Characteristics of Steel Sheet - Heat Change
1. Heat Change

Changes in steel sheets due to heat application can be divided into the following types:
Scale (iron scorch = Thin oxidized layer that appears on heated steel)
Change to molecular structure
Expansion and shrinkage

These changes to steel panels mainly occur through heating during body panel welding work, oxyacetylene torch heating, and friction heat from grinding or sanding.

As steel sheet is heated, the color changes until it starts to melt at 2,132 °F (1,500 °C).
Color
Temperature Range °F (°C)
Color
Temperature Range °F (°C)
Dark Red
1,112 (600)
Yellow
1,832 (1,000)
Red
1,292 (700)
Orange
2,012 (1,100)
Rose-pink
1,562 (850)
White
2,192 (1,200)
Yellowish Red
1,652 (900)
Bright White
2,282 (1,250) or higher


Heating Temperature and Material Change:

As a steel sheet is heated, its plasticity increases. However, if the temperature of the sheet exceeds a certain point, it becomes hard and brittle. Therefore, it is important not to exceed 1,333 °F (723 °C) during steel processing to prevent this structural change. At 392-482 °F (200-250 °C), blue heat brittleness happens to the sheet. If 1,328 °F (720 °C) is exceed, red brittleness occurs. Further, if it goes to over 1,652 °F (900 °C), the steel grains become large and rough causing oxidization and weakening of the material.

Brittleness

Generally, the more heat applied to a metal, the softer it becomes and the easier it is to work with. But, where soft steel is concerned, hardness and tensile strength peak then elongation decreases at a certain temperature.