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Hans Sumersperger 1496: Emperor Maximilian's splendid sword in the Vienna Treasury , blued and gilded, iron cut

The tempering or bluing is a heat treatment in which a material is selectively heated to affect its properties, in particular tensions reduce, but also for purely decorative purposes. Tempering is used on a large scale in the processing of steels, aluminum and other non-ferrous metals as well as alloys and also in glass production.

Process in steel processing

After hardening or welding steel , the workpiece can be tempered by heating it to temperatures below the transformation point A 1 (723 ° C) . Internal tensions are reduced in the process. Tempering is a process of heat treatment.

Tempering is usually used after hardening. Hardened steel becomes softer the higher it is tempered. The hardness is reduced and the toughness increases. By oxidizing the surface to form annealing colors used to assess the tempering temperature and uses of the steel can be used (yellow for tools for machining of iron, purple for the treatment of brass, blue wood tools). The two most important parameters of starting are the tempering temperature and the starting time. The heating and cooling also affects the tempering effect. In practice, the most common tempering temperatures are between 200 ° C and 550 ° C; the starting time can be between minutes and hours. The tempering temperatures and periods are interchangeable. Tempering with a short duration and high temperature has the same effect as a long-lasting tempering at a correspondingly low temperature. This interchangeability is described by the Hollomon-Jaffe parameter . Formally, it corresponds to the Larson-Miller parameter , which also describes creep effects . There is the possibility of using the residual heat for tempering (residual heat use of the desired residual workpiece core temperature after quenching from the hardening temperature) or complete re-heating to the tempering temperature. Tempering takes place in special tempering furnaces that cause the workpieces to be heated quickly through air circulation and which have an extraction system for the oil vapors that arise. Tempering can also be done in a salt bath ( saltpeter or nitriding salt bath ) or in a slightly heated hardening furnace.

Temper levels in steel

Annealing colors for
unalloyed tool steel
colour temperature
White yellow 200 ° C
Straw yellow 220 ° C
Golden yellow 230 ° C
Yellow brown 240 ° C
Brownish red 250 ° C
red 260 ° C
Purple 270 ° C
violet 280 ° C
Dark blue 290 ° C
Cornflower blue 300 ° C
Light Blue 320 ° C
Blue-gray 340 ° C
Gray 360 ° C

In general, four tempering stages are important in steel processing:

  1. Temperatures below 80 ° C
    Segregation of carbon atoms ( chemical symbol C) at lattice defects , carbon cluster formation , d. H. Precursors of precipitations of carbon atoms
  2. From 80 ° C to 200 ° C (1st tempering stage)
    Steels with more than 0.2% carbon content : Martensite changes into α + ε carbides . α is also known as cubic martensite. ε-carbides (FexC) contain less iron (Fe) than conventional carbides (at 120 ° C x = 2.4)
    Steels with less than 0.2% carbon content : No formation of ε-carbides, since the carbon atoms are more energy-efficient in the vicinity of dislocations. The martensite is not or only minimally distorted tetragonal, i. that is, there is no change in the crystalline structure.
  3. From 200 ° C to 320 ° C (2nd tempering stage) (for low-alloy steels between 200 ° C and 375 ° C)
    The remaining austenite decomposes. Carbides and ferrite areas α 'are formed, which in terms of their concentration still differ from the equilibrium phases Fe 3 C and α. Alloy additives such as B. Chromium can shift the decay to higher temperatures.
  4. From 320 ° C to 520 ° C (3rd tempering stage)
    An equilibrium structure of cementite and ferrite is established , combined with a relatively strong reduction in hardness.
  5. Temperatures over 500 ° C
    Increasing formation and coagulation of the cementite particles
  6. Temperatures above 450 ° C to 550 ° C (4th tempering stage) (special carbide formers and / or mixed carbides)
    In the case of alloys containing vanadium , molybdenum , chromium and tungsten , special carbides precipitate at these temperatures. H. Carbides of alloying elements. If these are finely distributed enough and correspond to certain compositions, they can lead to increases in hardness that even exceed the martensite hardness (secondary hardness maximum). Such alloys are commonly referred to as hot-work steels .

Temper embrittlement

Two embrittlement phenomena are observed in connection with tempering:

  1. "300 ° C embrittlement" or "blue brittleness" (200 ° C <T <400 ° C)
    Assumption: precipitation of carbon and nitrogen on the grain boundaries, aging phenomenon in steels with a higher C content;
    Consequence: reduced cold formability and toughness ;
    Avoidance: Avoid T-area or alloy with silicon.
  2. "500 ° C embrittlement" or tempering embrittlement (448.5 ° C <T <530 ° C)
    Cause: Enrichment of the austenite grain boundary with trace elements or carbides; especially with manganese, chromium-manganese and chromium-nickel steels;
    Consequence: reduced toughness : notched bar impact energy is reduced and transition temperature in the notched bar impact test increases;
    Avoidance: If this temperature range of the tempering temperature cannot be avoided, add molybdenum (already significant improvement at 0.05–0.1%, effect hardly exists at 0.2–0.3%) or tungsten.

Processes not related to steel

In the further processing of rolling ingots and extrusion billets made of aluminum and its alloys as well as other industrially important non-ferrous metals, tempering to the optimum temperature for rolling, drawing and pressing processes, as well as for drop forging, is an elaborate technique that also allows for recrystallization and recrystallization machining material.

Tempering is used in the manufacture of glassware in order to relieve existing stresses in the material through the cooling or reshaping process of glass . The glassware is heated to such an extent that it does not soften again, but the internal tensions can be balanced out. Then the temperature is slowly lowered and the glass is slowly cooled down to below a critical point. It can then be cooled further quickly. If this treatment does not take place, the glass breaks relatively quickly or shatters spontaneously in the event of temperature shocks.

See also

Individual evidence

  1. ^ Wilhelm Ostwald: Basics of Inorganic Chemistry, page 612
  2. ^ Ulrich Fischer: Metal table book . 41st edition. Verlag Europa-Lehrmittel Nourney, Vollmer, 2001, ISBN 3-8085-1721-2 , p. 128B.
  3. ^ Liu Cheng: Phase Transformation in Iron-Based Interstitial Martensites. PhD thesis, Delft University of Technology, the Netherlands, 1990.
  4. a b c E. Macherauch: Internship in materials science. 9th edition, Vieweg Verlag, Braunschweig / Wiesbaden 1990.
  5. H.-J. Eckstein: technology of heat treatment of steel. 2nd edition, VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig 1977.