4130 Rollcage

[JustinOlson]

I'm going to build a rollcage for my 77 280z. I've never welded chromoly before, but have done lots of reading about welding it. According to the Lincoln website you dont have to pre and post heat thin chromoly tubing with a wall thickness of less then 3mm.

http://www.lincolnelectric.com/knowl...hrome-moly.asp

I plan on using 1.75" X 0.095" 4130 tubing. I plan on using the recommended ER80S-D2 per the lincoln article. Would the lower tensile strength ER-70S-2 filler be better for weld ductility?

Should I practice welding up scrap chromoly tubing to get used to welding it. Does it weld much differently then mild steel? What amperage would and gas flow would you recommend on 1.75" X 0.095" 4130? Any advice to help me not put too much heat into the weld and make it brittle? That is my main concern with using chromoly.

I will be doing a 10pt cage with a main hoop much like this:


Regards,
Justin

[halbritt]

ER70S-2 preferred by some for welding thing 4130 tube. Without pre and post heat, you probably want as much resistance to cracking as you can get. There's not much loss in strength as the admixture with the parent material raises the tensile strength of the filler and you should be able to make up for any loss of strength in joint design and linear inches of weld. Some small mild steel gussets will also go a very long way in improving the strength of the cage.

As for how much current to use, you should use just enough to create the smallest HAZ possible. Definitely practice on some scrap pieces and make sure that you have good fitment. You might also want to backpurge as well. Also note that while 300-400F preheat is not required, some folks will preheat thinwall tube to ~100F to remove excess moisture.

[zapster]

you get all the fun stuff..

70s-2 is good for the job..

make sure you clean the tubing inside and out as far as you can get inside..
oils and yuck from manufacturing will be present and oil loves to "wick" to heat..
therefore contamination from within is possible..

also..
try to drill a hole in the piece of tubing where one will be totally sealed off..totally sealed off tubing needs a "blowhole" somewhere...

trust me i know..
wherever a piece of tubing is totally sealed off drill a hole in the tube where it will be welded to in the middle of that tube..
you need air circulation as far as you can go...
eventually you'll reach a point where a end somewhere will be open to the atmosphere..

do the cage right....

...zap!

[elderthewelder]

why not use 4130 filler??? I am not real familiar with the ER80S-D2 , actually have never used it, All I know is when I weld 4130 and 4340 at work we use the same 4130/4340 filler to match,. As for a purge, seal off as much as possible and insert a purge line, run it at 5 to 10 CFH, maybe a little more if it is a long run

[halbritt]

4130 filler is used on 4130 parent material when the part will receive post-weld heat-treatment for maximum strength. Without heat-treatment, the weld will cool quickly and harden as a result yielding very low ductility and a propensity towards cracking.

Normalized 4130, which is the state that most tube will be in before welding has a tensile strength of around 95Ksi with excellent toughness and elongation characteristics. Welding with ER70S-2 will yield an admixture that has an approximate tensile strength somewhere between 70 and 95Ksi, very likely close to 85Ksi, with near 20% elongation. This closely matches the strength and toughness of normalized 4130.

4130 can be heat treated to around 180Ksi, which sounds great, but there is a tradeoff in toughness (ductility). In any application where I've seen parts receive post-weld heat treatment, they're generally hardened and then tempered to around 130Ksi.

[elderthewelder]

Originally Posted by halbritt

4130 filler is used on 4130 parent material when the part will receive post-weld heat-treatment for maximum strength. Without heat-treatment, the weld will cool quickly and harden as a result yielding very low ductility and a propensity towards cracking.

Normalized 4130, which is the state that most tube will be in before welding has a tensile strength of around 95Ksi with excellent toughness and elongation characteristics. Welding with ER70S-2 will yield an admixture that has an approximate tensile strength somewhere between 70 and 95Ksi, very likely close to 85Ksi, with near 20% elongation. This closely matches the strength and toughness of normalized 4130.

4130 can be heat treated to around 180Ksi, which sounds great, but there is a tradeoff in toughness (ductility). In any application where I've seen parts receive post-weld heat treatment, they're generally hardened and then tempered to around 130Ksi.

Thanks for the education, It is much appreciated, come to think of it all my weldments with the 4130 filler got sent out for heat treat. So where does the ER80S-D2 come in to play VS the ER70-S? any post weld heat treat required with those??? don't think Er70 can be heat treated if I remember correctly

[halbritt]

People go back and forth on the topic of whether to use ER70S-2 or ER80D-2. Most folks doing a lot of 4130 for racing apps and airplanes suggest using ER70S-2. However, if a slightly stronger weldment is preferred at the expense of toughness (ductility) then ER80D-2 can be used, though some preheat may be required.

I did some poking around on the Internet to see what people recommend and came up with an excellent treatise on the topic that explains it much better than I could:

http://www.netwelding.com/welding%204130.htm

[TubularFab]

Great article!!! That ones been printed for the archive....

[PaulG]

Here's one more idea. Use 1.75 .095 MS DOM. Properly designed and built, that size should be fine although you'll need to check your regs as they may require 1/8. If your regs say .095 is fine, you'll save lots O money and have less worry about a weld brake. Just a thought.

