Preheats based upon thickness requirements are strictly for H2 cracking control. If there is no evidence of cracking you will find that your microstructure is probably even better suited to the service. Preheats slow down the cooling rate and help minimize shrinkage stresses, and evolves H2 out of the material. But this slowing down of the cooling rate runs counter to the what you actually want with low temp carbon steels, and that is a refined microstructure that demonstrates good low temp impact toughness.
If you have no cracks, you should be fine.