Transcript Lecture-12 - LearnEASY

ENMAT101A Engineering Materials and Processes
Associate Degree of Applied Engineering
(Renewable Energy Technologies)
Lecture 12 – The heat-treatment of plain-carbon
steels
High Carbon Steel is used in springs
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The heat-treatment of plain-carbon steels
Reference Text
Section
Higgins RA & Bolton, 2010. Materials for Engineers and Technicians,
5th ed, Butterworth Heinemann
Ch 12
Additional Readings
Section
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The heat-treatment of plain-carbon steels
Note: This lecture closely follows text (Higgins Ch12)
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Principles of hardening (Higgins 12.2)
If a piece of steel containing sufficient carbon is heated until its structure
is austenitic - that is, until its temperature is above the upper critical
temperature - and is then quenched, i.e. cooled quickly, it becomes
considerably harder than it would be were it cooled slowly.
There is insufficient time for the formation of Pearlite, so a new type of
grain forms: Martensite. This is also a BCC structure.
Martensite is a very hard grain structure.
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VIDEO: Crystals and Grain Structure
BBC (1973)
Part 3: Heat Treatment
1. What is a grain?
2. Recrystallisation
• Steel grains are too small to be visible - need a microscope approx 250 times
magnification.
• Ferrite: Light coloured. Made of iron. Gives ductility to the steel
• Pearlite: darker coloured. Layers of Iron + Iron Carbide. Hardness and strength to
the steel.
• 100% Pearlite: 0.83%C. Recrystallisation temperature 723C. Eutectic alloy.
• Normalising - cooled in air, grain size reduced and more uniform shape, toughness
increased due to smaller grains
• Quenching - increases hardness. Not enough time for pearlite to form, so a needle
like structure forms - martensite. Very hard and brittle.
• Tempering - (after quenching) restores toughness. Modifies the martensite needles
with small flakes of carbon. This gives keeps most hardness, adds toughness.
• 0.1%C steel (Mild Steel). Recrystallisation 900C. Not enough carbon to produce
martensite.
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Principles of hardening (Higgins 12.2)
If a piece of steel containing sufficient carbon is heated until its structure
is austenitic - that is, until its temperature is above the upper critical
temperature - and is then quenched, i.e. cooled quickly, it becomes
considerably harder than it would be were it cooled slowly.
There is insufficient time for the formation of Pearlite, so a new type of
grain forms: Martensite. This is also a BCC structure.
Martensite is a very hard grain structure.
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Principles of hardening (Higgins 12.2)
See Higgins Fig 12.1 (i)
Martensite:
Water quenching of 0.5% C steel
an irregular mass of needle-shaped
crystals. Actually the crystals are
discuss-shaped, and the needles
are cross-sections of these discs.
Water Quenched: Martensite
http://pwatlas.mt.umist.ac.uk
Martensite
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Principles of hardening (Higgins 12.2)
See Higgins Fig 12.1 (ii)
Tempered Martensite
Water-quenched from 850°C and
tempered at 400°C - tempered
martensite, the crystals of which
have become darkened by
precipitated particles of cementite
Tempered Martensite
http://pwatlas.mt.umist.ac.uk
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Principles of hardening (Higgins 12.2)
See Higgins Fig 12.1 (iii)
Martensite / Bainite
Oil quenched from 850°C - the
slower cooling rate during
quenching has allowed a mixture
of bainite (dark) and martensite
(light) to form.
Bainite is softer than martensite.
Martensite and Bainite
http://www.matcoinc.com
Bainite
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TTT diagrams (Higgins 12.2.1)
Read Higgins 12.2.1:
TTT curve:
TimeTemperatureTransformation
Hardness is
dependent on
the cooling rate.
Higgins
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TTT diagrams (Higgins 12.2.1)
Read Higgins 12.2.1:
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TTT diagrams (Higgins 12.2.1)
Read Higgins 12.2.cting cooling rates:
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TTT diagrams (Higgins 12.2.3)
Read Higgins 12.2.3
Higgins
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The hardening process (Higgins 12.3)
Read Higgins 12.3:
Hypo-eutectoid steel: Heat to 30-50°C above UCT temperature, and
then quenched at appropriate rate.
Hyper-eutectoid steel: Quenching from about 30°C above the LCT.
Since cementite is present, cooling from above the UCT tends to
precipitate as long, brittle needles along the grain boundaries of the
austenite. This is a poor structure so its formation is prevented by
continuing to forge the steel whilst the primary Cementite is being
deposited – (between UCT and LCT). This breaks the needles into
globules from which cooling can be done. If subsequent heat-treatment
goes more than 30°C over LCT the primary Cementite will dissolve into
the Austenite and precipitate back to needles on cooling.
