Preheat is used to control the cooling rate of the weld and HAZ. If you carbon arc without preheat that area will cool too fast and do whatever you don't want it to do - usually become more brittle than it should. Now you've got an area that's prone to do whatever you don't want it to do - usually crack.
And for what it's worth, current is the numero uno factor in controlling penetration (see Jackson's penetration equations). Unless you're welding copper, I've never heard of anyone preheating to increase penetration, and the copper guys just want a puddle at all, but pretty much everyone cares about the properties of the weld and HAZ.
Tommy,
Like Mike and Super have said, pre-heat is very important to all aspects of the process, starting with the arc gouging of parts for weld prep or repair.
Take for example T1 plate 2" thick on heavy equipment. It has cracked from whatever extreme forces were exerted on it. It is below freezing on some log yard and you are repairing it out in the field. You hit it with the air arc, it is basically the same principle as having an arc strike out of the weld area. You get a sudden heating and then cooling of the area giving you a severe change in the HAZ. Mechanical properties, grain growth, molecule alignment, etc.
Now, by the time you finish your gouging and start to weld there may be enough pre-heat, at least on the surface, that the welder says, I already have pre-heat, no need to add more time to this job. Let's get it welded and get back into the shop where I can get warmed up. So he welds it out without proper heat as well. Now he has really altered everything in the HAZ. A couple of days later the unit cracks again. Possibly in the repair weld, more than likely, though, it will be about 1" to the side of the previous weld. All because there was no pre-heat applied BEFORE beginning any work.
Just my two tin pennies worth.
Have a Great Day, Brent
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