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