1. Lecture 14 – The surface hardening of 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
Flame Hardening 1

2. The surface hardening of steels
Reference Text
Section
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann
Ch 14
Additional Readings
Section

3. The surface hardening of steels
Note: This lecture closely follows text (Higgins Ch14)

4. Principles of hardening (Higgins 14.1)
READ HIGGINS 14.1
Many metal components require a hard outer skin and tougher inner core. E.g. Machine elements like shafts, bearings, gears, cams etc.
There are two ways to achieve this;
1. Use low-carbon steel (tough) and add carbon to the outer skin.
2. Use carbon steel and heat only the surface before quenching.

5. Case-hardening (Higgins 14.2)
1. Use low-carbon steel (tough) and add carbon to the outer skin.
Higgins

6. Case-hardening (Higgins 14.2)
READ HIGGINS 14.2
Carburising
in solid media:
(pack carburising)
Higgins
Engineering Materials and Processes

7. Case-hardening (Higgins 14.2)
READ HIGGINS 14.2
Carburising in liquid media (cyanide)
Higgins
Engineering Materials and Processes

8. Case-hardening (Higgins 14.2)
READ HIGGINS 14.2
Carburising by gaseous media
Higgins
Engineering Materials and Processes

9. Heat-treatment after carburising (Higgins 14.3)
READ HIGGINS 14.3
Refining the core
Refining the case
Higgins
Engineering Materials and Processes

10. Case-hardening steels (Higgins 14.4)
READ HIGGINS 14.4
Higgins
Engineering Materials and Processes

11. 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 11

12. Nitriding (Higgins 14.5) READ HIGGINS 14.5
Requires Steel with alloys that form carbides with N
Lower temperature for diffusion (500oC for 40 to 100 hours)
Higgins
Engineering Materials and Processes

13. Nitriding (Higgins 14.5) READ HIGGINS 14.5
Engineering Materials and Processes

14. Nitriding (Higgins 14.5) READ HIGGINS 14.5 14.5.1 Heat treatment
Advantages and disadvantages of nitriding
Carbonitriding
Engineering Materials and Processes

15. Ion Nitriding (Higgins 14.6)
READ HIGGINS 14.6
Plasma nitriding and ion implantation.
Engineering Materials and Processes

16. Flame-hardening (Higgins 14.7)
READ HIGGINS 14.7
Localised heating/quenching
Higgins
Engineering Materials and Processes

17. Induction-hardening (Higgins 14.8)
READ HIGGINS 14.8
Powerful, high frequency current induces eddy currents in the surface of the component, heating it locally.
Higher frequencies heat to a shallower depth (skin effect).
Induction Heating
Engineering Materials and Processes

18. Summary (Higgins 14.9)
Higgins
Engineering Materials and Processes

19. 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

20. 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 20

21. Engineering Materials and Processes
GLOSSARY
Carburising
Plasma Ion
Nitriding
Pack carburising
Cyanide hardening
Gas-carburising
Flame hardening
Induction hardening
Carbonitriding
Engineering Materials and Processes 21

22. Define all the glossary terms.
QUESTIONS
Moodle XML: Some questions in Steel
Define all the glossary terms.
Describe why a part would need a hard skin and a soft core.
Use a table to summarise the advantages and disadvantages of the three carburising methods as shown in the video: Pack carburising, cyanide and plasma.
List advantages and disadvantages of nitriding
Engineering Materials and Processes 22