1. Engineering Materials and Processes Lecture 12 – The heat-treatment of plain-carbon steels
Prescribed Text:
Ref 1: Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. ISBN:
Readings:
Callister: Callister, W. Jr. and Rethwisch, D., 2010, Materials Science and Engineering: An Introduction, 8th Edition, Wiley, New York. ISBN
Ashby 1: Ashby, M. & Jones, D., 2011, Engineering Materials 1: An Introduction to Properties, Applications and Design, 4th edition, Butterworth-Heinemann, Oxford UK. IBSN:
Ashby 2: Ashby, M. & Jones, D., 2011, Engineering Materials 2: An Introduction to Microstructures and Processing, 4th edition, Butterworth-Heinemann, Oxford UK. IBSN:
Lecture (2 hrs):
Ref 1, Ch 1: Engineering materials;
Ref 1 Ch 2: Properties of materials.
Laboratory 1 (2 hrs): Hardness test
Callister: Ch 1, 2, 18-21
Ashby 1: Ch 1, 2
Ashby 2: Ch 1
High Carbon Steel is used in springs 1

2. The heat-treatment of plain-carbon steels
Reference Text
Section
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann
Ch 12
Additional Readings
Section
Engineering Materials and Processes

3. Note: This lecture closely follows text (Higgins Ch12)
The heat-treatment of plain-carbon steels
Note: This lecture closely follows text (Higgins Ch12)
Engineering Materials and Processes

4. 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.
Engineering Materials and Processes

5. VIDEO: Crystals and Grain Structure
1. What is a grain?
BBC (1973)
2. Recrystallisation
Part 3: Heat Treatment
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.
Engineering Materials and Processes 5

6. 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.
Engineering Materials and Processes

7. 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
Martensite
Engineering Materials and Processes 7

8. 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
Engineering Materials and Processes 8

9. 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
Bainite
Engineering Materials and Processes 9

10. TTT diagrams (Higgins 12.2.1)
Read Higgins :
TTT curve: Time-Temperature-Transformation
Hardness is dependent on the cooling rate.
Higgins
Engineering Materials and Processes

11. TTT diagrams (Higgins 12.2.1)
Read Higgins :
Engineering Materials and Processes

12. TTT diagrams (Higgins 12.2.1)
Read Higgins 12.2.cting cooling rates:
Engineering Materials and Processes

13. TTT diagrams (Higgins 12.2.3)
Read Higgins
Higgins
Engineering Materials and Processes

14. 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.
Engineering Materials and Processes

15. 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
Engineering Materials and Processes

16. 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 ( °C) are for hardness
Higher temperatures ( °C) for stressed parts that need strength, toughness, and general reliability.
Engineering Materials and Processes

17. Tempering (Higgins 12.4) Read Higgins 12.4 Tempering
Lovett
Engineering Materials and Processes

18. Tempering (Higgins 12.4) Read Higgins 12.4 Tempering
Refer Higgins Table 12.3
Heat treatments and typical uses of plain-carbon steels
Engineering Materials and Processes

19. Tempering (Higgins 12.4) Engineering Materials and Processes
Refer Higgins Table 12.3: Heat treatments and typical uses of plain-carbon steels
Higgins
Engineering Materials and Processes

20. Tempering (Higgins 12.4) Engineering Materials and Processes
Refer Higgins Table 12.3: Heat treatments and typical uses of plain-carbon steels
Higgins
Engineering Materials and Processes

21. 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 , , )
Higgins
(i) Martempering (ii) Austempering
Engineering Materials and Processes

22. 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.
Engineering Materials and Processes

23. 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
Engineering Materials and Processes

24. Jominy Test
(Higgins 12.7)
Higgins
Engineering Materials and Processes

25. Jominy Test
(Higgins 12.7)
Higgins
Engineering Materials and Processes

26. Heat Treatment Furnaces (Higgins 12.8)
READ HIGGINS 12.8
Engineering Materials and Processes

27. 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
Engineering Materials and Processes

28. Online Resources. Teach yourself phase diagrams Handout Wikipedia:
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:
h ttp://
S how this website on screen. Will be using this later.
Engineering Materials and Processes 28

29. Super saturated solution Critical cooling rate Tempering
GLOSSARY
Martensite
Bainite
Super saturated solution
Critical cooling rate
Tempering
Martempering Austempering
Ruling section
Jominy Test
Engineering Materials and Processes 29

30. Define all the glossary terms.
QUESTIONS
Moodle XML: Some questions in Steel
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?
List iron grain structures that are super-saturated with carbon.
Describe the difference between heat treatment of hypo and hyper-eutectoid steels. Why is hyper-eutectoid more complicated?
Describe the Jominy test. What does it measure?
Describe how Critical Cooling rate can be modified by %C or alloys elements.
Summarise the advantages and disadvantages of the three carburising methods shown in the video: Pack carburising , cyanide and plasma.
Engineering Materials and Processes 30