Heat Treatment of Steel 1 St… · Resulting microstructure: Equiaxed grains, relatively coarse equilibrium microstructure. Heat treatment process: For hypo-eutectoid steel heat about

Mechanical Engineering Department Materials Engineering 2 Laboratory

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Heat Treatment of Steel 1

Content 1 Why Study Phase Transformations in Steel? ........................................................................................... 2

2 Heat Treatments for Steel ........................................................................................................................ 2

2.1 Bases of a Heat Treatment ............................................................................................................... 2

2.2 7 Basic Heat Treatments for Steel .................................................................................................... 3

2.3 Process Annealing ............................................................................................................................. 3

2.4 Spheroidizing .................................................................................................................................... 3

2.5 Full Annealing ................................................................................................................................... 4

2.6 Normalizing ....................................................................................................................................... 6

2.7 Properties of Annealed vs. Normalized Carbon Steels ..................................................................... 6

2.8 Hardening ......................................................................................................................................... 6

2.9 Hardness Gain by Quenching ........................................................................................................... 7

2.10 Tempering......................................................................................................................................... 9

2.11 Case Hardening ............................................................................................................................... 10

2.12 Martempering ................................................................................................................................ 11

2.13 Austempering ................................................................................................................................. 11

2.14 Cryogenic treatments ..................................................................................................................... 13

3 Procedure ............................................................................................................................................... 13

Appendix A: Isothermal Transformation Diagrams ..................................................................................... 16

Appendix B: Continuous Cooling Transformation Diagrams ....................................................................... 18

1 Adapted from: Callister, W.; Engineering and Science of Materials, Wiley and Sons, 1998; and other sources.


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1 Why Study Phase Transformations in Steel? The development of a set of desirable mechanical characteristics for a material often results from a phase transformation that is accomplished by a heat treatment. The time and temperature dependencies of some phase transformations are conveniently represented on modified phase diagrams. It is important to know how to use these diagrams in order to design a heat treatment for some alloy that will yield the desired room temperature mechanical properties. For example, the tensile strength of an iron–carbon alloy of eutectoid composition (0.76 wt% C) can be varied between approximately 700 MPa (100 ksi) and 2,000 MPa (300 ksi) depending on the heat treatment employed.

One reason for the versatility of metallic materials lies in the wide range of mechanical properties they possess, which are accessible by various means. Four strengthening mechanisms were discussed in earlier course work, namely grain size refinement, solid-solution strengthening, cold work strengthening, and secondary phase dispersion strengthening. Additional techniques are available wherein the mechanical properties are reliant on the presence and morphology of a given microstructure.

The development of microstructure in both single- and two-phase alloys ordinarily involves some type of phase transformation— an alteration in the number and/or character of the phases.

The present discussion relies on a good understanding of the basic principles relating to the solid state phase transformations. Inasmuch as most phase transformations do not occur instantaneously, consideration has to be given to the dependence of reaction progress on time, or the transformation rate. Here, the discussion will focus on the development of two-phase microstructures for iron–carbon alloys, and the transformation diagrams that enable the determination of the microstructure that results from a specific heat treatment.

2 Heat Treatments for Steel

2.1 Bases of a Heat Treatment

A heat treatment is a sequence of controlled warming, heating, holding and cooling (Figure 1) which seeks to modify the microstructural constitution and its morphology. During a heat treatment every variable is controlled; i.e. nothing is left to chance. Each step (heating, holding, and cooling) is defined by the following:

Initial and final temperature

Rate of temperature change

Time at temperature

Warning: In steel, the starting microstructure has to be austenite, which should be homogeneous and uniformly heated.

Figure 1: Typical heat treatment sequence of warming, heating, holding and cooling.

Time

Tem

pera

ture

g homogeneous


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2.2 7 Basic Heat Treatments for Steel

a) Annealing ≡ Recocido a. Process annealing b. Spheroidizing c. Full annealing

b) Normalizing ≡ Normalizado

c) Hardening ≡ Endurecido a. Hardening b. Case Hardening

d) Tempering ≡ Revenido e) Martempering ≡ Martemperizado f) Austempering ≡ Austemperizado g) Cryogenic treatment ≡ Tratamiento

Criogénico

2.3 Process Annealing

Name: Process annealing, intercritical annealing, stress-relief annealing or just stress relieving.

