I have a 1998 Trek 830 20" frame that I am converting to a high powered E-bike. I need to weld one brace in on the frame. Is it possible to weld mild steel to chromoly steel? Will welding it weaken the chromoly tubing if I keep the amperage low? Will a Miller Mig work fine?

Thanks,
Tommy

Hard to say for sure based on the details provided but in a general sense, you should be fine.

Chromo used in bikes isn't normally hardened, so you won't affect it below the other welds or joints by welding in a bracket. You can pull alloy out, but that's really beyond the level of detail here.

Welding any tube can affect it structurally, so you probably wouldn't want to add a bracket in the most highly stressed part of the bike unless it was sufficiently strong. I mix in mild steel with Chromo in applications where stiffness is the main issue since it pretty much is the same stiffness. Drops would be an example.

Mig will work but you have to have the correct settings right out of the gate, and you have the cold start issue. A lot of chromo is migged including many stock car frames, but they had plenty of time to develop their proceedures on scrap. You are better off using a Chromo based wire, I had enough trouble getting that for TIG. You can also use a stainless all purpose. If it was up to me I would see if I could pick up some other old bikes in the trash and do a few coupons, just because of the settings.

Remember that you want loads tangent to the tubing since radial loads can crush thin tubing. In other words if your loads are runing up and down from 12 o'clock to 6, then you are better off (or at least should consider) two small tabs attached at 3 and 9 rather than on the radial axis. The tubing can be too fine to take a direct hit. Custom choppers and such can clean up the look with radial brackets but they often increase the wall thickness as much as four fold.

Not good advice! The only benefit of using e.g. 4130 wire would be if after all welding is done the whole thing was going to be heat-treated to some elevated tensile condition. The MIG equivelant of E70S wire is appropriate for what you are attempting.

Excellent advice from Peterpan1, however, that you obtain some old frame parts to practice on, adjust welder settings, etc.!

Edit: Make sure all parts to be welded are clean down to bare metal.

"Not good advice! The only benefit of using e.g. 4130 wire would be if after all welding is done the whole thing was going to be heat-treated to some elevated tensile condition. The MIG equivelant of E70S wire is appropriate for what you are attempting."

Good catch, mostly as regards using 4130 itself as a filler rod. I would consider any filler rod that is designed for high tensile applications with 4130 to be "4130 filler rod", though to say that gives rise to a classic top ten tig welding 4130 question.

As far as the heat treating is concerned, the same problems apply to the tubing itself. You are melting, slowly cooling, and not further heat treating the tubing at the weld also. The normal side of this argument that I have taken is to say you don't need materials in the filler that are stronger than strong enough. With enough fillet size in the joint you will be strong enough using the same stuff used for 1018, or something better, like er80s-d2 which i have to confess I have never seen for sale up here.

One reason people have given for using 4130 filler rod (not sure what it is actually made of, there are several versions of the stuff) is either A) it lays down better, B) it is stronger in a non-heat treated form, just as the tube has a higher tensile strength than a 1018, even if the tube is not fully hardened.

Reasons given for using the 1018 stuff or stainless are that it is more available, more easily laid down with gas welding, higher ductility.

I use er80s-d2 welding chromoly and chromoly to mild.. On my racecar chassis anyways. Ordered it from tig depot because my local supply shops didn't have it.

I was recently watching a video on welding aircraft frames made of 4130. They did both fusion welds and mild steel filler rod welds, sometimes in the same tube joint.

Welding braze-on's is not recommended. Brazing them is fine though.

Brazing is also easier, for me at least. Maybe if I had HF start I wouldn't care. A lot of MTBs and such seem to have welded tiny bits. I doubt they are built any tougher than the bikes I like to build.

Speaking of welded braze-ons, my understanding is that welding braze-on bits is not recommended on air hardening tubes; creates a hard spot on the tube which can lead to a fracture. 853 is definitely effected, and most likely TT Platinum as well. Cable stops on the top tube for example which attach on the thin butted section of tube are a no no.