Tempering is a process that is generally carried out on a steel that has already been quench hardened. The main reason for a tempering process is to remove some of the excessive hardness while maintaining strength. The process is as follows;
Re-heating (below the A1 temperature)
The steel is re-heated slowly and uniformly to a temperature below A1.
Holding (certain period)
The steel is held at this temperature to ensure that the carbon atoms saturated in the martensite diffuse to form a carbide precipitate and ferrite (a) and cementite (Fe3C).
Removes brittleness from a steel
- Improves ductility
- Improves toughness
- Improves strength- Provides a stable structure.
Depending on the actual temperatures used in the process, if a lower temperature is used then some of the original hardness can be maintained.
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= Increases toughness
Tensile and yield strength
If you click the links, it will show the difference in the tensile strength (σTS) and yield strength (σY) that can be achieved by carrying out a quench and temper process. When tempering the carbon in the saturated solution reacts with Fe to make Fe3C. This returns the solution back to equilibrium, increasing the strength of the steel.
Over tempered steel
As seen in the graph below, if the steel is over tempered then the Fe3C grains coarsen and the overall hardness of the steel drops considerably.
The 3 images show supersaturated C coming out of solution as small particles of carbide (not pearlite).