So ummm... "some guy" is saying that mig welding around the front trunnion to the receiver, plugs, can weaken the heat treating and cause the trunnion to explode
Is this common knowledge, I can't find any references to this online
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Exploding highly unlikely but bolt gap shrinkage is possible due to the amount of heat applied while MIG welding. The heat on the sides can turn the trunion bright red due to the amount of amperage required to get good weld penetration. When welding up the trunion, I do one spot and let it cool down before doing another more as a safety precaution.
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How can the bolt gap shrink, is it adding that much material?
Mig welded mine and I gave it time to cool off, I alternated sides too to give it time to cool as well now I'm weary my gun is going to explode :/
Normally welding ( depending on the RC and type of steel ) will heat and soften the metal .
The spots that are welded are two points around the front where the hand guard mates up. That area is fairly safe to weld but the spots where heat can be an issue is closer to the rails. This area is pretty thick and the other side is where the bolt/rollers mate up. The possible issue with using a MIG is wire feed rate and amperage. The MIG will feed wire no matter how slow you set the feed rate. Some have slowed the rate down to the slowest setting and raise the amperage up fairly high to get penetration hoping the wire speed is slow enough so there's no excess wire going into the molten puddle.
The advantage to TIG is amperage can be controlled with a foot pedal with no filler being added at all. Some welds can be done with just the TIG torch with no filler material and yield some nice looking spot welds with proper heat penetration but not to the point where the trunion is raised to cherry red. The excess heat can lower or destroy a trunion's hardness rating in this spot which can cause irregular wear and accelerated gap shrinkage. I would hope an experienced welder would MIG the trunion area with these considerations in mind. Even a TIG torch can ruin a trunion without having these concerns in mind.
Assuming there might have been too much heat on the trunnion , during a mig weld, is there a way to re-temper it with say an oven or mapp gas torch (with flame far away) to get the trunnion to 600 degs and let it cool at room temp?