That doesn't make sense. I know that ER80 is a better filler to use with 4130, but it would not take away the post welding heat treating requirement. The 4130 tube is still heated and taken from a normalized condition to an annealed condition making it a weaker tube. By heat treating after the weld it will bring back the strength of the tube.
It's not the joint that's weak, it's the heat affected zone around the weld that is affected. So I don't know why using a different filler would change this. You also have to remember that there are different levels of heat treating. I'm talking about a light heat treat to the area around the joint after welding which is then cooled slowly. It has to be the right balance of tensile strength and percent elongation. This is also assuming using normalized 4130.
What do you mean crumbles? Brittle failure, fatigues failure?
And it's not just the welding process of 4130 that makes it harder to work with, it's the pre and post weld work that needs to be done. The selection of what tubing to use is also critical. There isn't just one 4130, it can come in different heat treats meaning different strengths. There are a lot of different views on all of this and I'm not really sure why. It could be that people have done it and it has worked so they think that's the way to do it. I don't tend to hold the views of that kind of person very high, just because something has worked, doesn't mean it's the best or even a good way to do it. But I also look at everything as an engineer and need better reasoning than the fact that it has worked before.
Here's some good stuff on it:
Article about the John Force failure and normalized versus heat treated tubing.
http://competitionplus.com/index.php?option=com_content&task=view&id=4358&Itemid=24
Thread on Miller Motorsports Forum talking about that article.
http://www.millermotorsports.com/mboard/showthread.php?t=8889
I do agree with you that 4130 should only be welded with TIG, but also OA welding has also proven to be a good way to do it both in principle and in practice. I don't think backpurging is necessary with the thicknesses that are being used on roll cages and chassis. And a professional should always be doing this.
As for the NASCAR stuff, I could see the lower levels of circle track racing using the cheaper ERW tubing. But I would assume that the Nextel Series cars and higher level circle track racing using DOM. That's not saying that some people will cheap out and use ERW, but teams with bigger budgets should have no problem spending a little extra money to get a better chassis.
And I don't know why MIG is scary. With mild steel there is no downside to using MIG. I mean the HAZ is bigger than with TIG, but mild steel does not get affected by the heat the same way that 4130 does due to it's chemical structure. But in mild steel a high quality MIG weld will have the same strength as a high quality TIG weld when using the same filler. Obviously different fillers are going to change things.
Tim
