Thermal Processing of Metal
Transcript of 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
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AS SABAK ( (Surah 34 Verse 10
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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.
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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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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
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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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