When a welder strikes an arc, he is basically making a tiny, controlled bolt of lightning and then holding it steady in his hand. It sounds dramatic because it is.
A welding arc runs somewhere north of 10,000 degrees F. Depending on the process, the core of the arc can climb past 11,000 degrees F. For a little perspective: the surface of the sun is about 9,900 degrees F. So yes, the business end of a welder is genuinely hotter than the surface of the sun.
Because steel melts at only about 2,500 degrees F. That means the arc is running roughly four times hotter than it needs to be to turn solid steel into a liquid puddle. Here is the part most people miss: welding is not about having enough heat. It is about controlling heat you have way too much of.
Too little time in one spot and the two pieces never truly fuse, they just sit next to each other. Too much, and you warp the metal, burn through it, or cook the strength right out of it. The whole skill of the trade lives in that narrow window, and getting it wrong is a big reason welds crack.
Electricity wants to jump the gap between the welder and the metal. When it does, it rips the air in that gap into a glowing state called plasma, and all that resistance throws off ferocious heat and light. That heat melts the edge of your steel and the filler metal at the same time, so they flow together into one shared puddle. A second later it cools and freezes solid, and now it is one continuous piece of metal instead of two.
Done right, the joint ends up as strong as the steel around it, or stronger. That is the quiet magic of a good weld: two things become one thing, and you can't tell where the seam was. How hot an arc needs to be also depends on the metal being welded.
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