The ill affects of welds cooling rapidly can be surmised by comparing the cooling rates of three different operating conditions. The first case is when the part is heated to 200 degrees, the second case is where the base metal starts at 50 degrees with no preheat, and the last case is where the base metal starts at -10 degrees with no preheat. The delta T (the difference between the maximum and minimum temperatures) is going to determine the cooling rate. The larger the delta T, the faster the cooling to ambient temperature. Assume the base metal is heated to 3000 degrees when the arc is present and the weld puddle is formed. The weld cools from the molten temperature (assumed to be 3000) to the ambient temperature, either 200 degrees (with preheat), 50 degrees, or -10 degrees in the two cases where preheat is not used. What I'm driving at is the difference between -10 and 50 degrees isn't much, so the cooling rate from 3000 to ambient (delta T is 3010 versus 2950) isn't affected greatly. However, the difference between -10 and 50 and the jump up to 200 degrees is considerable. The cooling rate from 3000 down to 200 degrees is slow (delta T is 2800), while the cooling rate from 3000 to either -10 or 50 degrees is relatively quick. The danger of rapid cooling is the increased potential to form martensite even when the carbon equivalency is relatively low. How much martensite is a function of the carbon equivalency, the higher the CE, the greater the potential to form martensite at a given cooling rate. The 200 degree (or higher) preheat temperature is sufficiently high to ensure the cooling rate is retarded sufficiently to prevent the formation of brittle martensite.  

Don't overlook the beneficial affects of multiple pass welding. The successive weld beads will "temper" the brittle martensite of previous beads, but if the cooling rates are too high because the welds are allowed to cool between each weld bead, the benefit is compromised. If the root bead is made without the benefit of preheat, the next bead will temper the root bead if the interpass temperature is maintained and no martensite formed in the second bead (layer). However, if the interpass temperature is allowed to drop to ambient between passes there will be little tempering affect.

Once you have martensite present, you also have to be concerned with delayed hydrogen cracking. Now you've introduced an additional problem. The use of a EXX10 SMAW electrode ensures a hydrogen rich environment in the weld puddle. If EXX18 electrodes are used, they to will introduce diffusible hydrogen into the molten weld puddle as well unless they are properly stored and their exposure to ambient conditions is controlled to limit moisture "pick-up" by the hygroscopic limestone based flux covering.

What I haven't addressed is the influence base metal thickness has on the cooling rate. However, in my humble opinion, it is best to preheat the steel, even if it has a relatively low carbon equivalency, to prevent the potential for the formation of martensite. The contractor will save money by eliminating the use of preheat, but the consequences of a bad weld, i.e., delayed cracks that can form after the pronouncement the weld is "good" by the NDT crew far outweighs the money save by skipping preheat in very cold weather. Once again, it is important that the preheat be maintained for beginning to end of the welding cycle if preheating is going to provide the benefits expected.

There are other factors to be considered that were not presented in this short version of welding metallurgy 101. That is why there should be a metallurgist or welding engineer that has a thorough understanding of the conditions, base metals, and welding processes involved. They have the training required to understand what steps are necessary to ensure successful outcomes.

Best regards - Al