Temper colors of metals, determining the temperature by the color of the heated workpiece

Metal tarnish colors – is a spectrum of colors that form on the surface of a metal when an oxide film forms. These oxide films are created from the metal itself during heating. A key condition for the formation of such a film is the absence of exposure to water.

Such metal tarnish is a defect in the welded joint.

Origin of metal tarnish colors

In nature, tarnished colors can be observed on the surfaces of a number of minerals, including pyrite and chalcopyrite. It's logical to conclude that these changes are visible due to oxidation of the surface layer of the material. As a result, they become coated with a thin oxide film, which refracts the light striking it. The resulting interference effect "colors" the metal surface in different colors.

The brightness of the annealing colors depends on the thickness of the oxide film formed and the wavelength of light striking the surface of the material. The brightest shades can be seen on copper minerals. The resulting colors also depend on the metal's composition. If the element contains many metal ions, it will appear blue. If chromophores are present, you'll see red colors.

Artificial tarnish color of metal appears on its surface when exposed to high temperatures. The important condition is the absence of water or any other liquids.

As the temperature increases, the resulting oxide film shrinks, which is explained by diffusion (the process of "mixing" or penetrating particles of a chemical element into another material). Specifically, in the case of a metal oxide film, the interaction between oxygen atoms and the metal is observed.

It is worth noting that on alloy steels, the tempering color will appear at higher temperatures than on carbon steel.

Making artificial tarnished flowers

Blueing is a widely used technique in metalworking. Coating alloys with oxide films has been known and used for thousands of years.

Blued metal is rust-resistant, more durable under mechanical stress, and has a beautiful color even without additional coatings and paints.

Bluing is performed as follows:

• The workpiece is dipped or wiped with mineral oil;
• Heat on a metal sheet to the appropriate temperature (it may differ for different metals and alloys);
• Afterwards, they can perform hardening in cold oil (to avoid “tempering of the metal”).

The resulting oxide layer on the surface of a metal product is completely resistant to water and also has high resistance to mechanical stress.

Tables 1.

Oxide films form at different rates and are influenced by the following factors:

1. Hardening of the part (the presence of hardening accelerates the appearance of tempering);
2. The presence of contaminants (when heated, contaminants become carbonized and complicate the formation of a uniform layer of oxide film);
3. Roughness. A workpiece with uneven surfaces develops a dense film, and the resulting beautiful iridescent colors may be invisible. A polished part, on the other hand, quickly forms a uniform, thin layer of oxide on the surfaces;
4. Heating technologies. Depending on the equipment used to heat the parts, oxide films form at different rates and thicknesses. It's best to use equipment that allows for stable temperature control and maintenance.

Thin oxide films absorb light with shorter wavelengths but reflect those with longer ones. The color of metal parts changes depending on the temperature and density of the oxide film. The thicker the oxide film, the lighter the color. Blue or violet colors result when the longer wavelengths of the spectrum are reflected. If the oxide film reflects shorter wavelengths, the metal surface turns yellow. Light colors correspond to higher heating temperatures, while lighter colors correspond to lower temperatures. For this reason, many craftsmen often use annealing colors to determine the degree of hardening of parts, steel chips, and cutting tools used during turning.

Despite these factors, it is impossible to accurately determine the temperature of the metal using tempering colors, because the following factors influence the value of this indicator:

• heating time: the period of time during which a metal part heats up to the ambient temperature in the absence of heat transfer.
• the presence of various impurities in the metal;
• lighting features in the room where welding or hardening of workpieces was carried out;
• heating rate: the change in the temperature of a product per unit of time during its heating.

Among modern devices, pyrometers provide fairly accurate temperature monitoring. They operate by analyzing laser beams. These devices are equipped with special sensors that analyze reflected laser beams and display the metal temperature, which corresponds to the measured radiation characteristics.

Tarnishing technologies are widely used in the production of tools and equipment. This technique is especially common when working with copper, iron, aluminum, and brass.

Hardening improves the following parameters of the metal surface:

The color of the tarnished metal and its temperature or the temperature of the tarnished colors of the metal

As has already become clear from the material described above, the temperature and color of the metal changes throughout the heating of the workpiece. It is important to note that tempering temperature of metal The tarnish color varies for each individual alloy and type of metal. Therefore, there are numerous tables and lists of color-temperature relationships. Below are tables of metal tarnish colors for various alloys.

Steel tempering color scale

For carbon steels, the following relationship between colors and corresponding temperatures can be given:

Temper color temperature for carbon steels
Color	Temperature limits, °C
Citric	220 – 229
Yellow (straw color)	230 – 245
Gold	246 – 255
Earthy or brown	256 – 264
Scarlet or red-orange	265 - 274
Purple	275 – 279
Amethyst	280 – 289
Heavenly	290 – 294
Twitter	295 – 299
Indigo Crayola	300 – 309
Light blue	310 – 329
Aquamarine	320–339

 

On 12Kh18N10T stainless steel blanks, which contain 18% chromium, 10% nickel, and 1% titanium (taken from GOST 5632-2014), the annealing colors will change slightly differently depending on temperature. The main difference is the temperature. This is explained by the corrosion and heat resistance. Therefore, during heating and cooling, alloy particles and oxygen interact more slowly, slowing the formation of an oxide film.

The following table of metal tarnish colors shows the characteristics of color change in stainless steel products:

Temperament temperatures for stainless steels
Color	Temperature limits, °C
Light straw	300 – 399
Golden	400 – 499
Earthy or brown	500 – 599
Red or purple	600 – 699
Blue or black	700 – 779

Rainbow streaks may appear on the surfaces of stainless steel workpieces. These streaks can occur when the product is heated to boiling point (100°C). These rainbow streaks are caused by changes in the metal's crystal lattice.

Rainbow coloration on the surface of the workpiece does not indicate overheating of the stainless steel.