The color of metal heating is a change in the color of the material surface when heated, depending on temperature. This effect is used for visual control of temperature conditions in metallurgy and during heat treatment. Knowledge of heat colors helps to avoid overheating, preserve the structure of materials and ensure stable performance characteristics.
The importance of heat colors for metallurgy and materials science
The study of heat colors is important for metallurgy and materials science for several reasons:
- Temperature control: allows metallurgists to visually assess the temperature of the heated metal without using special devices.
- Development of new materials: helps to obtain valuable data on the behavior of materials at high temperatures, which is important when developing new alloys and composite materials with improved properties.
- Ensuring safety: in some industrial processes, it serves as an additional control tool, helping to prevent overheating and possible emergency situations.
The intensity and spectrum of radiation depend on the body temperature, its physical properties and the state of the surface. The higher the temperature, the more energy is emitted and the shorter the wavelength of maximum radiation. This explains why bodies heated to high temperatures emit light in the visible spectrum.
The physical basis of incandescale colors
The emission of light by heated bodies (thermal radiation or blackbody radiation) is a phenomenon based on the fact that as the temperature rises, the atoms and molecules of a substance begin to move faster and interact with each other more often, which leads to an increase in internal energy, which is then released in the form of electromagnetic radiation, including visible light.
When a body heats up, its atoms and molecules begin to emit electromagnetic waves of various lengths. At sufficiently high temperatures, this radiation falls into the spectrum visible to the human eye. The color of the radiation depends on body temperature:
- At low temperatures, the radiation is mainly in the infrared region, and the body appears dark or red;
- As the temperature rises, the radiation spectrum shifts towards shorter wavelengths, and the color changes from red through yellow and white to blue.
As the temperature rises, metal atoms and molecules begin to vibrate with greater intensity, which leads to an increase in the frequency and energy of the emitted electromagnetic waves. The radiation spectrum shifts from the infrared region towards visible light, which appears in the form of color shades.
Steel heat colors
As the metal heats up, certain shades of oxides contained in the thin surface layer (oxide film) disappear. First, the shades of purple decrease, then yellow, green and red. When the metal is heated to maximum, it begins to glow in pure white.
Incandescale colors are an important parameter when heat treatment steels and alloys, since the heating temperature directly affects the mechanical and physical properties of the material.
- Dark red (about 550—600°C) — the beginning of release and relief of internal stresses.
- Cherry red (approximately 600—700°C) — full release, softening of the structure.
- Bright red (700—800°C) is the beginning of relaxation and an improvement in plasticity.
- Yellow-red (about 800—900°C) — normalization and hardening of alloy steels.
- Light yellow (approximately 900—1000°C) — austenization before hardening.
- White heat (above 1000°C) is the beginning of melting and the risk of overheating.
As the temperature increases, the color becomes lighter and goes from red to yellow. Upon further heating, steel can reach white heat, which indicates a very high temperature close to the melting point. However, the exact determination of heat colors depends on the composition of the alloy and other factors.
The effect of material composition on heat colors
The colors of the heat may vary significantly depending on physical and chemical properties rafting. The main factors affecting surface color are:
- Thermal conductivity and heat capacity. Different materials have different thermal conductivity and heat capacity, which affects the rate of heating and cooling, as well as the temperature distribution within the alloy. Materials with high thermal conductivity, such as copper or aluminum, heat up faster and distribute heat more evenly, making the color less pronounced.
- Emission spectrum. The composition of the material determines its radiation spectrum when heated. Each chemical element and their compounds have an individual spectrum, which leads to differences in heat colors. For example, an alloy with a high carbon content will have richer reds and yellows than low-carbon steel.
- Oxide films. When heated, oxide films form on the surface of metals, which also affect the colors of the heat. These films can absorb or reflect specific wavelengths of light, changing the observed color. For example, aluminum's heat color may differ due to the formation of an oxide film on its surface.
- The structure of the material. Crystal structure and microstructure The material also plays a role in its hot colors. Grain size, differences in crystalline shapes, defects and impurity elements can affect the radiation spectrum when heated, especially in precision alloys.
Thus, the composition of the material is a key factor determining its heat color. Understanding these influences makes it possible to more accurately control heat treatment processes.
Methods for determining temperature by heat colors
Visual method
Visual assessment of temperature by heat color is the most affordable method that does not require special equipment. However, this method has a number of limitations:
- High requirements for the cleanliness of the material surface. To ensure good visibility of the surface, it is necessary to remove oxide films and other contaminants that may distort color perception.
- The need to compare the observed color with a well-known color scale or table that relates incandescant colors to specific temperature ranges. Such scales may vary depending on the type of metal or alloy.
- The possibility of error. The visual assessment of temperature is subjective and depends on lighting, viewing angle and individual characteristics of the observer's perception of color. Therefore, this method is often used in combination with other methods of measuring temperature.
The visual method is useful in situations where the use of measuring instruments is difficult or impossible. To improve accuracy, it is recommended to use reference scales that take into account specific material and observation conditions.
Pyrometers
To accurately measure temperature, pyrometers are used — devices that allow measuring temperature in a contactless way. They are widely used in metallurgy and other industries for precise temperature control of metals and alloys.
Types of pyrometers:
- Optical — they work by comparing the brightness of a heated body's radiation with the brightness of a reference light source at a certain wavelength. They can measure very high temperatures, but require adjustment and calibration.
- Infrared — register thermal radiation in the infrared range and convert it into temperature values. They are easy to use, provide quick measurement and can be adjusted to different temperature ranges.
- Colored — temperature is estimated based on the brightness ratio in several spectral ranges. Suitable for measuring temperature under conditions of strong radiation or light reflection.
The advantages of using pyrometers:
- Non-contact measurement helps to avoid damage to the measured object and ensures operator safety.
- High accuracy and speed of measurements make it possible to quickly obtain reliable temperature data.
- The ability to measure high temperatures that cannot be measured by contact methods.
The use of pyrometers is especially important in the production of precision alloys, where high accuracy of temperature control is required.
The importance of temperature control in the production of precision alloys
Accurate temperature control in the production of precision alloys is important for several reasons:
- The structure and properties of the material. Temperature affects the crystal structure and phase transformations in alloys. Incorrect temperature conditions can lead to the formation of undesirable phases, changes in grain size and other structural defects, which will adversely affect the mechanical and physical properties of the final product.
- The uniformity of the material. Precision alloys require high homogeneity of properties throughout the volume. Accurate adherence to the temperature regime helps to ensure an even distribution of components and avoid local inhomogeneities that can cause a decrease in strength, plasticity and some magnetic characteristics.
- Compliance with standards. Precision alloys are often used in critical equipment that requires strict compliance with established standards and technical requirements. Strict temperature control is an integral part quality assurance and the reliability of such materials.
PZPS produces a wide range assortment special steels and precision alloys, including various billets, hot-rolled and cold-rolled strips:
- made of soft magnetic alloys 27KH, 49K2FA-VI, 50N, 79NM, 80NM;
- made of precision alloys for elastic elements 17HNGT, 36NHTYU, 40KHNM;
- made of alloys with high electrical resistance X15YU5, X23U5, X20N80N, X15N60N;
- from alloys with a given temperature coefficient of linear expansion 29NK, 36N, 42N;
- made of corrosion-resistant steels 10X17N13M3T, 12X18N9, 12X18H10T;
- made of heat-resistant steel 20X13 and rafting HN78T.
At the PZPS, we use modern methods of temperature control to ensure the high quality of our products. For advice or to order precision alloys and special steels, please contact us through the site or by calling +7 812 740-76-57.
