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MME444 Heat Treatment Sessional

Week 02 – 04

Heat Treatment of Steels

Prof. A.K.M.B. RashidDepartment of MMEBUET, Dhaka

Purposes of annealing Refining grains Inducing ductility, toughness, softness Improving electrical and magnetic properties Improving machinability Relieve residual stresses

Purposes of normalising Modifying and refining cast dendritic structure Refining grains and homogenising the structure Inducing toughness Improving machinability

Purposes of hardening Improving hardness

Improving wear resistance

Purpose of tempering Relieving residual stresses Improving ductility and

toughness (at the sacrifice of some hardness or strength)

Common Heat Treatment of Steels

1. Annealing 3. Hardening2. Normalising 4. Tempering

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An unalloyed steel tool used for machining aluminum automobile wheels has been found to work well, but the purchase records have been lost and you do not know the steel’s composition. The microstructure of the steel is tempered martensite, and assume that you cannot estimate the composition of the steel from the structure.

Design a treatment that may help determine the steel’s carbon content.

Example 12.1

Design of a Method to Determine AISI Number

Example 12.1 SOLUTION

The first way is to heat the steel to a temperature just below the A1 temperature and hold for a long time. The steel overtempers and large Fe3C spheres form in a ferrite matrix. We then estimate the amount of ferrite and cementite and calculate the carbon content using the lever law. If we measure 16% Fe3C using this method, the carbon content is:

%086.1or 16100)0218.067.6(

)0218.0(CFe % 3 x

x

A better approach, however, is to heat the steel above the Acm to produce all austenite. If the steel then cools slowly (annealing), it transforms to pearlite and a primary microconstituent. If, when we do this, we estimate that the structure contains 95% pearlite and 5% primary Fe3C, then:

%065.1or 9510077.067.6

-6.67Pearlite % x

x

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Recommend temperatures for the process annealing, annealing, normalizing, and spheroidizing of 1020, 1077, and 10120 steels.

Example 12.2

Determination of Heat Treating Temperatures

Figure 12.4 Schematic summary of the simple heat treatments for (a) hypoeutectoid steels and (b) hypereutectoid steels.

Example 12.2 SOLUTION

From Figure 12.2, we find the critical A1, A3, or Acm, temperatures for each steel. We can then specify the heat treatment based on these temperatures.

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Design a heat treatment to produce the pearlite structure shown in Figure 11.16.

Example 11.8

Design of a Heat Treatment to Generate Pearlite Microstructure

Figure 11.16 Growth and structure of pearlite: photomicrograph of the pearlite lamellae ( 2000).

(From ASM Handbook, Vol. 7, (1972), ASM International, Materials Park, OH 44073.)

Example 11.8 SOLUTION

If we assume that the pearlite is formed by an isothermal transformation, we find from Figure 11.20 that the transformation temperature must have been about 675 oC.

Figure 11.20 The effect of the austenite transformation temperature on the interlamellar

spacing (in cm) of pearlite.

Interlamellar spacing of the pearlite:

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From the TTT diagram (Figure 11.21), our heat treatment must have been:

1. Heat the steel to about 750 oC and hold—perhaps for 1 h—to produce all austenite. A higher temperature may cause excessive growth of austenite grains.

2. Quench to 675 oC and hold for at least 103 s (the Pf time).

3. Cool to room temperature.

Figure 11.21 The time-temperature-transformation (TTT) diagram for an eutectoid steel.

Excellent combinations of hardness, strength, and toughness are obtained from bainite. One heat treatment facility austenitized an eutectoid steel at 750oC, quenched and held the steel at 250oC for 15 min, and finally permitted the steel to cool to room temperature. Was the required bainitic structure produced?

Example 11.9

Heat Treatment to Generate Bainite Microstructure

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After heating at 750oC, the

microstructure is 100% .

After quenching to 250oC,

unstable austenite remains for

slightly more than 100 s, when

fine bainite begins to grow.

After 15 min, or 900 s, about

50% fine bainite has formed

and the remainder of the steel

still contains unstable

austenite.

Thus, the heat treatment was

not successful !! The heat

treatment facility should have

held the steel at 250oC for at

least 104 s, or about 3 h.

Example 11.9 SOLUTION

A banitic structure can only be obtained during isothermal cooling of austenite, commonly known as austempering.

Figure 11.21 The time-temperature-transformation (TTT) diagram for an eutectoid steel.

A heat treatment is needed to produce a uniform microstructure and hardness of HRC 23 in a 1050 steel axle.

Example 12.3

Design of a Heat Treatment for an Axle

Figure 12.8 The TTT diagrams for a 1050 steel.