Objective: Relieving residual stresses product of prior manufacturing processes.

Resulting microstructure: No visible changes at the optical microscope, but a significant reduction in residual stress (see Figure 2).

Heat treatment process: Heat about 100⁰ C ~ 200⁰ C below A12 and hold; then cool in still air (Figure 3).

Figure 2: Details of the stress relive annealing heat treatment3.

2.4 Spheroidizing

Name: Spherodizing, spherodize annealing.

Objective: Improve the machinability of medium and high carbon steels.

Resulting microstructure: The morphology of the cementite, plates and/or continuous phase, is modified to small isolated sperulites in a ferrite matrix.

Heat treatment process: Heat about 50 ~ 100⁰ C below A12 and hold; then cool in still air (Figure 4).

2 A1: Eutectoid reaction line in the Fe-C equilibrium phase diagram. 3 Source: Metals Handbook, vol 4 Heat Treating, 9h Ed. ASM, USA, 1981.

Typical Residual Stress

Sources include forming

processes (stamping, forging

and rolling), machining,

welding, and severe heat

treatments, among others.

In practice you only reach the

relaxation stage of the

recrystallization process.

The process is heat activated and

can be modeled with the Larson-

Miller parameter:

P=T*(20 + log(t))


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Figure 3: Heat treatment process for stress relive annealing.

Figure 4: Heat treatment process for spherodizing4.

2.5 Full Annealing

Name: Full annealing, annealing.

Objective: Recrystallize the microstructure to "erase" the effects of prior cold work strain.

Resulting microstructure: Equiaxed grains, relatively coarse equilibrium microstructure.

Heat treatment process: For hypo-eutectoid steel heat about ~50⁰ C above A35 and hold; then cool in

furnace (Figure 5).

For hyper-eutectoid steel heat about ~50⁰ C above A12 and hold; then cool in

furnace (Figure 5).

4 Adapted from: D.R. Askeland & P.P. Phulé, The Science and Engineering of Materials, Intl. Student Ed. Thomson,

Canada, 2006. 5 A3: Austenite-Ferrite solvus line in the Fe-C equilibrium phase diagram.

Time

Te

mp

era

ture

A1

Slow In Air

Time

Tem

pera

ture

A1

Lento al Aire

Typical spheroidite of a

eutectoid steel (1080)

Slow In Air


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Figure 5: Heat treatment process for full annealing6.

Figure 6: Heat treatment process for normalizing7.

6 Source: Metals Handbook, vol 9 Metallography and Microstructure, 9h Ed. ASM, USA, 1981. 7 Source: Metals Handbook, vol 9 Metallography and Microstructure, 9h Ed. ASM, USA, 1981.

Time

Tem

pera

ture

A1

Slow In Furnace

A3

Hypo-eutectoids

Hyper-eutectoids

30% cold rolled

low carbon steel

Same steel

after full annealing

Time

Te

mp

era

ture

A3

Slow in Air

ACM

Hypo-eutectoids

Hyper-eutectoids

Fully annealed low

carbon steel (1008)

Same steel

normalized


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2.6 Normalizing

Name: Normalizing.

Objective: Recrystallize the microstructure to "erase" the effects of prior cold work and/or prior heat treatments.

"Normalizes" the microstructure for subsequent heat treatments; it is the best starting point for hardening.

Resulting microstructure: Equilibrium microstructure of equiaxed grain with relatively fine scale.

Heat treatment process: For hypo-eutectoid steel heat about ~50⁰ C above A36 and hold; then cool in

still air (Figure 6).

For hyper-eutectoid steel heat about ~50⁰ C above Acm8 and hold; then cool in

still air (Figure 6).

2.7 Properties of Annealed vs. Normalized Carbon Steels

The strength of normalized steels is significantly higher than that of annealed ones. However, the ductility and toughness are better for the annealed microstructures. Good ductility and toughness requiring mobility of dislocations, while resistance and hardness require the opposite. Figure 7 summarized these trends for the spectrum of carbon content in commercially available plain carbon steels. Although hardness of steel is not shown in Figure 7, it follow the same trend as the tensile strength.

