> This is my first post on here and maybe I am wrong. I have heard that
> if you use mig to weld 4130 you should anneal the welds. I have read
> this a number of times about frame building with 4130. You wouldn't
> have to heat treat the whole frame. Just anneal the welded parts.

First I would ask "Why?" then I would ask "How?"

Why would you need to anneal a MIG welded 4130 joint?

If the fear is a brittle joint, then why not use one of the preferred
methods of welding 4130? I would think that it is easier to find a
professional welder or someone at a trade school that will weld the
bike frame than going through a more complicated process.

How would you anneal just the weld joint and not the tubing near the
joint?

I'm not sure how one could do this. Controlling the heating and
cooling cycles such that they are isolated to the weld in the joint
doesn't seem practical in the garage workshop. I suppose if I had to
come up with a process I could think of something, but going back to
the first question, I don't think I'd be gaining anything for my effort.

I guess the main concern that I am hearing from posts like this is
regarding failure or premature failure of the bicycle frame. I think I
stated once before that the stresses imparted to the frame under
normal use would not exceed the yield strength of 4130. If one is
concerned about the stresses imparted on the welded joints under the
same conditions, I'd have to say initial weld quality rather than
post-weld processing would be more critical in determining the
strength of the welded joint.

Still, if the concern is that the joint will fail, then you also have
to consider the way steel fails. Most likely with a brittle joint one
would be looking for signs of metal fatigue like a crack in the weld.
This is something that can be monitored over time. You would see this
before a catastrophic failure.

Since I mentioned it, I'll also say that catastrophic failure is
unlikely. If you think you'll be riding down the road and the frame
will just spontaneously break apart, that's just not likely. Most
likely part of a weld will fail, but not the whole weld. Not every
single weld will fail at the same time in the same way. So if you're
monitoring your welds, you'll know if something needs to be repaired
before it fails.

That leads me to one of the best things about working with metal.
Metal can be repaired. Bad welds can be ground out and good metal put
in its place.

Of course, avoiding repair is why we have been having this discussion
about the best methods of welding 4130. Use the best methods with
skill and you don't have to worry about fixing it later.

It's a bicycle frame, not an aircraft or a race car frame or a roll
cage. If you weld the frame with gas or TIG, great. If you weld with
MIG, it works for Ducati motorcycles, so it should work for you if
you're a skilled welder. What is more important is that you go out and
try it. Even if the welding process is not ideal, you just might end
up with something ridable that will provide you with lots more smiles
per mile than any bicycle you've ever ridden. When you are in one of
those moments on your bike, I can guarantee that the last thing you'll
be thinking about is if you should have tried to anneal those MIG welds.

Jim