Degree of surface damage state caused by temperature
I (none)
The possibility of fire ignition is insignificant due to presence of streams or rivers.
II (very low)
Less than 460°C creep, while minimal deformation of steel members occurs.
III (low)
Creep, the time-dependent deformation of a material, may be significant in structural steel at temperatures in excess of 460°C. The maximum temperature ranges from 460°C to 538°C.
IV (moderate)
The yield strength is approximately 60 percent of the value at normal room temperature. The modulus of elasticity decreased appreciably from the value at normal room temperature. The maximum temperature ranges from 538°C to 700°C.
V (high)
Steel retains about 20 percent strength and stiffness. The steel surface is noticeably oxidized and possibly pitted, with some accompanying erosion and loss of cross-sectional thickness. Also, members heated beyond 700°C usually show large deflection or localized distortion when heated in a structural system. The maximum temperature ranges from 700°C to 1000°C.
VI (critical)
Extreme overheating beyond the rolling temperature can be expected to reduce steel properties to a larger degree. Evidence of pitting and flaking appears on the steel surface if this extreme heating has occurred. The high temperature strength loss and thermal expansion are extreme at these temperatures, and structural damage or collapse will likely precede any overheating material damage. The maximum temperature reaches more than 1000°C.