Figure 7: Typical mechanical properties of annealed and normalized carbon steels9.

2.8 Hardening

Name: Hardening, hardening by quenching.

(Simply calling it quenching, although wrong, is the industrial practice).

Objective: Increase strength and / or hardness of steel to optimize mechanical properties.

Resulting microstructure: A supersaturated solid solution consisting of 100% martensite (see figure 8).

8 Acm: Austenite-Cementite solvus line in the Fe-C equilibrium phase diagram. 9 Adapted from: D.R. Askeland & P.P. Phulé, The Science and Engineering of Materials, Intl. Student Ed. Thomson,

Canada, 2006.

0

20

40

60

80

100

120

140

160

180

200

220

0

100

200

300

400

500

600

700

800

900

1,000

1,100

0.0% 0.2% 0.4% 0.6% 0.8% 1.0%

Elo

nga

tio

n (

%)

Imp

act

Ener

gy (

N-m

)

Stre

ngt

h (

MPa

)

Carbon Content

Elongation (50 mm)

Tensile Strength

Impact Energy

Normalized

Annealed


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Figure 8: Morphology of marteniste in as-quenched carbon steel10.

Notice retained austenite in all cases (lighter areas).

Heat treatment process: For hypo-eutectoid steel heat about ~50⁰ C above A36 and hold; then quenched

into a specific medium (Figure 9).

For hyper-eutectoid steel heat about ~50⁰ C above Acm8 and hold; then

quenched into a specific medium (Figure 9).

In both cases the quenching temperature to be reached has to be low enough to ensure the completion of the martensitic transformation (see Figure 10 for the temperature dependence on carbon content).

Figure 9: Heat treatment process for hardening by quenching.

2.9 Hardness Gain by Quenching

Hardness of a properly quenched plain carbon steel is only a function of carbon content, as seen in Figure 10. Figure 10 also shows the increase of hardness, from the fully annealed to the quenched condition, as a per cent increase. Interesting to note is the fact that the highest gain in hardness, about 260%, corresponds to steel containing around 0.4 wt% C. By combining this finding with the fact that ductility decreases with carbon content, industry has recognized that those steels designed to enter service in a hardened condition should contain between 0.3 and 0.5 wt% C.

10 Source: Metals Handbook, vol 9 Metallography and Microstructure, 9h Ed. ASM, USA, 1981.

Time

Tem

pera

ture

A3

Lento In specific medium

ACM

Hypo-eutectoids

Hyper-eutectoids


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Figure 9: Martensitic transformation temperature dependence

on carbon content for carbon steels11.

Figure 10: Hardness gain after properly Figure 11. Incomplete quenching plain carbon steels12. austenitemartensite transformation13.

When the martensite transformation process is incomplete, i.e. the Mf temperature has not been reached, gamma will remain in the microstructure as “retained austenite” (see Figure 11), which appears as small “blocky islands" with sharp edges. This phase is metastable and will eventually transform. However, it is soft and will lower strength and toughness. Furthermore, upon transformation it will slightly expand, therefor distorting components in the range of x/1,000”, well in the scope of fits and tolerances of common mechanical components.

11 Adapted from: D.S. Clark & W.R. Varney, Physical Metallurgy for Engineers, van Nostrand, NY, 1962. 12 Source: D.S. Clark & W.R. Varney, Physical Metallurgy for Engineers, van Nostrand, NY, 1962. 13 Source: Metals Handbook, vol 9 Metallography and Microstructure, 9h Ed. ASM, USA, 1981.

-100

-50

0

50

100

150

200

250

300

350

400

450

500

550

600

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Tem

pera

ture

(⁰

C)

Carbon Content (wt%)

0

20

40

60

80

100

120

140

160

0.0 0.2 0.4 0.6 0.8 1.0

Tem

pera

ture

belo

w M

s(⁰

C)

Carbon Content (wt%)

Ms

Mf

1% Martensite

99% Martensite

90% Martensite

50% Martensite

10% Martensite


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2.10 Tempering

Name: Tempering

Objective: Reduce residual stresses resulting from the quenching and martensitic transformation process, and restore toughness and ductility of hardened steel.