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The hardening process (Higgins 12.3)
Read Higgins 12.3
When a hyper-eutectoid
steel has been correctly
hardened, its structure
should consist of small,
near spherical globules
of very hard Cementite
in a matrix of hard,
strong martensite.
(Figure 12.5)
Higgins
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Tempering (Higgins 12.4)
Read Higgins 12.4 Tempering
Fully hardened carbon steel is brittle. Tempering adds toughness but
maintains most of the hardness and strength.
As we have seen, the Martensitic structure in hardened steel consists
essentially of ferrite which is heavily super-saturated with carbon.
By heating to a high enough temperature, the carbon starts to precipitate
into tiny particles of Cementite.
Low tempering temperatures (200-300°C) are for hardness
Higher temperatures (400-600°C) for stressed parts that need strength,
toughness, and general reliability.
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Tempering (Higgins 12.4)
Read Higgins 12.4 Tempering
Lovett
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Tempering (Higgins 12.4)
Read Higgins 12.4 Tempering
Refer Higgins Table 12.3
Heat treatments and typical
uses of plain-carbon steels
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Tempering (Higgins 12.4)
Refer Higgins Table 12.3: Heat treatments and
typical uses of plain-carbon steels
Higgins
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Tempering (Higgins 12.4)
Refer Higgins Table 12.3: Heat treatments and
typical uses of plain-carbon steels
Higgins
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Isothermal Heat Treatments (Higgins 12.5)
The risk of cracking and distortion during the quenching of carbon steels
reduced martempering and austempering. These processes are known as
isothermal heat-treatments. (READ HIGGINS 12.5.1, 12.5.2, 12.5.3)
(i) Martempering
(ii) Austempering
Higgins
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Hardenability (Higgins 12.6)
• Quenching of thick sections can result
in an outer shell of martensite, the core
may be of bainite, or even fine pearlite.
• This is the 'mass effect' of heat
treatment.
• Plain-carbon steel has ‘a shallow depth
of hardening', or, ‘poor hardenability'.
Hardenability: The depth of martensitic
hardening produced by quenching.
This can leave the inside softer than the
outside – which may (or may not) be a
good thing.
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Hardenability
(Higgins 12.6.1)
12.6.1 Ruling section
Alloying elements help to
reduce the critical rate so
oil-quenching can be
used, or water
quenching can reach
deeper.
The limiting ruling
section is the maximum
diameter which can be
heat-treated (under
conditions of quenching
and tempering suggested
by the manufacturer) Higgins
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Jominy Test
(Higgins 12.7)
Higgins
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Jominy Test
(Higgins 12.7)
Higgins
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Heat Treatment Furnaces
(Higgins 12.8)
READ HIGGINS 12.8
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Video:
Heat Treatment:
BBC: 1981
Heat treatment [videorecording] / producer Brian Davies.
Video: Discusses the use of heat which changes the properties of
metals. Outlines different techniques including hardening,
tempering, annealing, normalising as well as a non-heat process,
cold-working.
Recommended viewing: All
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Online Resources.
Teach yourself phase diagrams
Handout
http://www-g.eng.cam.ac.uk/mmg/teaching/phasediagrams/i2a.html
Heat Treatment: BBC: Heat treatment [videorecording] / producer Brian Davies.
[B.B.C.], 1981.
Video: Discusses the use of heat which changes the properties of metals. Outlines different
techniques including hardening, tempering, annealing, normalising as well as a non-heat
process, cold-working.
Wikipedia:
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GLOSSARY
Martensite
Bainite
Super saturated solution
Critical cooling rate
Tempering
Martempering
Austempering
Ruling section
Jominy Test
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QUESTIONS
Moodle XML: Some questions in 10105 Steel
1.
2.
3.
4.
5.
6.
Define all the glossary terms.
Why are isothermal heat treatments of carbon steel limited to thin sections?
Why are there a range of different quenching fluids?
When a carbon steel is quenched, which grain structure causes hardness?
If a quenched steel is too hard, what process can be used to toughen it?
On the TTT curve for a particular carbon steel, what advantage is there in
avoiding the ‘nose’ of the curve – as isothermal heat treatments do?
7. List iron grain structures that are super-saturated with carbon.
8. Describe the difference between heat treatment of hypo and hyper-eutectoid
steels. Why is hyper-eutectoid more complicated?
9. Describe the Jominy test. What does it measure?
10. Describe how Critical Cooling rate can be modified by %C or alloys elements.
11. Summarise the advantages and disadvantages of the three carburising methods
shown in the video: Pack carburising , cyanide and plasma.
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