I realize that you were talking about backgouging and not welding MW...
Consider this... As you know, Carbon arc gouging is a concentrated brutal fast heating of metal that is blown out from the metal with pressurized air to form a groove where the metal used to be... The heat from the CAC is the same or more concentrated than the heat from welding... The sudden heating and cooling caused by the gouging is sure to cause some surface micro cracks if the metal isn't preheated to the same temperature used to preheat for welding on the other side with
3 inch thick steel and then left to rapidly cooling back down to ambient temperature... The preheating and flame cutting or gouging are parts of the preparation for welding but can be considered part of the welding operation... When flame cutting or gouging is required, preheating should be the same as when welding... It might not be quite as important since stresses are much smaller; however, the thermal shock on the metal can occur in gouging as well as in welding...
Then there's the possibility of moisture in either the CAC electrodes if they're not properly stored, and the possibility that the air itself being supplied to the torch not being properly filtered for moisture also can be a source to introduce Hydrogen into the gouged groove... A lot of variables to consider...
Now you only mentioned that the steel is a group 1 and group 2 grade which narrows the particular grade to only a few grades and since you don't want to be specific that's your perogative... So could you tell us if you're using a low hydrogen type of filler metal to weld with and if you are, what is the diffusable Hydrogen rate? Not that it really matters because what I'm trying to get you to understand is that you're asking for trouble if you don't use the same preheat temp that was used to weld the joint...
Sure the requirement to preheat when backgouging isn't listed in AWS D1.1 because it makes common sense to use preheat on both sides even to backgouge on the side opposite to the side where most of the weld deposit for the joint is located... And that's only provided that the person gouging doesn't find any porosity or trapped slag or even the possibility of voids caused by lack of fusion because if there is any of that appearing during the backgouging process, more metal would have to be removed... Meaning more heat input for an extended period of time will be required in order to gouge all of the crap that could be found... There's also the possibility that the gouging will be uneventful and for a much shorter period of time at temperature but nobody really knows until the gouging is performed...
Finally, when you mentioned:
"one side was welded the previous day, and cooled to ambient temperature, would you need to preheat to 225 before backgouging"you do realize that in order to properly diffuse hydrogen from welded metal that has been left to cool down to ambient temperature requires that at least 48 hours waiting for adequate H diffusion to occur before NDT... This is especially true with Q&T steels... The sudden heat input could effect the hardness of the weld HAZ and possibly the weld also... If you weld one side, then the following day backgouge the opposite side before all of the H is diffused from the weld metal you might cause more work to complete or repair the joint than what you bargained for... A hydrogen crack can take anything from a few hours to 24 hours to occur. After 24 hours cracking is still possible but less likely, although there have been some reported cases of cracking at 72 hours...
Remember the purpose of preheat: Reduce the risk of hydrogen cracking, Reduce the hardness of the weld heat affected zone, Reduce shrinkage stresses during cooling and improve the distribution of residual stresses... If preheat is locally applied it must extend to at least 3"/75mm from the weld location and be preferably measured on the opposite face to the one being welded...
So, this is just my opinion but there are many who would also be better to be safe than sorry afterwards... Because the lack of preheating, the opposite side to be back gouged wasn't performed properly resulting in cracking and/or excessive hardness requiring a repair procedure to be followed in order to fix something that could have easily been avoided and then costing significantly more to complete the joint than originally anticipated... Not a good position to be in for sure.
Respectfully,
Henry