chromoly tubing question

[tomk]

i have a build coming up that I want to use chromoly for I am wondering what the strength is, compared to erw mild steel tube. example, if I used 1" 14 gauge mild steel for prototype 1, how thin could i go if i used chromoly 1" tubing? 18, or 20 gauge? even thinner?

[Polar Express]

For what application? More details please:

Here's why: do you need flexibility or do you need it absolutely stiff? This is important to know because of the different heat treatments that are avail.

The 'stiffer' it is, the more brittle it can be, but, also the more abrasion resistance due to the higher Brinell hardness, but do get that higher abrasion resistance, you also get a less flexible material.

So, if a little flexibility is good for your application, you may need to plan for a different wall thickness to get your desired performance.

Off the top of his head, the machinist sitting next to me says without knowing all the details, he wouldn't plan on going smaller than 16 gauge... that was with a lot of 'assumptions'.

PE

[tomk]

I was thinking the same thing on thickness, it is a snowbike swingarm

[SNWMBL]

Also need to consider tube diameter, if you increase the OD you can reduce the ID and end up a little lighter.

[Wade]

I guess the racers all talk about building their mild steel roll cages with 0.120", and their chromoly cages with 0.060" tubing. So, their always talking about a cage that's half as heavy. But, just as strong.

But, for an application like a swing arm, I bet your going to work the material. So, it's going to find a weak spot in your welds and come apart. And they say another weak spot is if 4130 is welded to mild steel.

They talk about chromoly being not hard, but rigorous to weld. You can't use a TIG welder without high frequency start. Because the old style welders will leave a hardened defect where they strike. And, you can't cool the weld, or even have a breeze. And, where your weld ends, no craters for sure. And, then they talk a lot about not using 4130 welding rod, but using something more flexible. Aircraft guys quote E70S2 Tig rod. I guess that's the kind of rigor you have to maintain to use very thin walled tube.

[AKToad]

Keep in mind that 4130 is heavier than mild steel but it has a larger bending moment so you can use less of it to get the same strength. If you take .083 4130 and the same tube in a36 mild steel the A36 will be lighter. Be prepared to spend some time learning to weld if you go the 4130 route.

Use the ER70-s filler rod it's used for mild steel, it helps with the heat effected zone of the weld but the down side is it makes a galvanic couple and the weld will rust much faster than the tube.

[pernikm]

www.matweb.com Search on 4130 and 1040. Compare yield strength, elastic modulus, density and elongation. Difference in yield strength can be significant if heat treated, but there is little gain if the tubing is normalized. Densities will be almost identical since the two are almost all iron with trace metals. Generally speaking, with low carbon steels, you gain yield strength at the expense of elongation with heat treat. When welding, you will probably destroy the heat treat so you have to ask yourself are the weld zones in an area of high stress or will the ease of welding mild steel outweigh the lower strength. I can help you with stress calculations if necessary.