heat treating of steel - home - sharpschool...
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Heat Treating of
Steel
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Heat Treatment
Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material.
http://en.wikipedia.org/wiki/Heat_treat
The techniques include annealing, case hardening, induction hardening, precipitation strengthening, tempering and quenching.
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Properties most affected by Heat Treating
Hardness: the characteristic of a solid material expressing its resistance to permanent deformation (scratching, abrasion, wear)
Ductility:is the physical property of being capable of sustaining large plastic deformations without fracture (in metals, such as being drawn into a wire).
Brittleness: A material is brittle if it is liable to fracturewhen subjected to stress i.e. it has little tendency to deform (or strain) before fracture.
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Effects of Hardness
Keep in mind that as hardness of a material goes up, it becomes more brittle and less ductile.
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Heat Treatment
Regardless of the reason for the heat treating, the basic process is the same and has three steps.
1. Heat the metal to a specific temperature
2. Hold the metal at that temperature for a specific amount of time
3. Cool the metal in a specific manner
http://www.jjjtrain.com/vms/eng_heat_treat/eng_heat_treat_01.html
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Annealing
http://www.jjjtrain.com/vms/eng_heat_treat/eng_heat_treat_02.html
The metal is heated in the furnace and allowed to cool very slowly in the furnace after the furnace has been turned off.
Softens the metal and relieves stresses.
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Normalizing
http://www.jjjtrain.com/vms/eng_heat_treat/eng_heat_treat_03.html
Similar to annealing but the material is cooled in the air.
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Stress Relieving
http://www.jjjtrain.com/vms/eng_heat_treat/eng_heat_treat_04.html
Similar to normalizing but the material is not heated to as high of a temperature, therefore the material is not softened as much as it is in normalizing.
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Hardening
Sometimes referred to as quenching or is included in quench tempering.
http://www.jjjtrain.com/vms/eng_heat_treat/eng_heat_treat_05.html
Induction Hardening:
http://en.wikipedia.org/wiki/Induction_hardening
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Hardening
The steel is heated to its transformation temperature. This is the temperature in which the carbon more easily dissolves in the steel, a state called austenite
The steel is held at that temperature until it is thoroughly heated (usually 1 hour per inch of penetration).
It is then cooled very quickly, called quenching. This freezes the carbon in its dissolved state and results in a steel at its maximum hardness.
The three most common quenching agents are water, oil, and brine http://www.navaching.com/forge/quenchants.html
To achieve maximum hardness (martensite), the steel must be cooled from its critical temperature to less than 1000 deg. F in less than one second.
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Carbon Designations
Low Carbon Steel: .3% Carbon or less
Medium Carbon Steel: .3% - .6% Carbon
High Carbon Steel: .6% Carbon and up.
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Transformation Temperatures
The critical temperature (as well as the maximum obtainable hardness) vary depending on the amount of carbon.
phasediagram.tif
http://www.auto-ware.com/techref/heattreat.htm
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Case Hardening
A steel needs to be have carbon to respond to hardening processes. A low carbon steel does not respond. Sometimes low carbon steels are case hardened, a state where the outside casing is hardened and the inside remains soft. This provides good wear resistance yet low brittleness.
http://en.wikipedia.org/wiki/Case_hardening
Carburizing:http://en.wikipedia.org/wiki/Carburization
Nitriding:http://en.wikipedia.org/wiki/Nitridization
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Tempering
http://en.wikipedia.org/wiki/Tempering
Used to “draw” a hardness back to a specific level and reducing stresses and brittleness in the process. Hardening and Tempering is the best way to achieve a specific hardness somewhere between a steel’s original state and its maximum hardness.
It is a similar process to hardening except that the steel is heated to a lower temperature than the critical temperature.
Temperatures used for heat treating O-1 tool steel
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You are now prepared to complete the
Heat Treating Processes
Written Assignment