HEAT TREATMENT

Pure Iron

• In the pure state it is a very soft grey metal

of no commercial use

Wrought Iron

• Has approx 0.05% carbon• Used since about 2000 BC• Is stronger than most other pure metals.• Made into weapons, armour, cooking pots

and vessels• Main limitation to wider uses due to

processing (no way of making large items and no welding)

Cast Iron

• Between 2% & 4% carbon content• Standard grey cast iron very brittle due to

carbon rosettes in the structure acting as stress-raisers

• Possible to use heat treatment to improve the structure, this gives materials such as ductile iron and malleable iron (black heart)

Ductile iron used in drain grids

Grey cast iron showing the graphite flakes in a pearlite matrix

Ferrite

The structure of pure iron. Has a body-centred cubic (BCC) crystal

structure.It is soft and ductile and imparts these

properties to the steel. Very little carbon (less than 0.01% carbon will dissolve in ferrite at room temperature). Often known as α iron.

Microstructures

α γ

A photomicrograph of 0.1% carbon steel (mild steel). The light areas

are ferrite.

AusteniteThis is the structure of iron at high temperatures

(over 912 deg C).

Has a face-centre cubic (FCC) crystal structure.

This material is important in that it is the structure from which other structures are formed when the material cools from elevated temperatures. Often known as γ iron. Not present at room temperatures.

MICROSTRUCTURE OF AUSTENITEMICROSTRUCTURE OF AUSTENITE

CementiteA compound of iron and carbon, iron carbide (Fe3C).

It is hard and brittle and its presence in steels causes an increase in hardness and a reduction in ductility and toughness.

Pearlite

A laminated structure formed of alternate layers of ferrite and cementite.

It combines the hardness and strength of cementite with the ductility of ferrite and is the key to the wide range of the properties of steels.

The laminar structure also acts as a barrier to crack movement as in composites. This gives it toughness.

Two-dimensional view of pearlite, consisting of alternating layers of

cementite and ferrite.

Microstructure of Pearlite

Photomicrographs of (a) coarse pearlite and (b) fine pearlite. 3000X

Three-dimensional analogy to the structure of pearlite, i.e. the cabbage represents a single crystal of pearlite, and the water in the bucket the single crystal of ferrite.

Martensite

A very hard needle-like structure of iron and carbon.

Only formed by very rapid cooling from the austenitic structure (i.e. above upper critical temperature). Needs to be modified by tempering before acceptable properties reached.

The needle-like structure of martensite, the white areas are retained austenite.

MICROSTRUCTURE OF MARTENSITEMICROSTRUCTURE OF MARTENSITE

Electron micrographs of (a) pearlite, (b) bainite, and (c) tempered martensite, illustrating the

differences in cementite

0.1% Carbon Steel

the small amount of pearlite in the structure

Applications

A typical application of low carbon steel in a car body.

Effect of Carbon Content

Increasing the carbon content decreases the amount of ferrite and increases the proportion of pearlite in the structure.

• Heat-Treatment Heat treatment is a method used to alter the

physical, and sometimes chemical properties of a material.

The most common application is metallurgical. It involves the use of heating or chilling, normally

to extreme temperatures, to achieve a desired

result such as hardening or softening of a material. It applies only to processes where the heating

and cooling are done for the specific purpose of

altering properties intentionally.

Process Annealing — Eliminating Cold Work: A low-temperature heat treatment used to eliminate all or part of the effect of cold working in steels.

Annealing and Normalizing — Dispersion Strengthening: Annealing - A heat treatment used to produce a soft, coarse pearlite in steel by austenitizing, then furnace cooling. Normalizing - A simple heat treatment obtained by austenitizing and air cooling to produce a fine pearlitic structure.

Spheroidizing — Improving Machinability: Spheroidite - A microconstituent containing coarse spheroidal cementite particles in a matrix of ferrite, permitting excellent machining characteristics in high-carbon steels.

• Figure shows the microstructure of spheroidite, with Fe3C particles dispersed in a ferrite matrix (× 850). (From ASM Handbook, Vol. 7, (1972), ASM International, Materials Park, OH 44073.)

• Figure showsthe Dark feathers of bainite surrounded by light martensite, obtained by interrupting the isothermal transformation process (× 1500). (ASM Handbook, Vol. 9 Metallography and Microstructure (1985), ASM International, Materials Park, OH 44073.)

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

Figure shows The set-up for the Jominy test used for determining the hardenability of a steel.

Isothermal Heat Treatments

Austempering - The isothermal heat treatment by which austenite transforms to bainite.

Isothermal annealing - Heat treatment of a steel by austenitizing, cooling rapidly to a temperature between the A1 and the nose of the TTT curve, and holding until the austenite transforms to pearlite.

Quench and Temper Heat Treatments

Retained austenite - Austenite that is unable to transform into martensite during quenching because of the volume expansion associated with the reaction.

Tempered martensite - The microconstituent of ferrite and cementite formed when martensite is tempered.

Quench cracks - Cracks that form at the surface of a steel during quenching due to tensile residual stresses that are produced because of the volume change that accompanies the austenite-to-martensite transformation.

Marquenching - Quenching austenite to a temperature just above the MS and holding until the temperature is equalized throughout the steel before further cooling to produce martensite.