Figure 12.2 (a) The Fe-Fe3C phase diagram.

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Example 12.3 SOLUTION

1. Austenitize the steel at 770 + (30 to 55) = 805 oC to 825 oC, holding for 1 h and obtaining 100% .

2. Quench the steel to 600oC and hold for a minimum of 10 s. Primary ferrite begins to precipitate from the unstable austenite after about 1.0 s. After 1.5 s, pearlite begins to grow, and the austenite is completely transformed to ferrite and pearlite after about 10 s. After this treatment, the microconstituents present are:

%64100)0218.077.0(

0.0218)(0.5Pearlite

%36100)0218.077.0(

0.5)(0.77αPrimary

3. Cool in air-to-room temperature, preserving the equilibrium amounts of primary ferrite and pearlite. The microstructure and hardness are uniform because of the isothermal anneal.

Example 12.4

Design of a Quench and Temper TreatmentA rotating shaft that delivers power from an electric motor is made from a 1050 steel. Its yield strength should be at least 145,000 psi, yet it should also have at least 15% elongation in order to provide toughness. Design a heat treatment to produce this part.

Figure 12.8 The TTT diagrams for a 1050 steel.

Figure 12.11 The effect of tempering temperature on the mechanical properties of a 1050 steel.

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Example 12.4 SOLUTION

1. Austenitize above the A3 temperature of 770oC for 1 h. An appropriate temperature may be 770 + 55 = 825oC.

2. Quench rapidly to room temperature. Since the Mf is about 250oC, martensite will form.

3. Temper by heating the steel to 440oC. Normally, 1 h will be sufficient if the steel is not too thick.

4. Cool to room temperature.

Part 1: Design of a heat treatment cycle of a steel sample

Part 2: Conduct the heat treatment cycle

1. Analyse your steel sample to determine the carbon and other alloys, if any, contents.

2. Determine the approximate cooling rate and quenching medium required to obtain the desired properties. Finally select the heating temperature and holding time required and plot the heat treatment cycle of the process.

1. Once the heat treatment cycle is approved by the course tutor, conduct the heat treatment operation.

2. After heat treatment, prepare a metallographic sample from your heat treated steel sample and obtain micrographs in different magnifications.

3. Measure hardness of your heat treated sample in Rockwell C scale.

Week 2-4: Heat Treatment of Steels

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Student Group

Sample Description

DesiredProperties

1 AISI 1050The steel to be quenched and tempered to produce a minimum yield strength of 1000 MPa and a minimum of hardness VHN 40

2 AISI 1080The steel to be quenched and tempered to produce a structure having a tensile strength of at least 1050 MPa but a hardness below RC 40

3 AISI 1080Apply a suitable heat treatment to produce a structure containing pearlite and martensite

4 AISI 10120Apply a suitable heat treatment to produce a fully martensitic structure and then temper enough to have a hardness within the range of RC 50 – 55

5 AISI 10125Apply a suitable heat treatment to make the steel soft enough to be machined and have a hardness below RC 45

Work Schedule

Supplementary Tables and Figures

Ref: D. A. Askeland, The Science and Engineering of Materials,4th Ed., Chapman & Hall, 1988

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Figure 12.2 (a) The Fe-Fe3C phase diagram.

Figure 12.5 The effect of carbon and heat treatment on the properties of plain-carbon steels.

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Figure 12.4 Schematic summary of the simple heat treatments for (a) hypoeutectoid steels and (b) hypereutectoid steels.

Figure 11.19 The effect of interlamellar spacing (λ) of on the yield strength of pearlite.

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Figure 11.20 The effect of the austenite transformation temperature on the interlamellar spacing of pearlite.

Figure 12.13 Increasing carbon reduces the Ms and Mf temperatures in plain-carbon steels.

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Figure 11.21 The time-temperature-transformation (TTT) diagram for an eutectoid steel.

Figure 12.8 The TTT diagrams for a 1050 steel.

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Figure 12.8 The TTT diagrams for a 10110 steel.

Figure 12.16 The CCT diagram (solid lines) for a 1080 steel compared with the TTT diagram (dashed lines).

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Figure 12.17 The CCT diagram for a low-alloy, 0.2% C Steel.

Figure 12.11 The effect of tempering temperature on the mechanical propertiesof a 1050 steel.

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Figure 11.28 Effect of tempering temperature on the properties of and eutectoid steel.

Figure 12.14 Formation of quench cracks caused by residual stresses produced during quenching. The figure illustrates the development of stresses

as the austenite transforms to martensite during cooling.

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Figure 12.15 The marquenching heat treatment designed to reduce residual stresses ands quench cracking.