Steel blueness - what does that mean?

The blueing or "tempering" of steel is a process that is often used in steelworking. What this measure in the heat treatment, how it is performed, and what annealing colors in steel processing are, can be found in this article.

Purpose of the blueing

The blueing of a steel workpiece serves to relieve internal stresses in a workpiece. Incidentally, a similar technique with the same purpose is also used in other metals and glassmaking.

Internal stresses in the workpiece

Especially due to work such as hardening of steel, residual stresses can build up in the interior of the workpiece due to the structural displacements. Targeted tempering can relieve such stresses without damaging the workpiece. Otherwise, cracks or a general weakening of the material would threaten at residual residual stresses.

Starting procedure

The blueing is a heat treatment. After quenching (hardening process), the workpiece is reheated as soon as possible. The tempering temperature depends on the desired properties of the steel. Using the tempering temperature, later steel properties can be set relatively precisely.

In any case, the temperature must be below the so-called transformation temperature (723° C). If the temperature exceeds this value, this can lead to structural weakness of the steel. In extreme cases, the respective workpiece can even become completely unusable as a result.

Property changes due to tempering

The higher the tempering temperature, the more the steel loses its hardness. At the same time he gains toughness. Bluing allows you to selectively shift the balance between these two properties.

Of importance for the property changes are therefore the tempering temperature, and the duration of the action of this temperature (annealing time). In practice, tempering temperatures in the range of 300° C. to 550° C. are customary today. The tempering time can be very short (a few minutes) but also very long (up to several hours).

Annealing colors

Tempering colors are the temperature changes on the surface of the workpiece during tempering. They are formed by oxidation of the surface. Depending on the temperature, the color changes. At 200° C the surface is white-yellow, with increasing temperature it turns golden yellow to brown and purple (270° C) and then to violet and blue. At 360° C then shows a clearly gray color

Tips & Tricks

Tempering colors as described above apply only to unalloyed tool steels. Alloy components of higher alloy steels make this type of temperature control ineffective.

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