BrokenBiker
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never really.understood it, is it just the same as brazing but with a mig gun?
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rich the mechanic
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From what I understand it is just mig with the wire made from the same compositon as brazing rods although mega expensive and likes to fold up and best suited to a hand held wire feed torch. Saying that never seen it for real just what I have read/heard.
BrokenBiker
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i thought it involved a spool gun or push/pull gun but was just curious as to how it differs from mild steel mig etc. is the same gas used to shield, how do amps vary etc.
i was asked to mig braze a push bike for one of the local kids speedway club. the mother saw on my advert that i do weld repairs to gates and asked if i could fix the bike and had been told by a 'professional' welder that it needed to be mig brazed as normal mig would just blow holes. cleaned it back with a diamond burr, drilled holes at the ends of the crack and welded it as normal, has held up fine for the last 5 races.
i think the 'pro' was just using terms to try and get a few more notes out of her. i charged 40 quid for half hours work, we were both happy with it and i am pretty sure she will come back to me if anything else needs welding
rich the mechanic
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Above is all my info on mig brazing but welding a bike frame I know a tad more and that is chrome moly don't like mig much. I welded an extenstion to a fork steerer tube and cracked it regularly, couple of years later I went to buy a frame which I spotted a crack on and was given it for free for my trouble of going to look and a mate of a mate used to build his own frames and told me mig welds tend to crack but TIG or braze worked fine I only had acces to mig at the time so he gusseted and braced it for me still have it under the stairs 15 years on.
BrokenBiker
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i was a bit weary of it being chromoly or some exotic steel but was assured it was just mild. did a spark test and a discrete weld puddle and it seemed ok. turns out it was only a cheap bike that already had the crack when it was bought second hand.
i had contemplated tig with 309L rod and a purge but a) i couldn't find my 309L rods and b) i didn't have enough gas to purge it properly.
i had contemplated tig with 309L rod and a purge but a) i couldn't find my 309L rods and b) i didn't have enough gas to purge it properly.
If you haven't already found it... http://www.mig-welding.co.uk/brazing.htm Like TIG brazing it's fairly different to oxyfuel in that there's very little capillary action by comparison and aside from the not melting the parent material bit closer to welding with MIG or TIG than brazing. Note the bit in the link that mentions pure argon
Chromoly is an inanimate metal, it doesn't know what MIG is much less actively dislike it. Sorry, couldn't resist
Like most harder (stronger) flavours of steel it has enough carbon (and other stuff) in it to be hardenable by heat treatment. Generalizing MIG is a fast process with lower heat input than TIG and both are much lower heat input than oxyfuel. Because you put less heat into the part with MIG the surrounding (relatively cool) area effectively quenches the weld. What happens when you heat and quench a med carbon steel? With TIG the higher input results in everything getting warmer and/or the heat spreads further so it takes to longer to cool down and the result is less brittle. That said if you speak to some people in the EAA they'll point out that if you subject some weld coupons to fatigue testing TIG often lags behind oxyfuel with regards to fatigue life unless some pre or post heat was used
4130 chromoly predates the invention of both TIG and MIG and 'back in the day' would be welded with oxy fuel or MMA/stick depending on the part... in other words anyone that says it must be TIG welded is talking absolute rubbish*
Thin wall tubing and oxyfuel with mild steel filler obviously won out, for thicker sections and/or when a matching strength filler was called for then pre and post heat is needed regardless of welding process. MIG can work fine with a bit of preheat as long as the part isn't soo thin you're forced to throw a bunch of tacks at it and call it welded.
* unless they're reading from the NHRA rule book or the like in which case it's about the rules not metallurgy
Chromoly is an inanimate metal, it doesn't know what MIG is much less actively dislike it. Sorry, couldn't resist
Like most harder (stronger) flavours of steel it has enough carbon (and other stuff) in it to be hardenable by heat treatment. Generalizing MIG is a fast process with lower heat input than TIG and both are much lower heat input than oxyfuel. Because you put less heat into the part with MIG the surrounding (relatively cool) area effectively quenches the weld. What happens when you heat and quench a med carbon steel? With TIG the higher input results in everything getting warmer and/or the heat spreads further so it takes to longer to cool down and the result is less brittle. That said if you speak to some people in the EAA they'll point out that if you subject some weld coupons to fatigue testing TIG often lags behind oxyfuel with regards to fatigue life unless some pre or post heat was used4130 chromoly predates the invention of both TIG and MIG and 'back in the day' would be welded with oxy fuel or MMA/stick depending on the part... in other words anyone that says it must be TIG welded is talking absolute rubbish*
Thin wall tubing and oxyfuel with mild steel filler obviously won out, for thicker sections and/or when a matching strength filler was called for then pre and post heat is needed regardless of welding process. MIG can work fine with a bit of preheat as long as the part isn't soo thin you're forced to throw a bunch of tacks at it and call it welded.
* unless they're reading from the NHRA rule book or the like in which case it's about the rules not metallurgy
rich the mechanic
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I guessed it was a heat related but I know nowt about hardening and not much more on anealing. Blokey that sorted my frame for me told me that mig welds went brittle but TIG and brazing worked fine, he only gas brazed and was very clever at building bikes rather than being a welder. Thanks for a proper explanation at least I now know I could MIG Chrome moly and then give it a good heating after if I have to.
