Heat treatment of steel. Scheme, description

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Heat treatment of steel. Encyclopedia of radio electronics and electrical engineering

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Heat treatment is necessary when parts or tools need to be given new qualities: increased hardness, resistance to large shock loads, etc. It consists mainly of three processes: hardening, tempering and annealing.

Hardening. What seems to be easier: he heated the part, lowered it quickly into the water - and it is hardened. The process is simple if you know what temperature to heat up to, how to determine it and in what medium to cool the part (in water, oil, etc.).

The correctness of hardening of carbon steels is controlled according to the schedule shown in the figure. When heated above the upper boundary of the hardening zone (given by a thick line), all the original properties of the metal are lost. Carbon steels with a carbon content below 0,30% (grades from St. 0 to St. 30, see previous article) are not hardened at all.

Tool and alloy steels have hardening temperatures: for grades U7 - 800...820°C; for U8 - 730 ... 800 ° С (quenched in water); for U9-U13 from 760 to 780°C (hardened in water).

For chromium - 830 ... 860 ° C, chrome-silicon - 820 ... 860 ° C (quenched in oil, tempering temperature 150 ° C); chromium-tungsten - 800 ... 820 ° C (quenched in water, tempering temperature 150 ° C).

The cooling medium can be water and oil. In amateur practice, water, a 10% solution of table salt are used for carbon steels; for alloyed - water and mineral engine oil. The temperature of the medium is room temperature.

With a little skill, you can determine the heating temperature with the required accuracy by the color of the hot metal:

dark brown (visible in the dark) - 530...580°С;
brown-red - 580...650°С;
dark red - 650...730°С;
cherry red - 770...800°С;
light red - 830...900°С;
orange - 900...1050°C;
dark yellow - 1050...1150°С;
light yellow - 1150...1250°С;
bright white - 1250...1350°С.

When hardening miniature parts, in order not to overheat them, the part is placed on some blank, which is heated. The color is determined by the color of the blank.

If the part is not completely hardened (half shafts, nipper jaws, etc.), then the boundary of the hardened area is "washed out". The part is lowered into the cooling medium to a certain level (only the jaws of the nippers) and quickly sways up and down.

Vacation. The hardened steel part is brittle due to the presence of high internal stresses. With the help of a thermal process - tempering - brittleness can be reduced; while maintaining the hardness obtained during hardening.

Vacation is carried out at relatively low temperatures. The part heated for this purpose is cooled in the open air.

The tempering temperature is easy to determine by the so-called tint colors, which are colored oxide films that form at different temperatures on a well-cleaned surface of the tempered part:

light yellow (straw) - 220°С;
dark yellow - 240°С;
brown-yellow - 255°С;
brown-red - 265°C;
light blue - 285°С;
violet - 295°С;
cornflower blue - 330°С;
gray - 351°С.

When tempering small parts, they use heating on a blank. In this case, the tint colors are observed on the part itself.

The tempering temperatures of some tools, fixtures and parts are as follows: measuring - 150 ... 180 ° С; cutting metal from carbon steels - 180...200°C; hammers, dies, small drills - 200-225°C; drills for soft steels, punches, drills, cutters - 225...250°С; drills and taps for copper and aluminum, chisels - 250...280°С; tool for woodworking - 280...300°С; springs - 315...330°С; springs - 400...500°С; parts and tools operating under heavy loads - 500 ... 650 ° C.

Annealing. The steel annealing process is necessary when it is necessary to do something else from an old hardened tool or part (a hardened part cannot be machined due to its high hardness).

During annealing, a previously hardened part is heated to the desired temperature (see figure) and then cooled in the open air.

Heat treatment of steel

After annealing, the steel acquires all the qualities that it had before hardening, that is, it is easily processed.

Author: L. Erlykin

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