thermal processing of metal

8/12/2019 Thermal Processing of Metal

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AL- FATIHAH


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Garden of Knowledge & Virtue

ENGINEERING MATERIALS/

MATERIALSSCIENCE & ENGINEERING

MME 2501 MME 2503

THERMAL PROCESSING OF METAL

2

Dr Zahurin Halim Sem 2 1112


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AS SABAK ( (Surah 34 Verse 10

3Dr Zahurin Halim Sem 2 1112

And indeed We bestowed grace on Daud from Us: O

mountains, glorify with him Allahspurity repeatedly - and

you too O birds! And We made the iron soft for him.

[Dan demi sesungguhnya, Kami telah memberikan kepada Nabi Daud limpahkurnia dari Kami (sambil Kami berfirman): "Hai gunung-ganang, ulang-ulangilahmengucap tasbih bersama-sama dengan Nabi Daud, dan wahai burung-burung

(bertasbihlah bersama-sama dengannya)!" Dan juga telah melembutkan besi

baginya]


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LEARNING OBJECTIVE

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AFTER STUDYING THIS CHAPTER

YOU MUST BE ABLE TO:

State the purpose of the heat treatments.Describe briefly the following heat

treatment:

Annealing

NormalizingQuenching

Tempering

Describe the microstructure differencewith the difference in heat treatment.



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HEAT TREATMENT-

INTRODUCTION

Modern demands of good quality products have madeheat treatment an important process for mostengineering applications.

Heat treatment involves the improvement of propertiesof metals and alloys by changing their microstructure.

Thermal processing of metals is a metallurgy processthat involves heating or cooling

Heat treatment: heating and cooling procedure tomanipulate structural changes in most metal



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Heat treatment can be done to accomplish any

number of objectives:

To diffuse carbon and alloying elementsTo soften the materialsTo improve machineabilityTo harden the material

To increase the toughnessTo increase wear resistanceTo relief the stress

Materials undergo heat treatment with the objective

of:Before shaping - To softening a metal before formingAfter forming- To relief strain hardeningFinal finish - To achieve final strength and hardness

HEAT TREATMENT -

OBJECTIVE



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HEAT TREATMENT -TYPES

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Annealing

Normalizing

Quenching

Tempering



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HEAT TREATMENT OF STEEL



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PROCESS PURPOSE PROCEDURE PHASE(s)

Annealing

Quenching

Normalizing

Tempering

To soften

To harden

To refine the grain

structure

To reduces

brittleness of

martensite and to

reduces internal

stress caused by

quenching

Slow cool from-stable range

Quench to miss

I-T curves

Slow cool from

-stable range

Quench,

followed by

annealing

+ cementite(PEARLITE)

Martensite

+ cementite

(BAINITE)

Temperedmartensite



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SUMMARY: PROCESSING

OPTIONS



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ANNEALING

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The iron-carbon

phase diagram in the

vicinity of theeutectoid, indicating

heat treating

temperature rangesfor plain carbon steels



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Annealing: Heat to Tanneal, then cool slowly.


Types of

Annealing:

1. Full annealing

2. Processannealing

3. Spheroidizing

1. Full Annealing Steel heated to a hightemperature (10-38C)above the

upper transformation temperature.

Upon reaching this temperature

the steel changes to austenite andbecomes face centered cubic.

Material then is cooled in the

oven. Cooling rate 100F per hour.

A temperature (10-38C) above the

upper transformation temperatureis normally sufficient to produce

austenite without generating large

grain size.


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2. Process Annealing: Part is heated above 700C. Process annealing is

frequently used an intermediate heat treating step.Inprocess annealing part is not softening as full annealingbut time required is considerable less. No phase transformation occurs because thetemperature is below A1

3. Spherodizing:Spheroidizing is a quick annealing method identical to processannealing. Parts are heated to a temperature below or near thelower transfoRmation temperature and then cooled slowly. Thename spheroidizing comes microscopic appearance of the steel.

A photomicrograph of a pearlite steel has partially

transformed to spheroidize


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Garden of Knowledge & Virtue15

EFFECT AND PURPOSE OF

ANNEALING

Purpose of Annealing The material is easier to machine, after most ofmachining is completed the part is hardened byheating and quenching To provide a material that is easier to form

To relieve internal stresses To refine the crystal structure

Effect of Annealing :o More soften

o Make a material less hardo Make a material less brittle & more ductileo Reduce the amount of internal stresseso Reduce tendency to distort and crack



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NORMALIZING

Process involves heating the materialabove the upper critical temperature

and then cooling it slowly at room

temperature, when the material isremoved from the furnace it is set out to

cool slowly on a bench or a floor. Effect

materials on normalizing is same with

annealing except for a finer grain size.



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QUENCHING

Quenching is a controlled cooling process which causes metals to

harden. Material heated to a high temperature and plunged

quickly into the quenching medium and submerged until they are

cool.



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Water quenchingMost commonly used, inexpensive, convenientto use, provides very rapid coolingCan cause internal stresses distortion orcrackingWater quenching should be about roomtemperature for best result.Oil quenchingIs more gentle than water as a quenchingmediaIt is used for more critical parts, such as partswhich have thin sections or sharp edges ex:razor blades and knife.Less change of internal stresses, distortion, orcracking but does not produce as hard orstrong of steel as water does.

QUENCHING MEDIA



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Air quenchingLess drastic than either oil or water.Air does not cool as rapidly as either oil or water.Because of slower rate of cooling, there is lesschange of internal stresses, distortion andcracking.Generally used only on steels which have highalloy content. Special alloys such as chromiumand molybdenum are selected because they areknown to cause the material to harden eventhough a slower quenching method is used.Brine quenching

Brine itself is salt water(5-10%) salt in water.

Brine will cool the material slightly faster than

water.

QUENCHING MEDIA



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HARDENABILITY--STEELS

Ability to form martensite Jominy end quench test to measure hardenability.

24C water

specimen(heated to

phase field)

flat ground

4

1

Adapted from Fig. 11.10,

Callister 6e. (Fig. 11.10

adapted from A.G. Guy,

Essentials of Materials

Science, McGraw-Hill

Book Company, New

York, 1978.)

Hardness versus distance from the quenched end.

WHY HARDNESS CHANGES


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WHY HARDNESS CHANGESWITH POSITION

The cooling rate varies with position.

Adapted from Fig. 11.12, Callister 6e.

(Fig. 11.12 adapted from H. Boyer (Ed.)

Atlas of Isothermal Transformation

and Cooling Transformation Diagrams,

American Society for Metals, 1977, p.

376.)

HARDENABILITY VS ALLOY


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HARDENABILITY VS ALLOYCONTENT

Jominy end quenchresults, C = 0.4wt%C

"Alloy Steels"

(4140, 4340, 5140, 8640)

--contain Ni, Cr, Mo

(0.2 to 2wt%)--these elements shift

the "nose".

--martensite is easier

to form.

Adapted from Fig. 11.13, Callister 6e.

(Fig. 11.13 adapted from figure

furnished courtesy Republic SteelCorporation.)


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Effect of quenching medium:

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QUENCHING MEDIUMGEOMETRYMedium

air

oil

water

Severity of Quenchsmall

moderatelarge

Hardnesssmall

moderate

large

Effect of geometry:When surface-to-volume ratio increases:

--cooling rate increases--hardness increases

Positioncenter

surface

Cooling ratesmalllarge

Hardnesssmalllarge



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thermal processing of metal

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