Resulting microstructure: A matrix consistent of martensite (supersaturated solid solution) and a fine particle dispersion of ε carbide14 (see Figure 12).

Heat treatment process: After properly hardening by quenching, heat about 150~350⁰ C below A12 and

hold; then cool in still air (Figure 13).

Figure 12: Tempered martensite in a hardened and tempered medium carbon steel15.

Figure 13: Heat treatment process for tempering after hardening by quenching.

14 ε-carbide is a transition iron carbide with a chemical formula between Fe2C and Fe3C. 15 Source: Metals Handbook, vol 9 Metallography and Microstructure, 9h Ed. ASM, USA, 1981.

The image appears dark because

light in the optical microscope is

reflected by the carbides that

protrude from the surface by the

etching effect.

One can clearly see the ε carbides

in "high relief“ in this scanning

electron microscope image.

Time

Tem

pera

ture

A1

moderate indistinct


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Properly quenching and tempering a medium carbon steel, plain or low alloy, has a significant effect on the mechanical behavior of the material. Note in Figure 14 that the tensile strength of an AISI-SAE 1045 steel, when quenched and tempered (Q&T), is approximately 25% higher, and the loss in ductility is about half of the normalized condition. Additionally, experience shows that the fatigue behavior of the Q&T condition is always superior.

Figure 14: Mechanical Behavior of an AISI-SAE 1045

medium carbon steel after different heat treatments16.

2.11 Case Hardening

Name: Case Hardening

Objective: Only harden the surface of the steel to improve wear resistance without losing toughness at the core of the component.

Resulting microstructure: Dual: tempered martensite on the surface and “other structures” a few thousandths of an inch below (see Figure 15).

Heat treatment process: Several options are available

a) Flame Hardening: heating, by means of an open flame, the surface of a part into the homogenous austenite region; then quench.

b) Induction Hardening: heating, by means of an induction coil, the surface of a part into the homogenous austenite region; then quench.

c) Carburizing: If the level of carbon in the steel is not enough to produce martensite, you can first carburize or carbo-nitride the surface before the heat treatment.

d) Ion-nitriding: hardens the surface without subsequent heat treatment (Figure 16).

16 Source: TechnoForgeIndia; http://www.technoforgeindia.com/Quality_clip_image012.jpg, 2008.


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e) If the steel is hardenable17 enough, cooling in still air will suffice to produce 100% martensite. Residual heat in the core of the part can temper the martesinte properly; otherwise, a normal tempering process follows the hardening stage.

Figure 15: 0.4% C steel induction hardened, Figure 16: 0.7% C steel double-nitrided quenched in water and tempered for 1 hr at 525°C for 9.5 hr (surface hardness: ~ 58 HRC)18. (surface hardness: ~65 HRC)19.

2.12 Martempering

Name: Martempering

Objective: Hardening of steels with high propensity to crack by thermal shock.

Resulting microstructure: Tempered martensite and lower bainite (the retained austenite tends to transform into lower bainite)

Heat treatment process: Homogenize and quench into salt bath to control martensite formation followed by normal tempering (see Figure 17).

2.13 Austempering

Name: Austempering

Objective: Hardening of medium carbon steels while generating excellent toughness.

Resulting microstructure: Middle and/or lower bainite.

Heat treatment process: Homogenize and quench into isothermal salt bath at bainite formation temperatures (see Figure 18). The process is typically very long, therefore it is only used in particular applications.

17 Hardenability of machine steels will be addressed elsewhere. 18 Source: D.S. Clark & W.R. Varney, Physical Metallurgy for Engineers, van Nostrand, NY, 1962. 19 Source: Metals Handbook, vol 4 Heat Treatment, 9h Ed. ASM, USA, 1981.


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Figure 17: Martempering process is used to harden steels

with high propensity to crack by thermal shock20.

Figure 18: Austempering process is used to harden steels

while generating excellent toughness19.

20 Adapted from: ASM Handbook, vol 4: Heat Treating, 9h Ed. ASM International, USA, 1991.

Eutectoid Temperature

Ms

M50%

Mf

Surface

Center

Slow, homogenous martensiteformation

Tempering


Ms

M50%

Mf

Surface

Center Slow, homogenous bainite formation


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2.14 Cryogenic treatments

Name: Cryogenic treatments, cold tempering, cold treatment, cryogenic tempering.