[JustinOlson]

So I've been doing a lot of reading trying to figure out the requirements of SCCA and NHRA cages. I'm looking at building a cage that satisfies both requirements so that I can take it to the drag strip and run fast, but also have no issues passing tech for track events.

SCCA Tubing Requirements for Roll Cages:

Up to 1500 lbs. 1.375 x .095 DOM / Seamless / Alloy
1501-2200 lbs. 1.500 x .095 DOM / Seamless / Alloy
2201-3000 lbs. 1.500 x .120 DOM / Seamless / Alloy
1.625 x .120 DOM / Seamless / Alloy
1.750 x .095 DOM / Seamless / Alloy


NHRA Tubing Requirements for Roll Cages:
1.625 x .118 DOM
1.625 x .083 4130 Chromoly.

With these requirements in mind I created a table to show the weights of different materials when used in the cage:



The two lightest options are 1.625" x .125" mild steel at 132lbs and 1.75" x .095" 4130 chromoly at 108lbs. So for $100 difference I will have a cage that 24lbs lighter and will pass tech in both NHRA and SCCA. I am capable of tig welding and feel 4130 is the best option in this case.

My only major issue is how to sport the main roll hoop as NHRA and SCCA do it differently:

SCCA:


NHRA:


I figured I could take the SCCA main roll hoop with diagonal bar and add the feet like the NHRA have going in. Would this satisfy both requirements? Input welcome.

Regards,
Justin

[zapster]

do you have a N.H.R.A. rulebook?
theres detailed diagrams of cage requirements in it..

if you do just go by whats in the book..

if not what you want to do will be good and still pass tech..

...zap!

[zapster]

one thing i do see upon further review is the bars on the rear..
near where your head goes?..

they should be welded to the framerails in the rear..where the back seat was..
i dont think pads on top of the wheel wells is a good idea..
you'd better check..or you'll be if you have to change it when its done..

the bars should be straight from the loop to the framerails..

...zap!

[zapster]

these pics were taken from the 2006 edition of the N.H.R.A. rulebook..
at the bottom of the "rear" pic it states..
"all cars with a o.e.m. frame must have the roll cage welded to the frame"


Attachment 4787

Attachment 4788


any other questions just ask..i got the rulebook right here...

...zap!

[TubularFab]

Zap - those braces are going to the top of the spring/strut perches, which is probably the strongest place in that area.

Justin - The one thing I don't like about the SCCA picture you have is the angled back main hoop. I'd keep that as vertical as possible... The SCCA diagonal in the main hoop is a must though. I also agree with Zap on the rear diagonals - straight, no bends! It looks to me that the SCCA cage is the stronger design. I'd try to build it so SCCA spec and add in whatever extra braces the NHRA requires...

[JustinOlson]

The main issue is head room with the rollbar. As you can see from the pictures, the wheel wells are right behind the main hoop mounting points. I agree that a vertical main hoop would be stronger, but there is a choice I must make.

Have a vertical main hoop with curved harness bar for seat clearance, or have the main hoop, diaginal and harness bar all in the same plane slightly angled back.

I'm going to be making seat mounts this next week for the car so I can see where the cage will be placed from that point. I'll drop the seats as far as I'm able to.

I agree that the rear downtubes should be straight. I will be going to the shock tower tops tho, as there isn't really any "frame" in this unibody datsun.

Regards,
Justin

[zapster]

I agree that the rear downtubes should be straight. I will be going to the shock tower tops tho, as there isn't really any "frame" in this unibody datsun.



in that case sub frame connectors are a must have..
your gonna twist it apart if you dont..

scca racing with all the turns and such..
i'd do it anyway..

...zap!

[zapster]

Justin..

Heres a couple of other things to ponder...
They may be of help because you have no "frame"

Attachment 4801

Attachment 4802


...zap!

[PaulG]

Justin,

I wasn't trying to talk you out of 4130, but if your trying to save a few pounds, then 1.50 .095 4130 should be fine. I assume yor car will be under 2200lbs?

That also gets you a little more head room.

[JustinOlson]

I'm not sure that I can get my car under 2200lbs, but the fact of the matter is that I still would need to run 1.625+" diameter tubing for NHRA.

I just read this on another thread and its making me question if 25lbs is worth it to go 4130:

"Remember that cages mean to prevent intrusion. When your brain hits your skull in a 45g collision it won't matter that your cage was super rigid, you'll just be a well preserved corpse with jelly for internal organs. Energy absorbtion is the name of the game.

While AISI 4130 steel typically has a higher tensile strength that is only a part of the picture. All steels have a very similar stiffness (modulus of elasticity) and Mild steel (AISI 1020 for this example) will deform 600% more before fracture than Chromoly.
(My reference is the ASM International Metals Handbook H.E. Boyer, T.L Gall)
What you should discuss is energy absorbtion. A stiff but brittle metal will not absorb as much energy before fracturing as a similarly stiff but more ductile one. The differences in specifications laid out by NHRA, SCCA, NASA, etc reflect the difference in materials properties in an attempt to place them on a relatively level playing field with respect to energy absorbtion. THAT is a fact which reflects a larger picture."

I will be tying together the suspension and subframes to the cage. After the cage if finished I'm going to do a tube frame front end so I can get away from this 35 year old front suspension. It looks like C5 or C6 corvette front suspension is what I will be using.







Regards,
Justin