"Blue brittle" phenomenon:

Cold knowledge: When some carbon steels (especially those with high nitrogen content) are processed or subjected to stress in a specific temperature range (about 200°C - 300°C), their strength and hardness will increase abnormally, but at the same time, their plasticity (toughness) will drop sharply, becoming very brittle and easy to break like glass. This temperature range is exactly the range where the oxide film on the surface of the steel begins to appear blue, so it is called "blue brittle".

Why is it cold? This is not a generally recognized material property. For steel pipe manufacturing (such as hot straightening, hot bending) or pipes in service at specific temperatures, it is very important to avoid this "blue brittle zone", otherwise catastrophic brittle fracture may occur.

The "rainbow color" of the weld is not just aesthetics:

Cold knowledge: After welding, the surface of the weld and heat-affected zone of carbon steel pipes sometimes has beautiful yellow, blue, purple and other oxidation colors (rainbow colors). This is not just the result of high-temperature oxidation. These colors actually reflect the different peak temperature ranges that the steel experiences during the cooling process.

Why is it cold? An experienced welder or inspector can roughly judge the temperature history of the weld area by observing these colors. For example:

Light yellow: ~430°C

Dark yellow: ~460°C

Brown: ~510°C

Purple: ~550°C

Blue: ~600°C

Gray-black: higher temperature or thicker oxide scale

This helps to indirectly assess the welding heat input and possible microstructural changes in the heat-affected zone.