Temper colors are produced at temperatures well below that necessary for hardening. This would of course normally give a uniform color to the entire part, not the mottled effect of Color Case Hardening. For hardening to take place it is necessary the part be above its critical temp & then quenched. This temp varies according to the carbon content of the base steel. The higher the carbon content the lower the critical temp. The listed 1400° is no doubt above the critical temp for the steel being used for those results. Often times the temp will be raised somewhat for the Carburising stage & then dropped for the hardening phase, but, not below that critical temp. After carburizing the carbon rich case does not need quite as high a temp to harden as it needed to absorb the carbon. Most of the warpage & size change occurs at the quench & the higher the temp above that which is an absolute requirement the more likely it is to occur.
As I stated I have not been involved in any Color Hardening, but it is my understanding to get good colors some form of Animal Charcoal is essential. Just a pure Heat Temper will not give these mottled effects.