Objective: Fundamentally, transform all austenite into martensite.

But there are other applications where it is not entirely clear what is being achieved, beyond excellent improvement in wear resistance.

Heat treatment process: Several commercially available process have been tried, including:

Quench - double temper - cryogenic treatment

Quench - cryogenic treatment - temper

3 Procedure 1) Preparation:

a. Set one box furnace to 850⁰ C b. The instructor will explain the general procedure for the session. c. Mark each AISI-SAE 434021 Charpy impact test sample as not to confuse them. d. Take the initial hardness of each of the 4 samples (at least 3 replicas).

2) Heat treatment: a. Place all samples at 850⁰ C for 30 minutes. b. Cool the different samples as follows:

i. Quench into still water ii. Quench into agitated oil

iii. Quench into still oil iv. Cool in still air

21 Adapted from: ASM Handbook, vol 4: Heat Treating, 9h Ed. ASM International, USA, 1991.

Table 1: Properties of AISI-SAE 4340 low alloy steel in different metallurgical conditions.

AISI-SAE

Grade Condition

Tensile Strength Yield Strength Elong.

Red. in Area

Hard-ness

Izod Impact Energy

MPa ksi MPa ksi % % HBN J ft-lb

4340

Normalized at 870⁰ C (1,600⁰ F)

1,280 186 860 125 12 36 363 16 12

Annealed at 810⁰ C (1,490⁰ F)

745 108 475 69 22 50 217 52 38

Oil quenched from 830-845° C (1,525-1,575 °F) Hardness HRC after tempering for 2 hrs at:

⁰ C 204 260 316 371 427 482 538 593 649

⁰ F 400 500 600 700 800 900 1,000 1,100 1,200

HRC 55 52 50 48 45 42 39 34 31


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3) Property measurement: a. Take the hardness of each sample (at least 3 replicas). b. Preform an impact energy test on each sample according to the procedure detailed in

the following section. 4) Impact Test Procedure

The Charpy pendulum impact test is VERY DANGEROUS. The safety measures in the operation of the machine must be strictly adhered to. The following points should be observed:

• Always operate the machine between 2 persons (never alone). One person manipulates the specimens and the other operates the impact machine.

• Always place the protection guard in the correct position. • Always warn before releasing the pendulum with the established protocol; that is, the

operator of the pendulum says aloud: - all hands out!, and never operate the pendulum pin lever until all the others present clearly answer: - hands out!

• Always use gloves and tongs to manipulate the specimens; even those at room temperature.

• Never touch cold media (dry ice: solid CO₂) or hot media (muffles and ovens). • Samples have to measured in advance, before placing them into the temperature

conditioning media.

Test execution:

a. The machine operator: i. Verify that the anvil is free of debris and releases the machine break.

ii. Rises the pendulum and secures it in its ratchet. iii. Move the energy pointer to its starting position (the maximum capacity of the

machine). b. The sample manipulator:

i. Take a sample using the tongs and place it on the anvil of the machine. a) It is important to monitor that the notch on the specimen should be

centered and "looks" in favor of the direction of the impact. c. The machine operator:

i. Following the established protocol, the operator of the pendulum says aloud: - all hands out!, and waits until all the others present clearly answer: - hands out!

ii. Only then he release the pendulum using the ratchet lever (the pendulum drops and hits the specimen).

iii. When the pendulum starts its descent (after impacting the specimen), applies the brake of the machine.

iv. Takes the reading of the machine indicator. v. Records the energy reading in the Charpy Impact Test Record.

d. The sample manipulator: i. Collects the sample pieces either from the cage or the anvil of the machine,

and places them on the measurement table. ii. Takes the measurement of the flat shear area of the fracture surface by

means of a Vernier caliper. iii. Takes the measurement of the lateral expansion by means of a Vernier

caliper. iv. Records the readings in the Charpy Impact Test Record. v. Identifies and sets the samples aside for later reference.


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5) Plot the results into the provided chart. 6) Analyze and discuss your hardness and impact energy results contrasting with the provided

metallographic images and stress-strain curves of an AISI-SAE 4340 heat treated as per the experiment just finished.


