above martensitic temperature3) Isothermal hold4) Cooled to room temperature in air • Isothermal heat treatment
process to produce Bainite• Alternative to quenching and tempering• Improved ductility and impact strength for particular hardness• Decreased cracking and distortion quenching• Particularly advantageous for thin sections (<3/8"){thicker sections have non-uniform properties due to different cooling rates}
Austempering
William F. Smith. Structure and Properties of Engineering Alloys. McGraw-Hill Publishing Co. 1981