Flame hardening is NOT suitable for surfaces of low carbon steel components due to

Concept:

The steel is being divided as low carbon steel, high carbon steel, medium carbon steel, high carbon steel on the basis of carbon content. Low carbon steel has carbon content of 0.15% to 0.45%.

Low carbon steel is the most common form of steel as it’s provides material properties that are acceptable for many applications. It is neither externally brittle nor ductile due to its lower carbon content. It has lower tensile strength and malleable. 

Low-carbon steel is the most widely used form of carbon steel. These steels usually have a carbon content of less than 0.25 wt.%. They cannot be hardened by heat treatment (to form martensite) so this is usually achieved by cold work.

Low carbon steel (Soft but ductile), the metal is easy to form, but it requires significant quantities of flame-distributed energy to heat treat the surface and harden the material. The carbon absorbs thermal energy, but, due to the lower content rating, greater temperatures are needed if the required case hardening depth is to be realized.

Advantageously, this low carbon candidate is less prone to cracking and distortion than its carbon-rich relative, which is a desirable feature, considering the localized nature of an oxy-acetylene flame hardening torch.