T (Time) T(Temperature) T(Transformation) diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy. When austenite is cooled slowly to a temperature below LCT (Lower Critical Temperature), the structure that is formed is Pearlite. As the cooling rate increases, the pearlite transformation temperature gets lower. The microstructure of the material is significantly altered as the cooling rate increases. By heating and cooling a series of samples, the history of the austenite transformation may be recorded. TTT diagram indicates when a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved.

TTT DIAGRAMTTT DIAGRAM

TTT DIAGRAMTTT DIAGRAM

The TTT diagram is a kinetic (non equilibrium) phase diagram.

Martensite begins to form at 430 C, but does not complete to 100% at RT. Thus, there is retained Austenite left. Some C may deplete out into cementite at GB

AUSTENITE PEARLITE

As indicated when is cooled to temperatures below LCT, it transforms to other crystal structures due to its unstable nature. A specific cooling rate may be chosen so that the transformation of austenite can be 50 %, 100 % etc. If the cooling rate is very slow such as annealing process, the cooling curve passes through the entire transformation area and the end product of this the cooling process becomes 100% Pearlite. In other words, when slow cooling is applied, all the Austenite will transform to Pearlite. If the cooling curve passes through the middle of the transformation area, the end product is 50 % Austenite and 50 % Pearlite, which means that at certain cooling rates we can retain part of the Austenite, without transforming it into Pearlite.

Upper half of TTT Diagram(Austenite-Pearlite Transformation Area)

If a cooling rate is very high, the cooling curve will remain on the left hand side of the Transformation Start curve. In this case all Austenite will transform to Martensite. If there is no interruption in cooling the end product will be martensite.

Lower half of TTT Diagram (Austenite-Martensite and Bainite Transformation Areas)

Tempering, in metallurgy and engineering, low-temperature process in the heat treatment of steel by which a desirable balance is obtained between the hardness and toughness of the finished product. Steel articles that have been hardened by quenching, a process of heating to about 870° C (about 1600° F) and cooling rapidly in oil or water, become hard and brittle. Reheating to a lower temperature decreases the hardness somewhat but improves the toughness. The proper balance between hardness and toughness is controlled by the temperature to which the steel is reheated and the duration of the heating. This temperature is controlled by an instrument for measuring high temperatures, known as a pyrometer, or, historically, by observing the color of the oxide film formed on the metal during heating.

TEMPERINGTEMPERING

In materials science, quenching is the rapid cooling of a work piece to obtain certain material properties. It prevents low-temperature processes, such as phase transformations, from occurring by only providing a narrow window of time in which the reaction is both thermodynamically favorable and kinetically accessible. For instance, it can reduce crystallinity and thereby increase toughness of both alloys and plastics (produced through polymerization).

QUENCHINGQUENCHING

Continuous cooling transformation (CCT) diagram

There are two types of CCT diagrams

I) Plot of (for each type of transformation) transformation start, specific fraction of transformation and transformation finish temperature against transformation time on each cooling curve

II) Plot of (for each type of transformation) transformation start, specific fraction of transformation and transformation finish temperature against cooling rate or bar diameter for each type of cooling medium 53

Definition: Stability of phases during continuous cooling of austenite

Determination of CCT diagram type I

CCT diagrams are determined by measuring some physical properties during continuous cooling. Normally these are specific volume and magnetic permeability. However, the majority of the work has been done through specific volume change by dilatometric method. This method is supplemented by metallography and hardness measurement.

In dilatometry the test sample (Fig. 1) is austenitised in a specially designed furnace (Fig. 2) and then controlled cooled. Sample dilation is measured by dial gauge/sensor. Slowest cooling is controlled by furnace cooling but higher cooling rate can be controlled by gas quenching.

54

MF, Martensite finish temperature

M50,50% Martensite

Pearlite finish

MS, Martensite start temperature

Metastable austenite

Metastable austenite +martensite

Martensite

50% Transformation

Har

dnes

s, H

RC

Tem

pera

ture

Log time

Ae1

Austenite +pearlite

Austenite+upper bainite

to=Minimum incubation period at the nose of the TTT diagram, t’o=minimum incubation period at the nose of the CCT diagram

t0

Pearlite start

A

F ED

C

B

Distance from quench end

AB

CDEF

Jominy sample

Martensite Pearlite+MartensiteFine pearlitepearlite

Coarse pearlite

a

bc

d

Fig. 8: CCT diagram ( ) projected on TTT diagram ( ) of eutectoid steel

t’0

55

Upper Bainite Bainite Pearlite

Temperature vs time (sec, then min)

TTT diagram for isothermal transformation of steel W 1 (1% C)A = austenite,B = bainite, P = pearlite Ms = start of martensite transformation,M50 = 50% M,

Quenching in a liquid bath at 700°C; holding time 4 min. During this interval the C has separated out, partly as pearlite lamellae and partly as spheroidized cementite. Hardness 225 HV. Quenching to 575°C, holding time 4 s. A very fine, closely spaced pearlite as well as some bainite has formed. Note that the amount of spheroidized cementite is much less than in the preceding case. Hardness 380 HV. Quenching to 450°C, holding time 60 s. The structure consists mainly of bainite. Hardness 410 HV. Quenching to 20°C (room temperature). The matrix consists of, roughly, 93% martensite and 7% retained austenite. There is some 5% cementite as well which has not been included in the matrix figure. Hardness 850 HV.

The pearlite spacing goes as

v α λ-2

or

λ α 1/v2