Lecture 14 The surface hardening of steels - Live AND Learn · The surface hardening of steels...
Transcript of Lecture 14 The surface hardening of steels - Live AND Learn · The surface hardening of steels...
Lecture 14 – The surface hardening of steels
Flame Hardening
The surface hardening of steels
Reference Text Section
Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann
Ch 14
Additional Readings Section
The surface hardening of steels
Note: This lecture closely follows text (Higgins Ch14)
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.
Case-hardening (Higgins 14.2)
1. Use low-carbon steel (tough) and add carbon to the outer skin.
Higgins
Case-hardening (Higgins 14.2)
Engineering Materials and Processes
READ HIGGINS 14.2
14.2.1 Carburising
in solid media:
(pack carburising)
Higgins
Case-hardening (Higgins 14.2)
Engineering Materials and Processes
READ HIGGINS 14.2
14.2.2 Carburising in liquid media (cyanide)
Higgins
Case-hardening (Higgins 14.2)
Engineering Materials and Processes
READ HIGGINS 14.2
14.2.3 Carburising by
gaseous media
Higgins
Heat-treatment after carburising (Higgins 14.3)
Engineering Materials and Processes
READ HIGGINS 14.3
Refining the core
Refining the case
Higgins
Case-hardening steels (Higgins 14.4)
Engineering Materials and Processes
READ HIGGINS 14.4
Higgins
VIDEO: Crystals and Grain Structure
BBC (1973)
Engineering Materials and Processes
1. What is a grain?
2. RecrystallisationPart 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.
Nitriding (Higgins 14.5)
Engineering Materials and Processes
READ HIGGINS 14.5
• Requires Steel with
alloys that form carbides
with N
• Lower temperature for
diffusion (500oC for 40 to
100 hours)
Higgins
Nitriding (Higgins 14.5)
Engineering Materials and Processes
READ HIGGINS 14.5
Higgins
Nitriding (Higgins 14.5)
Engineering Materials and Processes
READ HIGGINS 14.5
14.5.1 Heat treatment
14.5.2 Advantages and disadvantages of nitriding
14.5.3 Carbonitriding
Ion Nitriding (Higgins 14.6)
Engineering Materials and Processes
READ HIGGINS 14.6
Plasma nitriding and ion implantation.
Flame-hardening (Higgins 14.7)
Engineering Materials and Processes
READ HIGGINS 14.7
Localised heating/quenching
Higgins
Induction-hardening (Higgins 14.8)
Engineering Materials and Processes
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 Heatinghttp://www.automotive-business-review.com
Summary (Higgins 14.9)
Engineering Materials and Processes
Higgins
Video:
Engineering Materials and Processes
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
Handout
Wikipedia:
Online Resources.
Teach yourself phase diagrams
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.
GLOSSARY
Carburising
Plasma Ion
Nitriding
Pack carburising
Cyanide hardening
Gas-carburising
Flame hardening
Induction hardening
Carbonitriding
Engineering Materials and Processes
QUESTIONSMoodle XML: Some questions in 10105 Steel
1. Define all the glossary terms.
2. Describe why a part would need a hard skin and a soft core.
3. Use a table to summarise the advantages and disadvantages of the three
carburising methods as shown in the video: Pack carburising, cyanide and
plasma.
4. List advantages and disadvantages of nitriding
Engineering Materials and Processes