HEAT_TREATMENT.pptx

7/28/2019 HEAT_TREATMENT.pptx

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Some Issues on Heat treatment

1. Introduction

The key to improve the material property is to change

its structure at atomic level. This can be achievedthrough (i) alloying and controlled heat and cooling

basically heat treatment

The basic steps of heat treatment are:

Heat -> Soaking -> Cooling


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Temperature Time of soaking Rate of cooling

Medium of cooling

- Different combinations of the above parameters

- Different compositions of materials and initialphases of materials

Different heat treatments


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2. Purpose of heat treatment

(1)Soften the metal prior to shaping;

(2)Relieve the effects of strain hardening that occursduring cold forming;

(3)Achieve the final strength and hardness required in

the finished product as one of the end

manufacturing processes.


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Heat treatment

Body heat treatment

Surface heat treatment

3. Classification of heat treatment processes

(1) annealing, (2) martensite formation in steel, (3)

precipitation hardening, (4) surface hardening


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4. Annealing


-Reduce hardness and brittleness

-Alter microstructure for desired mechanical properties

-Soften metals to improve formability

-Recrystalize cold worked (strain hardened) metals-Relieve stress from shaping


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4. Annealing

Full annealing

Normalizing

Process Anneal

- Recrystallization anneal

- Recovery Anneal

Stress-relief annealing


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5. TTT curve

principle for Martensite Formation

- eutectoid composition

- preheat or heat up alloy to austenite.

- austenite to various phases : cooling rate.

Pearlite, P; Bainite, B: alternative forms of

ferrite-carbide mixtures

Martensite, M


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MetalsPhase diagram for Iron and Carbon

Fe3C

Eutectic

Eutectoid


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5. TTT curveprinciple for Martensite Formation


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5. TTT curveprinciple for Martensite Formation

Martensite: hard and brittle

BCT + carbon

Tetragonal

Ms: the temperature M starts to form.

Ms depends on alloyed element; some are lower than

room temperature


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5. Heat treatment to form Martensite phase

Austenitizing: heat up to a certain temperature to

form

Quenching: cooling media:

brine: the fastest; air: the slowest

Tempering: heat up to temperature below eutectiodsoak for one hour & slow cooling

BCT to BCC


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5. Heat treatment to form Martensite phase

Austenitizing

quenching - tempering


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5. Hardenability

- The relative capacity of a steel to be hardened by

transformation to martensite; hardness penetration

- Hardenability = Hardness ?

- alloying elements increase the hardenability: to

make TTT curve right: to increase the time to start

the transformation for Austenite to Martensite


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Heat treatment

6. Precipitation hardening

A necessary condition for metals that can be heat treated

to be hard is - Martensite formulation, which

includes the following points:

(a) The composition of metal, in the range 0.1-0.8 % C;

(b) The Ms should be above the room temperature;

(c) The TTT curve should allow the possibility that the

cooling trajectory passes on the left of the nose.


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- Further, heat treatment for non-ferrous metals, e.g.,

aluminum, copper, magnesium, do not follow the

martensite formation. This calls for a new process.

- Necessary condition may not be met in practice for all

compositions of steel

Precipitation Hardening process

- Formation of fine particles (precipitates) that act to

block the movement of dislocations

- thus strengthen the metal


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(1)Composition of the metal has two phases at the room

temperature, see Figure 1a (next slide).

(2) When the temperature arises, one phase should bebe dissolved, i.e., sloping solvus line

Composition C (next figure) satisfies above conditions

Necessary condition for the precipitation process


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Figure 1a, b



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3 steps of precipitation/age hardening:

(1)Solution treatment

- alloy heated above Ts into alpha-phase and held to

dissolve the beta phase

(2) Quenching

- to the room temperature to create a supersaturated

solid solution

- It is noted that the structure of martensite is of highly

distorted BCC plus excessive carbon dissolved; Figure

2. Supersaturated structure has that same feature.



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(3) Precipitation treatment (aging)

- Reheat the material to the temperature a little bit above

the room temperature, Tp, but below Ts, to cause

precipitation of fine particles of the beta phase.

- high strength and hardness achieved in this step

- Temp. and time for the step are variables.

Higher temp. hardness peaks quicklylower temp more time to harden, but hardness is more

- Over-aging reduction in hardness



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Figure 2


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7. Surface heat treatment

1) Thermo chemical treatment

2) Composition of part surface altered by addition of

other elements

3) Adding of carbon, nitrogen, or other elements

Carburizing Nitriding

Carbonitriding


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Pack carburizing- Pack carbonaceous

materials (charcoal); Very thick hard

outer layer

Gas carburizing- Diffuse Hydrocarbon

fuels (propane in a furnace); thin hard

outer layer

Liquid carburizing- Diffuse molten saltbath containing sodium cyanide,

barium chloride, and other compounds;

medium sized hard outer layer

- Carburizing

- Low hardness

- Ductile

- Capable of

withstanding stress


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Carburizing -> HRC 60, Thickness: 0.025 4 mm

Nitriding -> HRC 70, Thickness: 0.025

0.05 mmCarbonitriding -> HRC 70, Thickness: 0.07-0.5 mm

Chromizing and Boronizing -> HRC 70


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8. Heat treatment methods and facilities

1. Heat treatment furnaces

2. Surface hardening methods- flame hardening

- induction heating

- Laser beam heating

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