Heat Treatment

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Heat Treatment of

Steel Alloys

Heat Treatment of

Steel Alloys

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YELLOW ALERT

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Heat Treatment of Steel Alloys

Diffusive vs. Displacive Transformations of

Pure Iron (Fe)

Role of Dissolved Carbon in Fe

Transformations

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Diffusion Process

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Other diffusion

mechanisms

Other diffusion

mechanisms

Interstitial

diffusion

Grain

boundary

Surface

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Speed of the interface

Temperature

Diffusive Transformation of FCC to BCC in Pure Fe

Above 914° C pure Fe is face centered cubic (FCC).

Below 914° C the thermodynamically stable phase of pure Fe is body centered cubic (BCC).

Note that the speed of the “interface” in this transformation is zero at 914° C.

Why this shape?

Increasing driving force

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Nucleation in the Diffusive Transformation of f.c.c.-> b.c.c. in Pure Fe

Nucleation is very important

The more nuclei : The more Volume Transformed In a diffusive transformation: – Volume transforming per second increases

linearly with the number of

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Grain Boundary

Nucleation

Grain Boundary

Nucleation

The grain boundaries in the f.c.c. pure Fe

are the most common site for

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Homogeneous vs.

Heterogeneous Nucleation

Homogeneous vs.

Heterogeneous Nucleation

The critical radius, r*_het, of a

heterogeneous nucleus is much larger than the

critical radius, r*_hom, of a homogeneous nucleus of the same phase.

For the same critical radius the

heterogeneous

nucleus contains far fewer atoms.

Crystal radius

Absolute temperature

homogeneous

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Diffusive

Transformation of

f.c.c b.c.c. in Pure Fe

Diffusive

Transformation of

f.c.c b.c.c. in Pure Fe

The overall rate of transformation depends both on nucleation and growth

The semi-schematic diagram below shows that the rate of transformation starts below the equilibrium temperature, 914°C, and increases until approximately 700°C.

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Time-Temperature-Transformation

(TTT) Diagram

Time-Temperature-Transformation

(TTT) Diagram

The standard practice to display diffusive transformations is with the “Time-Temperature-Transformation” (TTT) diagram. It is also known as the “Isothermal-Transformation” diagram or “C-curve”. The TTT diagram for the

diffusive f.c.c.->b.c.c. transformation of pure Fe is shown at the right.

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The two curves are related

Consider the 1% transformation line (1% of the fcc to transform to bcc)

1) The transformation rate is zero both at 914 and –273 C so the time required for the transformation is infinite at these temperatures

2) The transformation rate is a maximum at 700 C so the time for the 1% transformation must be a minimum at 700 C

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Displacive Transformation of f.c.c. ->

b.c.c. in Pure Fe

Displacive Transformation of f.c.c. ->

b.c.c. in Pure Fe

If we quench f.c.c. Fe from 914°C at a rate of about 105_°Cs-1_{, we expect to}

prevent the diffusive The TTT diagram for the

diffusive f.c.c.->b.c.c. transformation from taking place.

In reality, below 550°C the Fe will transform to b.c.c. by a displacive

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Martensite Plates form in f.c.c. Lattice

The displacive transformation of f.c.c. -> b.c.c. in pure Fe is shown schematically.

Lens shaped crystals of b.c.c. Fe nucleate at the grain

boundaries of the f.c.c. Fe and grow out into the f.c.c. crystal.

The lens shaped crystals stop when they hit the next grain boundary.

This kind of transformation is called a Martensitic

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Martensite transformation

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Complete TTT Diagram for Pure Fe

The is shown below. The “M_s” stands for “Martensite

Start Temperature” and the “M_f” stands for “Martensite Finished Temperature”.

If a sample is cooled fast enough to prevent the diffusive transformation from taking place, then martensite will be formed as schematically shown at the left.

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Martensite Transformation in Steels

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The Martensite in Steel is Not Cubic

The crystal structure of 0.8% Carbon martensite is shown below.

To make room for the carbon atoms the lattice

stretches along on crystal direction. This produces a face centered tetragonal unit cell.

Note that only a small proportion of the labelled sites actually contain a carbon atom

.

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BCT formation

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Fe-C Interstitial Solid Solution in Austinite

The Carbon atoms fit into interstitial spaces in the FCC Austinite structure schematically shown below.

Note the distortion of the Fe atoms [0.258-nm diameter] around the Carbon atoms [0.154-nm diameter] since the voids are 0.104-nm diameter.

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Fe-C Interstitial Solid Solution in Ferrite & Martensite

The Carbon atoms cannot fit into interstitial spaces in the BCC ferrite structure like they can in the FCC Austinite and

produce a BCT ( schematically shown below).

Note in the BCT the Carbon atoms force the unit cell to be alongated in the c-direction. The largest interstitial void in BCC iron has a diameter of 0.072-nm.

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Isothermal Transformation

Experiments

Isothermal Transformation

Experiments

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An Example

(Assume a Eutectoid Low Carbon Steel)

An Example

(Assume a Eutectoid Low Carbon Steel)

(a) Water-quench to room Temperature.

(b) Hot-quench at 690°C & hold 2 hr; water-quench (c) Hot-quench at 610°C

& hold 3 min; water-quench (d) Hot-quench at 580°C & hold 2 sec; water-quench (e) Hot-quench at 450°C &

hold 1 hr; water-quench All martensite Pearlite Pearlite 50% pearlite + 50 martensite Bainite

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Another one...

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Formation of Bainite

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Perlite + Martensite

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Bainite + Martensite

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Martensite

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Hypoeutectoid Phase Diagram

If a steel with a composition x% carbon is cooled from the

Austenite region at about 770 °C ferrite begins to form. This is called proeutectoid (or pre-eutectoid) ferrite since it forms

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Hypoeutectoid Isothermal Transformation Curve

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Quenched & Tempered

Steel Alloys

Quenched & Tempered

Steel Alloys

Heat Treatment of Steel Alloys (Tempering)

Microstructure of Fe-C Martensites

Mechanical Properties of Fe-C Martensites

Microstructural Changes in Martensite with

Tempering

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Tempering

Tempering is the process of heating a martensitic steel at a temperature below the eutectoid transformation

temperature. This makes it “softer” and more “ductile”.

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Microstructure of Fe-C Martensites

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Mechanical Properties of Fe-C

Martensites

Mechanical Properties of Fe-C

Martensites

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Microstructural Changes in Martensite with Tempering Microstructural Changes in Martensite with Tempering

Martensite is a metastable structure, and it decomposes

when reheated.

In lath martensites of low-carbon plain-carbon steels

there is a high dislocation density, and these

dislocations provide lower energy sites for carbon

atoms than there regular interstitial positions. This

process can take place between 20° and 200°C.

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Microstructural Changes in

Martensite with Tempering

Microstructural Changes in

Martensite with Tempering

For martensitic plain-carbon steels with more than 0.2% carbon tempering produces Cementite, Fe₃C.

The shapes are diffenent at different temperatures. The important point is that the Fe matrix returns to its BCC form found in Ferrite. The electron micrographs below show the microstructure for two treatments.

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Variation of Hardness with

Tempering Treatment

Variation of Hardness with

Tempering Treatment

The curves below show the reduction of hardness for various treatments of a quenched low-carbon plain-carbon steel with 0.35% carbon.