Lecture 14 – The surface hardening of steels Prescribed Text: Ref 1: Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. ISBN: 9781856177696 Readings: Callister: Callister, W. Jr. and Rethwisch, D., 2010, Materials Science and Engineering: An Introduction, 8th Edition, Wiley, New York. ISBN 9780470419977 Ashby 1: Ashby, M. & Jones, D., 2011, Engineering Materials 1: An Introduction to Properties, Applications and Design, 4th edition, Butterworth-Heinemann, Oxford UK. IBSN: 9780080966656 Ashby 2: Ashby, M. & Jones, D., 2011, Engineering Materials 2: An Introduction to Microstructures and Processing, 4th edition, Butterworth-Heinemann, Oxford UK. IBSN: 9780080966687   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

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) READ HIGGINS 14.2 14.2.1 Carburising in solid media: (pack carburising) Higgins Engineering Materials and Processes

Case-hardening (Higgins 14.2) READ HIGGINS 14.2 14.2.2 Carburising in liquid media (cyanide) Higgins Engineering Materials and Processes

Case-hardening (Higgins 14.2) READ HIGGINS 14.2 14.2.3 Carburising by gaseous media Higgins Engineering Materials and Processes

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

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

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

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

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

Nitriding (Higgins 14.5) READ HIGGINS 14.5 14.5.1 Heat treatment 14.5.2 Advantages and disadvantages of nitriding 14.5.3 Carbonitriding Engineering Materials and Processes

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

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

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 http://www.automotive-business-review.com Engineering Materials and Processes

Summary (Higgins 14.9) Higgins Engineering Materials and Processes

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

Online Resources. Teach yourself phase diagrams Handout Wikipedia: 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: h ttp://www.matweb.com S how this website on screen. Will be using this later. Engineering Materials and Processes 20

Engineering Materials and Processes GLOSSARY Carburising Plasma Ion Nitriding Pack carburising Cyanide hardening Gas-carburising Flame hardening Induction hardening Carbonitriding http://www.youtube.com/watch?v=1GFst2IQBEM Engineering Materials and Processes 21

Define all the glossary terms. QUESTIONS Moodle XML: Some questions in 10105 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 http://www.youtube.com/watch?v=1GFst2IQBEM Engineering Materials and Processes 22