1. With increasing percentage of carbon the ductility decreases and brittleness increases.
2. Plain carbon steel cannot be deep hardened on heat treatment, because due to effect of heat, only surface of steel get hardened while the inner layer of the steel remains soft.
3. During the use the mechanical properties of plain carbon steels get deteriorated at higher temperature. Thus the use of plain carbon steel to make various machine parts has limitations of temperature.
4. The corrosion resistance of plain carbon steel has limited use in manufacturing various machine parts, though strengths and welding characteristics may be suitable.