Engineering Materials and ProcessesLecture 12 – The heat-treatment of plain-carbon steels

High Carbon Steel is used in springshttp://cnhuaxing.en.made-in-china.com

The heat-treatment of plain-carbon steels

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Reference Text Section

Higgins RA & Bolton, 2010. Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann

Ch 12

Additional Readings Section

The heat-treatment of plain-carbon steels

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Note: This lecture closely follows text (Higgins Ch12)

Principles of hardening (Higgins 12.2)

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If a piece of steel containing sufficient carbon is heated until its structureis austenitic - that is, until its temperature is above the upper criticaltemperature - and is then quenched, i.e. cooled quickly, it becomesconsiderably harder than it would be were it cooled slowly.

There is insufficient time for the formation of Pearlite, so a new type of grain forms: Martensite. This is also a BCC structure.

Martensite is a very hard grain structure.

VIDEO: Crystals and Grain StructureBBC (1973)

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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.

Principles of hardening (Higgins 12.2)

Engineering Materials and Processes

If a piece of steel containing sufficient carbon is heated until its structureis austenitic - that is, until its temperature is above the upper criticaltemperature - and is then quenched, i.e. cooled quickly, it becomesconsiderably harder than it would be were it cooled slowly.

There is insufficient time for the formation of Pearlite, so a new type of grain forms: Martensite. This is also a BCC structure.

Martensite is a very hard grain structure.

Principles of hardening (Higgins 12.2)

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See Higgins Fig 12.1 (i)

Martensite:Water quenching of 0.5% C steelan irregular mass of needle-shapedcrystals. Actually the crystals are discuss-shaped, and the needles are cross-sections of these discs.

Water Quenched: Martensitehttp://pwatlas.mt.umist.ac.uk

Martensite

Principles of hardening (Higgins 12.2)

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See Higgins Fig 12.1 (ii)

Tempered MartensiteWater-quenched from 850°C and tempered at 400°C - tempered martensite, the crystals of which have become darkened by precipitated particles of cementite

Tempered Martensitehttp://pwatlas.mt.umist.ac.uk

Principles of hardening (Higgins 12.2)

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See Higgins Fig 12.1 (iii)

Martensite / BainiteOil quenched from 850°C - the slower cooling rate during quenching has allowed a mixture of bainite (dark) and martensite (light) to form.Bainite is softer than martensite.

Martensite and Bainitehttp://www.matcoinc.com

Bainite

TTT diagrams (Higgins 12.2.1)

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Read Higgins 12.2.1:

TTT curve: Time-Temperature-Transformation

Hardness is dependent on the cooling rate.

Higgins

TTT diagrams (Higgins 12.2.1)

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Read Higgins 12.2.1:

TTT diagrams (Higgins 12.2.1)

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Read Higgins 12.2.cting cooling rates:

TTT diagrams (Higgins 12.2.3)

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Read Higgins 12.2.3

Higgins

The hardening process (Higgins 12.3)

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Hypo-eutectoid steel: Heat to 30-50°C above UCT temperature, and then quenched at appropriate rate.

Hyper-eutectoid steel: Quenching from about 30°C above the LCT. Since cementite is present, cooling from above the UCT tends to precipitate as long, brittle needles along the grain boundaries of the austenite. This is a poor structure so its formation is prevented by continuing to forge the steel whilst the primary Cementite is being deposited – (between UCT and LCT). This breaks the needles into globules from which cooling can be done. If subsequent heat-treatment goes more than 30°C over LCT the primary Cementite will dissolve into the Austenite and precipitate back to needles on cooling.

Read Higgins 12.3:

The hardening process (Higgins 12.3)

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Read Higgins 12.3

Higgins

When a hyper-eutectoid steel has been correctly hardened, its structure should consist of small, near spherical globules of very hard Cementitein a matrix of hard, strong martensite.(Figure 12.5)

Tempering (Higgins 12.4)

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Read Higgins 12.4 Tempering

Fully hardened carbon steel is brittle. Tempering adds toughness but maintains most of the hardness and strength.

As we have seen, the Martensitic structure in hardened steel consistsessentially of ferrite which is heavily super-saturated with carbon.

By heating to a high enough temperature, the carbon starts to precipitate into tiny particles of Cementite.

Low tempering temperatures (200-300°C) are for hardnessHigher temperatures (400-600°C) for stressed parts that need strength, toughness, and general reliability.

Tempering (Higgins 12.4)

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Read Higgins 12.4 Tempering

Lovett

Tempering (Higgins 12.4)

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Read Higgins 12.4 Tempering

Refer Higgins Table 12.3Heat treatments and typical uses of plain-carbon steels

Tempering (Higgins 12.4)

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Refer Higgins Table 12.3: Heat treatments and typical uses of plain-carbon steels

Higgins

Tempering (Higgins 12.4)

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Refer Higgins Table 12.3: Heat treatments and typical uses of plain-carbon steels

Higgins

Isothermal Heat Treatments (Higgins 12.5)

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The risk of cracking and distortion during the quenching of carbon steels reduced martempering and austempering. These processes are known as isothermal heat-treatments. (READ HIGGINS 12.5.1, 12.5.2, 12.5.3)

(i) Martempering (ii) Austempering Higgins

Hardenability (Higgins 12.6)

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• Quenching of thick sections can result in an outer shell of martensite, the core may be of bainite, or even fine pearlite.• This is the 'mass effect' of heat treatment.• Plain-carbon steel has ‘a shallow depth of hardening', or, ‘poor hardenability'.

Hardenability: The depth of martensitichardening produced by quenching.

This can leave the inside softer than the outside – which may (or may not) be a good thing.

Hardenability

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Higgins

(Higgins 12.6.1)

12.6.1 Ruling sectionAlloying elements help to reduce the critical rate so oil-quenching can be used, or water quenching can reach deeper.

The limiting rulingsection is the maximum diameter which can be heat-treated (underconditions of quenching and tempering suggested by the manufacturer)

Jominy Test

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Higgins

(Higgins 12.7)

Jominy Test

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Higgins

(Higgins 12.7)

Heat Treatment Furnaces

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(Higgins 12.8)

READ HIGGINS 12.8

Video:

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Heat Treatment: BBC: 1981Heat 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

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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

MartensiteBainiteSuper saturated solutionCritical cooling rateTemperingMartemperingAustemperingRuling sectionJominy Test

Engineering Materials and Processes

QUESTIONSMoodle XML: Some questions in 10105 Steel

1. Define all the glossary terms.2. Why are isothermal heat treatments of carbon steel limited to thin sections?3. Why are there a range of different quenching fluids?4. When a carbon steel is quenched, which grain structure causes hardness?5. If a quenched steel is too hard, what process can be used to toughen it?6. On the TTT curve for a particular carbon steel, what advantage is there in

avoiding the ‘nose’ of the curve – as isothermal heat treatments do?7. List iron grain structures that are super-saturated with carbon.8. Describe the difference between heat treatment of hypo and hyper-eutectoid

steels. Why is hyper-eutectoid more complicated?9. Describe the Jominy test. What does it measure?10.Describe how Critical Cooling rate can be modified by %C or alloys elements.11.Summarise the advantages and disadvantages of the three carburising methods

shown in the video: Pack carburising , cyanide and plasma.

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