Chrome-moly or mild steel?

Lets say a guy (like me) had a good MIG and lots of experience with it, and a new DC TIG (scratch start) with zero experience with it.

If you were to advise him in building rear axle trailing arms with tubular steel, would you advise A) DOM mild steel and MIG, B) Chromoly and MIG with mild steel wire, or C) Chromoly with TIG and the proper filler?

How much stronger/lighter is the 4130? Application is the 86 El Camino rear axle.

CM is ~ 1/2 the weight for 2x the strength?

Unless you want to just burn some tube to get the feel before starting, I'd go CM and TIG. Everything else, MS and MIG.

But really, it's a trucklet, is light weight on the light end really worth it?

If it were me, for that application, mild steel and mig. You can mig moly tube if you use the correct wire.

Assuming the most likely failure mode of the part is a crack developing at the weld, I doubt option B would be any stronger than A. Specially if the parts in B are not heat treated after welding.

From what I remember (from a Carroll Smith book maybe) for 4130 you need to use the correct filler material and then heat treat afterwards to have 4130 strength.

pappatho wrote: From what I remember (from a Carroll Smith book maybe) for 4130 you need to use the correct filler material and then heat treat afterwards to have 4130 strength.

Or vibration stress relief

Just some numbers:

Tensile (minimums) - 4130(CM)85ksi vs. 1080 (MS)63.8 ksi - CM is only 33% stronger

Yield (minimums) - 4130 (CM) 70 ksi vs. 1080 (MS) 53.7 ksi - CM is only 30% stronger

We test our MS and it will typically break at 36% higher than the minimum from good quality manufacturer.

DO NOT USE ASTM A36

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Density - CM is .283#/ci vs. MS is .283#/ci

They're both steel. The chemistry is different.

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Typical roll cage material:

1-5/8" x .083"wt (typical CM Spec) = 1.36#/foot of rollcage tubing

1-5/8" x .095"wt (typical MS Spec for non NHRA use) = 1.55#/foot of rollcage tubing - 13% heavier than CM

1-5/8" x .118"wt (typical MS Spec for NHRA) = 1.92#/foot of rollcage tubing - 41% heavier than CM

If you use 70 feet of tube for a cage, a CM cage is only 39.2# lighter. In a fast car, that is a lot of weight.

Don't use CM. If you don't know what you are doing with CM, the chances of cracks is much higher than MS. And like wheels777 showed, it's only worth it if you are going for full on championship winning race car. For anything else, I take the easy to work, very yielding before it breaks, cheaper, easy to weld, doesn't require post-welding heat treatment mild steel.

If you can weld chromoly steel and it works for extra strength and lightness for the extra expense, use it. You just need to MIG weld it, especially since it's on your car. Do a lot of practice with TIG welding until you're good at it, then practice TIG welding chromoly steel.

It's only lighter if you use thinner wall, and FWIW, rear axle links aren't where I want to save weight / invite risk.

Chromoly is really best left to serious full race stuff.

I second the above.
- CM isn't lighter, just stronger, so you might be able to make parts with less material (thus lighter) if you design carefully and are really pushing the limits.
- MIG welding CM will result in possible cracking especially on fatigued parts.
You're wasting money with CM unless you're going to make the parts differently than you would with mild steel, and TIG welding CM is a must.

JohnyHachi6 wrote: - MIG welding CM will result in possible cracking especially on fatigued parts. TIG welding CM is a must.

Not true.

The issue with chromoly is its two strengths are pretty close to each other. Once you get to the point where it may bend, it's just about ready to break.

You CAN MIG chromoly but it's trickier. I was told to always braze.

Also, IIRC chromoly cages are illegal in most sanctioning bodies in the US.

Tensile strength on MiG wire is typically 70 ksi. Tensile on CM is 85 ksi MINIMUM. IF you weld CM with MiG, the weld is the weak link. Possibly not an issue in compression, potential issue in tension.

