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close this bookAnnealing, Hardening, Tempering - Course: Working techniques of heat treatment of steel. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 27 p.)
close this folder6. Tempering
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View the document6.1. Types of Tempering

(introduction...)

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