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