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Appendix A: Isothermal Transformation Diagrams Consider the iron–iron carbide eutectoid reaction:

30.76 % 0.022 % 6.67 %cooling

heatingwt C wt C Fe C wt Cg (A0)

which is fundamental to the development of microstructure in steel alloys. Upon cooling, austenite, having an intermediate carbon concentration, transforms to a ferrite phase, having a much lower carbon content, and cementite (Fe3C), with a much higher carbon concentration. Pearlite is one microstructural product of this transformation (Figure A1). The mechanism of pearlite formation is analogous to the eutectic ledeburite formation, and is discussed elsewhere.

Temperature plays an important role in the rate of the austenite-to-pearlite transformation. The temperature dependence for an iron–carbon alloy of eutectoid composition is indicated in Figure A2a, which plots S-shaped curves of the percentage transformation vs. the logarithm of time at 3 different temperatures. For each curve, data were collected after rapidly cooling a specimen composed of 100% austenite to the temperature indicated; that temperature was maintained constant throughout the course of the reaction. A more convenient way of representing both the time and temperature dependence of this transformation is in the Figure A2b.

In a Time-Temperature-Transformation (TTT) diagram (Figure A2 b) the vertical and horizontal axes are, respectively, temperature and the logarithm of time. Two solid curves plotted represents the time required at each temperature for the initiation or start of the transformation; the other is for the transformation conclusion. The dashed curve corresponds to 50% of transformation completion.

As can be recognized in Figure A2, the transformation rate at some particular temperature is inversely proportional to the time required for the reaction to proceed to 50% completion. That is, the shorter this time, the higher the rate. Thus, at temperatures just below the eutectoid reaction line (corresponding to just a slight degree of undercooling) very long times (on the order of 105~6 s) are required for the 50% transformation, and therefore the reaction rate is very slow. The transformation rate increases with decreasing temperature such that at 540 ⁰C (1,000⁰ F) only about 2~3 s are required for the reaction to go to 50% completion.

Several constraints are imposed on using TTT diagrams like the one in Figure A2b. First, each alloy has a particular diagram, similar in shape, but showing different times and temperatures. In addition, these plots are accurate only for transformations in which the temperature of the alloy is held constant throughout the duration of the reaction. Conditions of constant temperature are termed isothermal; thus, such plots are referred to as isothermal transformation diagrams, or sometimes as isothermal Time-Temperature-Transformation (TTT) plots.

Very significant to notice is that, although the constitution of the alloy may be similar in all cases (except the very rapidly quenched one), the specific morphology of the microstructures obtained depend heavily on the transformation temperature. Figure A3 illustrates such morphological changes for a eutectoid steel.

When transformation of a eutectoid carbon steel (0.76 wt% C) occurs above the critical temperature (that corresponding to the “nose” of the TTT cure) perlite will form; an alternating lamellar structure of ferrite and cementite. When the transformation occurs below the critical temperature, the structure formed is also dual phase, ferrite and cementite, but in this case it is an array of feathery o needle like structures called bainite.

Figure A1: Typical perlite microstructure in carbon

steel.


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When steel is cooled at very high rates to a temperature where diffusion is no more possible, the nature of the transformation changes to one characterized only by small atomic displacements controlled by temperature alone. Such displacive transformations form a single phase termed martensite, characterized by being a highly stressed supersaturated solid solution.

Figure A2 a): Isothermal fraction transformed vs. time for the austenite-to-pearlite

transformation for a eutectoid iron–carbon alloy

(0.76 wt% C).

Figure A2 b): Demonstration of how an isothermal Time-

Temperature-Transformation (TTT) diagram is generated

from fraction transformed vs. time data.

Figure A3: The expected microstructures of an

isothermally transformed eutectoid steel (0.76 wt% C)22.

22 Adapted from: D.S. Clark & W.R. Varney, Physical Metalurgy for Engineers, van Nostrand, NY, 1962.

a)

b)

Coarse Perlite

Fine Perlite

Upper Bainite

Lower Bainite

Martensite

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 1 10 100 1,000 10,000

Au

sten

ite

to P

erlit

e Tr

ansf

orm

atio

n

Time (s)

600⁰ C 650⁰ C 675⁰ C

300

350

400

450

500

550

600

650

700

750

800

0 1 10 100 1,000 10,000

Tem

per

atu

re (

⁰C)

Time (s)

~0% 50% ~100% Eutectoid Temp.


