Flame Carburization

Flame carburization (such as oxyacetylene flame hardening) gets Rockwell C hardness up almost a factor of 2 from ~35 for mild steel to 51-56 according to this industrial process thesis from India -

http://ethesis.nitrkl.ac.in/1397/1/final_thesis_jaykant_gupta_(combined_123).pdf

55-60 is the hardness of 'very hard steels' such as tool steel