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HardeningQuenchingAnnealingTemperingNormalizingAustempering

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HARDENING

Six major mechanisms are available to increase the strength of metals

Solid-solution strengthening Strain Hardening Grain size refinement Precipitation Hardening Dispersion Hardening Phase Transformation

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Solid solution StrengtheningIn solid solution strengthening, a base metal

dissolves other atoms, either as substitutional solutions, where the new atoms occupy sites in the host crystal lattice, or as interstitial solutions, where the new atoms squeeze into “holes” between the atoms of the base lattice.

The amount of strengthening depends on the amount of atoms involved.

Since distortion of the host structure makes dislocation movement more difficult, the greater the size difference, the more effective the addition.

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Produces an increase in strength by means of plastic deformation under cold-working conditions.

Grain Size Refinement Because grain boundaries act as barriers to

dislocation motion, a metal with small grains tend to be stronger than the same metal with larger grain.

Thus grain size refinement can be used to increase strength, except at elevated temperatures, where grain growth can occur & grain boundary diffusion contributes to creep and failure.

It is important to note that grain size refinement is one of the few processes that can improve strength without a companion loss of ductility & toughness

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Strain Hardening

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PRECIPITATION HARDENING In precipitation hardening or age hardening,

strength is obtained from a non equilibrium structure that is produced by a three-step heat treatment (For detail pl see 5.4).

DISPERSION HARDENING Strength obtained by dispersing second phase

particles throughout a base material is known as dispersion hardening. To be effective, the dispersed particles should be stronger than the matrix, adding strength through both their reinforcing action & the additional interfacial surfaces that present barriers to dislocation movement.

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Involves those alloys that can be heated to form a single phase at elevated temp & subsequently transform to one or more low-temp phases upon cooling.

When this feature is used to increase strength, the cooling is usually rapid & the phases that are produced are usually of a non equilibrium nature.

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Phase Transformation

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QUENCHING Austenitizing – again taking a steel with 0.6%

carbon or greater and heating to the austenite region.

Rapid quench to trap the carbon in the crystal structure – called martensite. Quench requirements determined from isothermal transformation diagram (IT diagram).

Depending on how fast steel must be quenched (from IT diagram), the heat treater will determine type of quenching required:Water (most severe)OilMolten SaltGas/ Air (least severe)Adding 10% sodium hydroxide or salt will have

twice the cooling rate!

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Annealing

primary purpose is to soften the steel and prepare it for additional processing such as cold forming or machining. If already cold worked - allows recrystallization.

What does it do?1. Reduce hardness2. Remove residual stress (stress relief)3. Improve toughness4. Restore ductility5. Refine grain size

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Annealing Process Steps:

1. Heat material into the austenite region (i.e. above 1600F) – rule of thumb: hold steel for one hour for each one inch of thickness.

2. Slowly furnace cool the steel – DO NOT QUENCH

3. Slow cooling allows the C to precipitate out so resulting structure is coarse pearlite with excess ferrite.

4. After annealing steel is quite soft and ductile.

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Other forms of Annealing

Normalizing – use when max softness not require and cost savings desired (faster than anneal). Air cooled vs. furnace cooled.

Process Anneal – not heated as high as full anneal.

Stress Relief Anneal – lower temp (1,000F), slow cooled. Large castings, weldments.

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TEMPERING

Steel is usually harder than necessary and too brittle for practical use after being hardened. Severe internal stresses are set up during the rapid cooling of the metal.

Steel is tempered after being hardened to relieve the internal stresses and reduce its brittleness.

Tempering consists of heating the metal to a specified temperature and then permitting the metal to cool.

The rate of cooling usually has no effect on the metal structure during tempering.

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TEMPERING Therefore, the metal is usually

permitted to cool in still air. Temperatures used for tempering

are normally much lower than the hardening temperatures.

The higher the tempering temperature used, the softer the metal becomes.

High-speed steel is one of the few metals that becomes harderinstead of softer after it is tempered.

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NORMALIZING

Ferrous metals are normalized to relieve the internal stresses produced by machining, forging, or welding.

Normalized steels are harder and stronger than annealed steels. Steel is much tougher in the normalized condition than in any othercondition.

Parts that will be subjected to impact and parts that require maximum toughness and resistance to external stresses are usually normalized.

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NORMALIZING

Normalizing prior to hardening is beneficial in obtaining the desired hardness, provided the hardening operation is performed correctly.

Low carbon steels do not usually require normalizing, but no harmful effects result if these steels are normalized.

Normalizing is achieved by heating the metal to a specified temperature (which is higher than either the hardening or annealing temperatures), soaking the metal until it is uniformly heated, and cooling it in still air.

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AUSTEMPERING Austempering is an isothermal treatment that

produces a bainite structure in some plain-carbon steels.

The process provides an alternate procedure to quenching and tempering for increasing the toughness and ductility of some steels.

In the Austempering process the steel is first austinitized, then quenched in a molten salt bath at a temperature just above the Ms temperature of the steel, held isothermally to allow the austenite-to-bainite transformation to take place, and then cooled to room temperature in air.

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AUSTEMPERING The final structure of an austempered eutectoid

plain-carbon steel is bainite. The advantages of austempering are : Improved ductility and impact resistance of

certain steels over those values obtained by conventional quenching and tempering and

Decreased distortion of the quenched material. The disadvantages of austempering over

quenching and tempering are the need for a special molten salt bath and the fact that the process can be used for only a limited number of steels.

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