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Appendix B: Continuous Cooling Transformation Diagrams Isothermal heat treatments are not the most practical to conduct because an alloy must be rapidly cooled from a high temperature23 to- and maintained at an elevated temperature. This is only accomplished when the samples are small and cooled into a salt bath. Therefore, most heat treatments for steels involve the continuous cooling of a specimen to room temperature. An isothermal transformation diagram (TTT) is valid only for conditions of constant temperature; hence, this diagram must be modified for transformations that occur as the temperature is constantly changing.

For continuous cooling, the time required for a reaction to begin and end is delayed. Thus the isothermal curves are shifted to longer times and lower temperatures, as indicated in Figure B1 for an iron–carbon alloy of eutectoid composition (0.76 wt% C). A plot containing such modified beginning and ending reaction curves is thus termed a continuous cooling transformation (CCT) diagram. Note that Figure B1 shows both diagrams, TTT and CCT superimposed.

Figure B1: Superimposition of isothermal (dashed line) and continuous cooling (solid line) transformation

diagrams for a eutectoid iron–carbon alloy (0.76 wt% C).

Normally, bainite will not form when an alloy of eutectoid composition or, for that matter, any plain carbon steel is continuously cooled to room temperature. This is because all the austenite will have transformed to pearlite by the time the bainite transformation has become possible. Thus, the region representing the

23 In any case, the starting temperature must correspond to a homogeneous equilibrium single phase structure. In

the case of a steel, this temperature has to correspond to a 100% austenite constitution.

0

100

200

300

400

500

600

700

800

0 1 10 100 1,000 10,000 100,000

Tem

per

atu

re (

⁰C)

Time (s)


Continuous Cooling Transformation

Constant Temp. Transformation

Ms

M50%

Mf

Ms: 1% martensite; M50%: 50% martensite; Mf: 99% martensite

A

B


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austenitepearlite transformation terminates just below the nose as indicated by the curve A-B in Figure B1.

Carbon and other alloying elements will shift the pearlite (as well as the proeutectoid ferrite phase) and bainite noses to longer times, thus decreasing the critical cooling rate. In fact, one of the reasons for alloying steels is to facilitate the formation of martensite so that totally martensitic structures can develop in relatively thick cross sections. Figure B2 represents the CCT diagram for an AISI-SAE 4340 low alloy steel (see Table B1).

Figure B2: Continuous cooling transformation diagram for an AISI-SAE 4340 low alloy steel showing several superimposed cooling curves and the resulting microstructural phases24.

24 Source: Callister, W.; Engineering and Science of Materials, Wiley and Sons, 1998

A B


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Cooling curves at different rates are superimposed and labeled as to explore the possible microstructures in this alloy. The transformation starts after a time period corresponding to the intersection of the cooling curve with the beginning reaction curve (nucleation of the new phase) and concludes upon crossing the completion transformation curve (growth of the new phase).

The microstructural products for the slow cooling (0.006⁰ C/s) and moderately rapid cooling (0.02⁰ C/s) rates in Figure B2 are fine and coarse pearlite, respectively. For any cooling curve passing through AB in Figure B2, the transformation ceases at the point of intersection. With continued cooling, the unreacted austenite begins transforming into bainite. Any remaining austenite will transform into martensite upon crossing the Ms line.

Table B1: Chemical composition of the AISI-SAE 4340 low alloy steel.

Element Content (wt%)

Carbon, C 0.370 - 0.430

Chromium, Cr 0.700 - 0.900

Manganese, Mn 0.600 - 0.800

Molybdenum, Mo 0.200 - 0.300

Nickel, Ni 1.65 - 2.00

Silicon, Si 0.150 - 0.300

Phosphorous, P 0.035 max

Sulfur, S 0.040 max

Iron, Fe 95.20 - 96.26

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Heat Treatment of Steel 1 St… · Resulting microstructure: Equiaxed grains, relatively coarse equilibrium microstructure. Heat treatment process: For hypo-eutectoid steel heat about