EM025 CHAPTER 1 ENGINEERING MATERIAL

Solution:

Z 1000°C 700°C 600°C
Austenite
Phase Cementite + Austenite Ferrite + Cementite
Composition

Explanation Ductile and non-magnetic Soft and ductile Hard and brittle

Microstructure
Sketch

1.5 HEAT TREATMENT

LEARNING OUTCOMES

a) Define heat treatment.
b) Explain various heat treatment process.
c) Describe the effect of heat treatment processes on material.

INTRODUCTION

• Metal engineering products are produced through various processes such as jointing,
cutting, casting and press work.

• Production process may create internal stress.

• Heat treatment is recommended to remove internal stress.

• Although most people don’t know what heat treatment is, it’s actually an essential part
of the manufacturing process.

• That’s because heat treating allows a metal piece to be improved in order for the
material to better withstand wear and tear.

HEAT TREATMENT

Microstructure before Microstructure after

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• Heat treatment can be defined as heating a metal or alloy to a specific temperature and
then cooling it to harden the material.

• This process is to change the micro structure and mechanical properties of steel without
changing its form.

• Rough and non-uniform micro structure is treated to form a fine and uniform structure.

• Types of heat treatment are:
- annealing,
-normalizing,
- hardening,
- tempering

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HEAT
TREATMENT

ANNELING HARDENING NORMALIZING TEMPERING

• Component are heated to specific temperature level and remain at that temperature
for a period of time.

• Cooling of component to room temperature is done either in the furnace with power
off or outside the furnace, in air or liquid media (water, salt solution or mineral oil).

• Heating temperature level and duration depend :
- carbon content
- types of heat treatment
- desired mechanical properties.

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• This process is used on metals like copper, aluminum, silver, steel, and brass to making it
more workable and less likely to fracture or crack.

• Done to relieve the internal stress due to cold work like rolling, knocking, bending,
machining and grinding. Examples:
➢ Work-hardened materials undergone a stamping process or cold drew bar stock.
➢ The metal wire that been drawn for one size to a much smaller size.
➢ Machining operations that create high amounts of heat or metal displacement.
➢ Welded components create residual stresses and recreate uniform physical properties.

• Primary purpose:
➢ reduce hardness
➢ improve toughness
➢ restore ductility
➢ refine grain size
➢ remove residual stress (stress relief)

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• These materials are heated to a certain temperature, and then are slowly air-dried.
• Process:

i. Heat in furnace to annealing temperature (260 °C to 760 °C).
ii. Power off, cooled slowly in furnace for certain period of time.

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• After this process, the microstructure formed either ferrite+pearlite, pearlite,
pearlite+cementite or martensite depending on the percentages of carbon content in the
steel.

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• This process is used on metals like iron based alloys, nickel based alloys, copper,
aluminum and brass to making it more ductility and reduced hardness.

• Done to relieve the internal stress due to thermal or mechanical hardening processes.
Examples:
➢ Stainless steel stampings in the automotive industry following the work hardening
that occurs during their forming process.
➢ Nickel-based alloys in the nuclear industry following the thermal microstructure
alteration that occurs following welding.
➢ Carbon steel after it is cold-rolled to reduce the brittleness caused by work hardening.

• Primary purpose:
➢ Increase tensile strength, ductility and also refines the grain structure.
➢ Reduces hardness.
➢ Easily serviceable and can be fully machined

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• These material is heated to an elevated temperature, and then it is cooled in contact with

air at room temperature. This process of cooling the metal with air is called air quenching.
• Process:

➢ Heated to about 50-80 °C above the upper critical temperature in furnace.
➢ After heating – cooled to room temperature outside furnace.

• During heating process, pearlite is transformed into austenite.

• This process is used on carbon content of the steel to increase the hardness of a metal.
• Done to relieve the internal stress due to thermal or mechanical hardening processes.

Examples:
➢ Machine cutting tools (drill bits, taps, lathe tools) need be much harder than the

material they are operating on in order to be effective.
➢ Knife blades – a high hardness blade keeps a sharp edge.
➢ Bearings – necessary to have a very hard surface that will withstand continued stresses.
➢ Armor plating - High strength is extremely important both for bullet proof plates and for

heavy duty containers for mining and construction.
➢ Anti-fatigue - Martensitic case hardening can drastically improve the service life of

mechanical components with repeated loading/unloading, such as axles and cogs.
• Primary purpose:

➢ Increase hardness and strength.
➢ Improve scratch resistance.

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• These materials are heated to hardening temperature, held at temperature, then "quenched"
(rapidly cooled), often in oil or water.

• Process:
➢ Heated to about 800 - 900°C above the upper critical temperature in furnace.
➢ After heating – cooled in quench media such as oil, water, salt water and polymer quench.

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• The austenite microstructure is transformed into needle-like martensite microstructure.

Austenite Martensite

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Quench Media
Rapid cooling of a work piece to obtain certain material properties.

1. Water:
A good agitation, bubbles can be prevented from sticking to the steel, and prevent
soft spots. However, water is corrosive and can cause distortion or cracking.

2. Salt Water:
More rapid than water because bubbles are broken easily. However, salt water is
even more corrosive than plain water, and hence must be rinsed off immediately.

3. Oil:
Used when a slower cooling rate is desired and this reduces the likelihood of
cracking. Quenching results in fumes, spills, and sometimes a fire hazard.

4. Polymer Quench:
Are capable of producing repeatable results with less corrosion than water and less
of fire hazard than oil. But, are possible only with constant monitoring of the chemistry.

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• This process is done immediately after hardening. It is used on to ferrous alloys, such as

steel or cast iron to increase the toughness by decreasing the hardness.
• Done to relieve the internal stress due to thermal or mechanical hardening processes.

Examples:
➢ Heating a carbon steel and rapidly quenching it can leave it too hard and brittle. It can

restore some of its ductility.
➢ Welds can create a localized and can leave undesirable mechanical properties and

residual stress that can promote hydrogen cracking.
➢ Materials can become work hardened through processes such as punching, bending,

forming, drilling, or rolling and have a high amount of residual stresses.
• Primary purpose:

➢ Improve hardness, strength and toughness.
➢ Restore ductility.
➢ Prevent hydrogen cracking.
➢ Remove residual stresses.

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• After being through hardening process, these materials are heated to tempering temperature, held
at temperature, cooling often in air.

• Process:
➢ Heated to about 300-700°C below the upper critical temperature in furnace.
➢ After heating – cooled air.

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• The martensite microstructure is transformed into fine martensite microstructure.

Martensite Fine Martensite

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TYPES EXPLANATION
Annealing
Normalizing To keep out inside strength after mechanical process.
Hardening Restructure the micro structure and recovered mechanical properties.
Tempering Cooling process inside the furnace.

Changing the rough micro structure and scattered after cold work .
It can present a good finishing by machine compared to non normalizing steel.
Cooling process outside of the furnace.

Aim for mechanical properties such as hardness, tough and unscratched.
This steel can obtain forced, rubbing or friction.
Cooling process in quench media.

After hardening process, material become hard but brittle.
Tempering was done by reheat after hardening.
Cooling process in air.

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