The preheat must be sufficient to evaporate the surface moisture.
Sure, the water will eventually evaporate at 0 degrees F through sublimation, but do you really want to wait that long?
I've encountered problems on numerous jobs as a welder when there was surface moisture on the steel. Any welder that works in the field has encountered the problem at one time or another. The problem will occur anytime the temperature dips below the dew point, so it can happen in the spring, summer, fall, or winter. The problem is even more acute when the joint involves lapped members that were assembled and left in place overnight. The moisture wicks up between the two members by capillary action. The daytime temperature can easily be higher than the minimum preheat temperature, so the question is, "How hot do I heat the carbon or high strength low alloy steel to completely evaporate the moisture?"
The preheat listed in Table 3.2 of D1.1 are the minimum preheat temperatures. The emphasis is on minimum. The next key words are carbon and high strength low alloy steels. You need to bracket the discussion if you want to maintain a realistic conversation. Every family of base metals responds to the affect of heat differently. What works with carbon and HSLA steels may not be considered good welding practice for other alloy systems such as austenitic stainless steel subject to sensitization for example.
Merely heating the joint with lapped members may not be sufficient to evaporate the moisture between the members in a timely manner. The question is, “How long do you want to delay the initiation of welding operations waiting for the moisture to evaporate?”
As a welder, I’ve had many situations where the welding foreman (he was foreman because he couldn’t pass the test) moaned and groaned that I was wasting time preheating the steel on a warm humid summer morning. I am so easy to get along with, I asked him if I should dispense with the preheat?
“You don’t need to preheat, it’s plenty warm enough. It is above 50 degrees and the plate is only 1 inch thick!” he said.
Eager to comply, I said, "Just hang around for a couple of minutes, I want you to check my parameters and the weld before you leave.”
After completing the weld, I lifted the hood and asked him to check my weld. “While you’re at it, you might want to go fetch the carbon arc so I can gouge this Sxxx out and weld it right!”
He never did question me about “preheat” again. Next job, next idiot, repeat lesson as needed.
You can wait several months if you want to wait for sublimation, wait an hour at 60 degrees F, or heat the joint and surrounding area to a sufficiently high temperature to reduce the time needed for complete evaporation to something reasonable. The other consideration is that the area heated must be sufficient to ensure any remaining moisture will not evaporate and migrate toward the weld zone during the welding operation. Any moisture migrating toward the weld can easily manifest itself as porosity or potentially, as previously suggested, provide a source of hydrogen as the moisture decomposes into atomic hydrogen and oxygen in the presence of the welding arc.
Is there any one temperature that is best? I would hesitate to say "yes" to any one temperature. I would offer that if is dependent on the job conditions. An open groove weld may only require the application of sufficient heat to warm the surfaces above the dew point and several minutes to allow for evaporation. However, a thicker joint with lapped members, think of paddle plate semi rigid moment connections, may require higher localized preheat to ensure the evaporation is complete and doesn’t necessitate delaying operations for a half hour or more while the moisture evaporates from between the lapped members.
While the inspector was trying to be helpful, his suggestions were not welcome. In situation such as that described, the inspector could easily keep his opinions to himself and let the contractor learn the lessons the hard way by removing the resulting porosity, excavating the cracked joints, while the inspector simply collects the overtime checks as compensation for the time spent observing the repairs and reinspection. Life as an inspector can be difficult or it can be easy. I prefer the easy life, I love hard work. I can sit around and watch the contractor struggle all day without ever having to wipe sweat from my brow.
Best regards – Al