- Around 600 degrees Celsius, a slight red color begins to appear
- At 700 degrees Celsius, it turns into a light orange color
- At 800 degrees Celsius, it becomes red
- At 900 degrees Celsius, it turns yellowish-red
- At 1000 degrees Celsius, it turns a whitish-red color
This is not an accurate method and it may vary depending on the type of steel being used. These colors are only applicable for certain types of steels (probably carbon steel). The color of the flame can be different for different types of metals at the same temperature.
The relationship between the heating temperature of steel and its color:
In 1893, Wien studied the relationship between the maximum wavelength λmax and temperature T, which is λmaxT=2898μm•K.
Therefore, the temperature can be judged based on the color of the flame (i.e., the wavelength of light).
Empirical observation shows that dark red indicates 600°C, red indicates 900°C, orange-yellow indicates 1100°C, yellow indicates 1300°C, light yellow indicates 1400°C, yellow-white indicates 1500°C, and bright white (with a hint of yellow) indicates 1600°C.
There is a type of temperature-sensitive paper developed by Nichiyu Giken Kogyo Co., Ltd. that can be placed on the heated metal to show its temperature changes through different colors.
By observing the color changes of the paper on different parts of the metal, one can determine their respective temperatures and record them accordingly to make a color chart for further use.
The Relationship Between the Color of Steel Heating and Temperature
| Fire color | Temperature ℃ |
| Dark brown | 520——580 |
| Dark red | 580——650 |
| Dark cherry | 650——750 |
| Cherry blossom | 750——780 |
| Light cherry blossom | 780——800 |
| Light red | 800——830 |
| Orange-yellow with a hint of red | 830——850 |
| Light withered | 880——1050 |
| Yellow | 1050——1150 |
| Light yellow | 1150——1250 |
| Yellow-white | 1250——1300 |
| Bright white | 1300——1350 |
The relationship between the tempering color and temperature of carbon steel.
| Tempered color | Temperature ℃ |
| Light yellow | 200 |
| Yellow-white | 220 |
| Golden yellow | 240 |
| Yellow-purple | 260 |
| Dark purple | 280 |
| Blue | 300 |
| Dark blue | 320 |
| Blue-gray | 340 |
| Blue-gray light white | 370 |
| Black-red | 400 |
| Black | 460 |
| Dark black | 500 |
This seems to require a lot of experience, as the temperatures can be different during the day and night. The thermometer is not always easy to use and may not be very accurate.
There can also be differences between the temperature of the flame and the temperature of the object being measured.
