As the carbon content of steel increases its critical temp becomes lower. I was a machinist, not a metallurgist, but I highly suspect what Dr Gaddy was doing was reaching the higher critical temp for the low carbon steel & then as it absorbed carbon dropped the temp to reflect the lower critical temp of the now higher carbon content. I believe it is necessary to be above the critical temp for hardening to take place, & also some minimum amount of carbon must be present. I do not know what the absolute minimum is but know that regular heat treating is not generally done on stells from around 1020 & down. These steel are normally carburized & hardened (Case Hardened).
The carbon content in the case on carburized steel can become quite high, so would have a lower critical temp than most carbon steel alloys.
