Heat Treating of

Steel

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.

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.

Effects of Hardness

Keep in mind that as hardness of a material goes up, it becomes more brittle and less ductile.

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

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.

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.

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.

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

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.

Carbon Designations

Low Carbon Steel: .3% Carbon or less

Medium Carbon Steel: .3% - .6% Carbon

High Carbon Steel: .6% Carbon and up.

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

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

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

You are now prepared to complete the

Heat Treating Processes

Written Assignment