Lecture 14 The surface hardening of steels Flame

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, 5 th ed, Butterworth Heinemann Ch 14 Additional Readings Section

The surface hardening of steels Note: This lecture closely follows text (Higgins Ch 14)

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. 1 Carburising in solid media: (pack carburising) Higgins Engineering Materials and Processes

Case-hardening (Higgins 14. 2) READ HIGGINS 14. 2. 2 Carburising in liquid media (cyanide) Higgins Engineering Materials and Processes

Case-hardening (Higgins 14. 2) READ HIGGINS 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 BBC (1973) Part 3: Heat Treatment 1. What is a grain? 2. Recrystallisation • 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 723 C. 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 900 C. Not enough carbon to produce martensite. Engineering Materials and Processes

Nitriding (Higgins 14. 5) READ HIGGINS 14. 5 • Requires Steel with alloys that form carbides with N • Lower temperature for diffusion (500 o. C for 40 to 100 hours) Higgins Engineering Materials and Processes

Nitriding (Higgins 14. 5) READ HIGGINS 14. 5 Higgins Engineering Materials and Processes

Nitriding (Higgins 14. 5) READ HIGGINS 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 http: //www-g. eng. cam. ac. uk/mmg/teaching/phasediagrams/i 2 a. 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: Engineering Materials and Processes

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

QUESTIONS Moodle 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