During the heating process of steel, its color will show different characteristics as the temperature changes.
1. Low temperature stage (about 200-500 °C)
The temperature is around 200-300 °C, and the steel surface begins to show a light yellow color . This is because at this temperature range, the surface of the steel begins to undergo slight oxidation, resulting in a very thin oxide film, resulting in color changes.
As the temperature rises to 300-400 °C, the steel color gradually changes to yellowish brown. This is due to the further deepening of oxidation degree and the increase of oxide film thickness.
When the temperature reaches 400-500 °C, the steel surface appears purple. The color change at this stage is due to the intensification of the thermal motion of the atoms inside the steel, and the energy level transition of the electrons produces a specific wavelength of light.
2. Medium temperature stage (about 500-900 °C)
When the temperature is within 500-600 °C, the steel surface becomes a deep purple. At this time, the oxidation reaction of the steel is more intense, the oxide film continues to thicken, and the intensity of thermal radiation is also increasing.
When the temperature rises to 600-700 °C, the steel takes on a dark blue color. This is because at this temperature, the spectral distribution of thermal radiation changes, and the blue light composition increases relatively.
At 700-800 °C, the steel color shows light blue. At this time, the organizational structure of the steel begins to change, such as the pearlite begins to change to austenite, and this organizational change will also affect the characteristics of thermal radiation, which will affect the color of the steel surface.
When the temperature reaches 800-900 °C, the steel surface appears bright red. This is because at high temperatures, the energy of steel thermal radiation is mainly concentrated in the red light band, and the oxidation and organizational transformation of steel are rapid.
3. High temperature stage (about 900-1500 °C)
At 900-1000 °C, the steel appears orange-red. At this time, the austenitic structure inside the steel is basically formed, and with the increase of temperature, the thermal radiation energy is further increased.
When the temperature reaches 1000-1200 °C, the steel surface is yellow. At this stage, the thermal radiation intensity of steel is very high, and there may be decarbonization and other phenomena at high temperatures.
At 1200-1300 °C, the steel appears white yellow. At this time, the steel is in a critical state of high-temperature liquid or semi-liquid, the wavelength of thermal radiation is shorter, and the color is more white.
When the temperature is 1300-1500 °C, the steel surface is white. This is because at this extremely high temperature, the energy radiated by the steel heat is evenly distributed throughout the visible light band, showing a white color.
Experienced operators can roughly judge the temperature by looking at the color of the steel and prepare for the next operation.
