Annealing, Hardening, Tempering - Course: Working techniques of heat treatment of steel. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 27 p.) 6. Tempering (introduction...) 6.1. Types of Tempering

Annealing, Hardening, Tempering - Course: Working techniques of heat treatment of steel. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 27 p.)

6. Tempering

The steel becomes hard an brittle by the quenching process after heating. So high stresses can occur in the structure of the material that cracks are produced and the material slivers to pieces like glass.

In order to eliminate those negative effects and give the material the "useful hardness", it is tempered after having been hardened, i.e. it is heated once again. The toughness of the material is increased again at a justifiable decrease of the hardness and strength.

Tempering temperatures relate to the purpose of use of the workpiece.

The higher the tempering temperature, the lower the hardness and the tougher the steel.

When a blank steel is heated, a 0.2 mm thick oxide layer is produced on the surface, this oxide layer becomes discoloured in dependence on the temperature.

Apart from temperature gauges, the temperature can also be estimated by the colour.

Tempering colours for plain carbon steels:

Tempering colours	Temperature °C	Temperature K	Examples for use
Pale yellow	210	483	Steel scriber
Light-yellow	220	493	Measuring instruments
Yellow	230	503	Chisels of any kind
Dark-yellow	240	513	Twist drill, files
Yellowish brown	250	523	Milling cutter, reamers,
Brownish red	260	533	screw taps, metal saw blades
Red/purple	270	543	Screw drivers, woodworking tools
Violet	280	553	Hot-cross chisel, centrepunch, mandrils
Dark-blue	290	563	Springs, surgical instruments
Cornflower blue	300	573
Light-blue	310	583	Rivets, axes, hand saw blades,
Greyish blue	320	593	scynthes
Grey/greyish green	330	603	Household knives

6.1. Types of Tempering

Tempering from outside:

The cold workpiece is slowly heated by means of appropriate heat sources and after having reached the tempering temperature (between 200°C and 500°C) it is cooled down.

Tempering from inside:

The workpiece is shortly quenched after having been hardened so that only outer layer is cold. The residual heat penetrates from inside, after having reached the tempering temperature it is cooled down.

The same effect can be reached when the workpiece is cooled down only on one side and the heat contained in the other part runs after again.

After hardening the point to be tempered must be finished quickly by means of polishing linen cloth in order to recognize the temper colours!

The approximate value for the tempering time for compact workpieces:

1 to 1.5 hours for 20 mm thickness of workpiece

Hints:

- The tempering effect is a combination of time and temperature. If the tune is enlarged, the temperature can be decreased.

- For tools, longer tempering at lower temperature is better than shorter tempering at higher temperature.

- High-alloy steels require longer tempering times than unalloyed steels.

What is the purpose of tempering?
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________

Which influence is exercised by the tempering temperature onto the useful hardness?
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________

Why it is possible to determine the temperature by means of tempering colours?
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________