Zomby Woof wrote:

JohnyHachi6 wrote: - MIG welding CM will result in possible cracking especially on fatigued parts. TIG welding CM is a must.

Not true.

I'm not an expert, but I've heard and read that the rapid heating and cooling caused by MIG welding vs. TIG is no good for CM joints.

Knurled wrote: The issue with chromoly is its two strengths are pretty close to each other. Once you get to the point where it may bend, it's just about ready to break. You CAN MIG chromoly but it's trickier. I was told to always braze. Also, IIRC chromoly cages are illegal in most sanctioning bodies in the US.

I remember reading in one of Carroll Smith's books that you "DO NOT BRAZE CM!" It has to do with the grain structure of CM vrs MS.

Oooh... I'm liking this.

I think I will do DOM/MIG, but the tech is making me chub up a little.

Zomby Woof wrote: If it were me, for that application, mild steel and mig. You can mig moly tube if you use the correct wire.

Good to know. I was partly wondering at what point does one need CM? This will be a 400 hp street car. Emphasis will be placed on durability. If DOM MS can get me there, I'll do it.

We use up to 9% chrome in high pressure boilers for the superheater tubes that get up to 1050f and 2000psi. It has to be heated up to 450f prior to welding on it, it has to be welded hot, and requires a 12-14hr post-weld heat treatment before use. This is appropriate use of chrome-moly steel.

NASCAR/NHRA/SCCA & most others require mild steel because they have no way to confirm that the builder performed the post-weld heat treatment correctly on chrome-moly tubing. Mild steel doesn't require heat treatment during initial construction, and it can be repaired in the field without heat treatment.

curtis73 wrote: Good to know. I was partly wondering at what point does one *need* CM? This will be a 400 hp street car. Emphasis will be placed on durability. If DOM MS can get me there, I'll do it.

I don't think you likely ever need CM. If done correctly CM parts can be lighter than MS parts. There would have to be some very tight packaging requirements or other odd circumstance where a mild steel part couldn't be made to work (at a weight penalty).

Ok. You think .125" DOM MS will do the trick? What diameter? 1.5"?

1-5/8" will make the car more sellable and more usable if it could see drag strip service.

jimbbski wrote:

Knurled wrote: The issue with chromoly is its two strengths are pretty close to each other. Once you get to the point where it may bend, it's just about ready to break. You CAN MIG chromoly but it's trickier. I was told to always braze. Also, IIRC chromoly cages are illegal in most sanctioning bodies in the US.

I remember reading in one of Carroll Smith's books that you "DO NOT BRAZE CM!" It has to do with the grain structure of CM vrs MS.

I seem to recall reading that when F1 cars still had steel structures, they were all tightly fitted, fillet brazed chromoly.

I'm pretty sure the human race will die out arguing over how to properly weld 4130.

Our FSAE car was 4130. We pre/post treated all of the welds on the A-arms as they were made from 5/8 .028 wall and the actual weld would get stressed. For 95% of the main chassis the welds were done cold and not post treated. Most of the loads on a chassis challenge the welds to a relatively low loading condition. The majority of tube size requirements were driven by stiffness needs and when analyzed for a 2x load condition still had >2x safety factor for the lowered weld strength. It was not possible to underside the tubes more to optimize node loading due to buckling and stiffness requirements. A few suspension attachment welds we did isolated post treat to recover some fatigue life. In my time wirh the team we never had a car experience a non impact caused chassis structural issue. A arm sets where rebuilt/rewelded constantly. A few of the cars where quite old with significant hours.

Small aircraft are 4130 an tend to be brazed. Perhaps an AP mechanic could chime in on why. I an not familiar with the detailed design considerations of aircraft frames.

I used mild primarily due to cost. Its usually 25-30% of the cost of 4130 which can be a huge cost difference when you are buying 300lbs.

I swear an old friend on mine that built aircraft told me he gas welded aircraft tubing, which I assumed was CM.