Whenever your root (first) pass doesn’t go just as it should, a hot pass may be in order. A person that is afraid of welding too hot with the root pass and blowing out the bottom of the key hole, the tendency is to run too cold and a convex weld will result. The toes will be incompletely fused or slag may be trapped in the sides of the weld.
The Hot pass is a technique which helps resurface the root pass, and make it usable without a lot of grinding and clean -up of the weld. Some welders employ the hot pass over the root pass every time, whether it needs it or not, which typically won’t hurt anything if it is done correctly. A hot pass should not be blistering hot. Rather it should be just a few amps over the root pass. It should use enough amps though to melt out the top of the root pass and reshape the convex weld into a concave one, where the sides of the weld flow down toward the center of the weld, and where they weld is smooth on the surface. With the hot pass, though, too much amperage is a danger because it will create a Heat Affected Zone in the root of the weld. Carrying too much heat into the root will cause the weld to become brittle and will be a breaking point when stress is applied. A hot pass should be rapid, and just enough metal should be deposited to dish the weld sides out, making it concave.Applying to much weld metal only creates another full pass, which may be run too hot. The idea of the hot pass includes helping to push out root passes that may have some suck back, because as the weld progresses, a true hot pass makes the thin root pass become molten again, and sink down. This sinking down actually forces the bottom of the root pass out a little further.
A hot pass can be made with MIG TIG or Stick. With MIG, the wire speed may be left alone, and the volts bumped up. With stick a change of 10-20% amperage is all that is needed, along with using a 7018 on top of the 6010 root pass. With TIG the hot pass is likely the easiest to do, since the amount of filler metal can be controlled precisely. There is a school of thought that teaches the hot pass should only be done at a minimally higher temperature, if any change at all is done, and they refer to the hot pass as “cooking”. But true cooking is simply going over the top passes at a high rate of speed, at a high amperage just to improve weld appearance without adding much if any filler metal. When used properly the Hot pass will salvage a bad weld, and not create more harm than it does good.