I've had success welding CroMoly frames before with a MIG welder and with flux core. It was, however, when I was rather inexperienced, and I think it was more to do with making a structure out of the weld wire than it was any good welding. I used a higher current than would have been wise for that thin a material and did a series of brief welds (about 1 second each) because it would burn through any longer than that.

I'll add this, though: I bought the Audel Welding Pocket Reference by James Brumbaugh and Rex Miller. On the "effects of alloying steels", it says, "chromium, vanadium, molybdenum, tungsten, and other elements increase strength by forming dispersed second-phase carbides" (p. 246). In Chapter 13 on Reactive and Refractory Metals (molybdenum is a refractory metal), For the most part, it's about welding straight molybdenum with which they practically forbid the use of CO2 as a shield gas (only straight argon or helium) without additional explanation and imply that the metal to be welded needs to be cleaned and prepared more vigorously than average.

As far as I can tell, doing cheap-easy welds on CroMoly like I did completely ruins its advantageous properties as an alloy (in general, enhanced hardness and corrosion resistance). Brazing is generally recommended as an alternative, and the Audel book notes, "when brazing, be sure to select a brazing filler metal that is suitable for the application. Temperatures for brazing filler metals range from 1200 to 4500F" (p. 260). But finding a "suitable" brazing filler might prove challenging because it's a pretty high-stress point.

So my advice is to MIG weld it and take into account that you're turning the frame into plain steel. It seems to be in a rather accessible place, so I'd use some scrap steel to distribute the forces at that point to a larger welded section (i.e. weld on a small plate so the length of weld is about twice as long as the crack).

Of course, this is the cheap-easy way to do it: the "right" way is probably to TIG weld with a CroMoly filler metal and the proper shield gas.

Fuck you, I love you,
---Jason Olshefsky

I am not familiar with the bike you've got, but I have had a reasonable amount of success welding bike frames. Clean it well and think about adding a gusset or other brace to distribute the force. Unless it was broken in a crash I would assume that the existence of a crack means that the frame needed reinforcement to begin with.

There is nothing wrong with brazing, especially if you add some bracing while you're at it. If it is a question between brazing or buying the frame they've got then you have nothing to lose by trying. I have a vintage frame that is brazed, not sure what vintage, but it's doing just fine as my chopper.

Whether you go Mig or Tig I'd add a gussett to reinforce the area, so you don't have a repeat..

Second the brazing. Brazing is the classic traditional method.
Use an oxy-acetylene torch just getting the area glowing orange, then apply fluxed brass rod to the work under the flame and the material will flow beautifully right into the crack.
I too have had success with this....fixing the cracked frame of my 1949 Schwinn "ACE" with brazing.
But mind that the tensile strength and engineered strengths of the original will never be the same... still it'll be a workable fix.

Regardless of joining method, you'll need to add gussets or other strengthening, otherwise this will just happen again.
Brazing is kinder to your alloy frame if you can do that w/o overheating.....

Thanks for all the replies. I ended up taking it back to the local bike shop. They brazed it for $65 and I think they used a flux with brass in it. I don't recall exactly as it's been a few weeks. So far, it's holding up on some test rides, though I don't see any sign of bracing or gussets. One oddity I had to ask the shop about was a small pinhole near the welding site that wasn't there previously. Their explanation was that the pinhole would redistribute the stress and keep a crack from spreading. Which, prompts the question as I type this, if I've got a crack spreading how is a pinhole going to help any?
After reading the other posts, I'm thinking I should take it to the burn and have fun on it. It's a sweet ride and a good bike. I have been considering whether I wanted to give it the playa treatment and degrade the machine. However, it's unlikely I would trust this bike on a tour and it's likely to break again at some point, so might as well have fun. Hopefully it will last the burn.