welding a roll cage

[Fheckro]

. . .  In the SCTA rules it states that for unitized construction the support bars shall have a 1/4in thick support pad on top and bottom of the floor (in a sandwich construction).

There is some confusion in the rule book about this. Contact "Kiwi" Steve Davies, chief car inspector. But wait till he has had time to decompress after Bonneville.  Let us know what you find out.

Ok just contacted Steve:

Hi Steve,

I am building a Chevy Monza for /BGALT class. The rules state that "on unitized construction and monocque cars, the roll cage structure and braces
shall have 1'4-in thick support pads on the top and bottom of the floor (or sill, in a sandwich construction) ...ect.

My question is... does this "top and bottom"  apply to welded cages?

the reason i ask :

I have some rather beefy frame connectors running right up close under the floor and would like to incorporate them into the roll cage equation.  I would like to
weld the cage tubes to the frame connectors and pass them up through a support pad on  top of the floor, welding that pad and tube to the floor skin.

Thanks

Fred


Reply:

Hi Fred, the top and bottom refers to bolted cages, what you are proposing sounds good, send me some progress photos, (not too large).
Cheers, Steve.

[Mobile Welder]

use ers80d2 wire for chrome moly if your going to use gmaw process. Pre-heat for any thickness' over 3/8" gtaw would be the preferred process.Joint prep/fittment is a little higher on the priority list here too. chrome moly has a fairly high carbon content to go along with the chromium and molybdeum(sp?). Anywho, i'd use 4130 which is the most common c.m used and is readily welded. Brazing C.M. is also quite strong,but you probabley arent alowwed to do that on a roll cage. I wouldnt oxy-fuel weld it unless you have an oven to let it come down slow.

[Fheckro]

Mig welds can be brittle. Tig welds can be brittle. It depends upon base material, filler material, dilution rates, etc.

[/quote]

I will be using mild steel tubing and ER70S-2 filler. I have a P&H 300 TIG machine. If you are going to be in the Spartanburg SC area any time this spring feel free to drop by. I have beer.

[johnneilson]

use ers80d2 wire for chrome moly if your going to use gmaw process. Pre-heat for any thickness' over 3/8" gtaw would be the preferred process.Joint prep/fittment is a little higher on the priority list here too. chrome moly has a fairly high carbon content to go along with the chromium and molybdeum(sp?). Anywho, i'd use 4130 which is the most common c.m used and is readily welded. Brazing C.M. is also quite strong,but you probabley arent alowwed to do that on a roll cage. I wouldnt oxy-fuel weld it unless you have an oven to let it come down slow.

Actually, 4130 alloy was developed for airframe construction and O2/Acetelyne welded.
The problem with TIG is the "heat affected zone" is so small that you have a very narrow band of hard material to create a stress riser and fail. That is why 4130 electric welded joints should be heated cherry red with a torch and allowed to cool in still air.

In field repairs on 4130 can be done with a mig and very strong (don't even think about using flux core).

John

[Gwillard]

use ers80d2 wire for chrome moly if your going to use gmaw process. Pre-heat for any thickness' over 3/8" gtaw would be the preferred process.Joint prep/fittment is a little higher on the priority list here too. chrome moly has a fairly high carbon content to go along with the chromium and molybdeum(sp?). Anywho, i'd use 4130 which is the most common c.m used and is readily welded. Brazing C.M. is also quite strong,but you probabley arent alowwed to do that on a roll cage. I wouldnt oxy-fuel weld it unless you have an oven to let it come down slow.

Actually, 4130 alloy was developed for airframe construction and O2/Acetelyne welded.
The problem with TIG is the "heat affected zone" is so small that you have a very narrow band of hard material to create a stress riser and fail. That is why 4130 electric welded joints should be heated cherry red with a torch and allowed to cool in still air.

In field repairs on 4130 can be done with a mig and very strong (don't even think about using flux core).

John

Using proper weld procedures the heat affected zone of a Tig weld is slightly larger than that of a Mig weld when the deposition is equal. When Tig welding 4130 in thickness under 1/8" preheat is not needed nor is post-weld heat treatment. In fact, if done wrong, attempts at PWHT can turn a good joint into a bad joint.
Using a 70,000-80,000 psi tensile low carbon filler leaves a weld that is more ductile than the 4130 base metal. Joint failure will typically take place in the heat affected zone just off the weld bead where the normalized 4130 has been annealed by the heat.
Yes, field repairs can and have been done using Mig. One manufacturer of production aircraft even used Mig to weld their 4130 tube frames. The problem with using Mig is that the lower heat input results in a narrow HAZ and faster cooling rates, which in turn can lead to a hard and brittle HAZ. For this reason Mig welding of safety critical 4130 components should be done only by someone who knows exactly what is happening from a metallurgical point and can take proper precautions the ensure a sound weld.

[Gwillard]

Mig welds can be brittle. Tig welds can be brittle. It depends upon base material, filler material, dilution rates, etc.

I will be using mild steel tubing and ER70S-2 filler. I have a P&H 300 TIG machine. If you are going to be in the Spartanburg SC area any time this spring feel free to drop by. I have beer.
[/quote]

If I get down that way I will certainly stop by and help you lower the population of beer.

[johnneilson]

Gwillard,

Can we agree that even a cup of coffee can be screwed up?

I can see that using the LC filler would leave a more ductle weld joint in an application that doesn't have proper heat treatment on the final assembly.
The issue of using CM32 filler rod is when the final assy is heat treated and the weld carries the base material strength, like control arms and suspension components.
I do know also of many who use SS filler rod on 4130, again a softer more ductle joint (too much nickle to heat treat properly).

Wasn't the issue of materials (heat treatment) in 4130 chassis for funny cars and top fuel an issue just a few years ago? Something like using a harder material than cond N?

What vintage is your P&H machine? If you need any parts, let me know. I have one that came over on the Santa Maria.

John

[SPARKY]

"I wouldnt oxy-fuel weld it unless you have an oven to let it come down slow."

The reason to use oxy-fuel is the recomended procedure is because

The reason gas welding is prefered in aircaft 4130---is because it heats such a larger area the heat sink cools MUCH slower.

[Gwillard]

Gwillard,

Can we agree that even a cup of coffee can be screwed up?

I can see that using the LC filler would leave a more ductle weld joint in an application that doesn't have proper heat treatment on the final assembly.
The issue of using CM32 filler rod is when the final assy is heat treated and the weld carries the base material strength, like control arms and suspension components.
I do know also of many who use SS filler rod on 4130, again a softer more ductle joint (too much nickle to heat treat properly).

Wasn't the issue of materials (heat treatment) in 4130 chassis for funny cars and top fuel an issue just a few years ago? Something like using a harder material than cond N?

What vintage is your P&H machine? If you need any parts, let me know. I have one that came over on the Santa Maria.

John

The reason for the caution against using a 4130 filler was because of the nature of this forum. While you are correct in that CrMo filler can be used to retain the strength of the base metal, most fabricators here do not have the facilities to perform a proper PWHT.
Certain SS fillers, such as 310, can be used to weld 4130 and will give a substantial increase in ductility. The downside is higher purchase price and lower tensile strength than a low carbon filler.

[Fheckro]

 The problem with using Mig is that the lower heat input results in a narrow HAZ and faster cooling rates, which in turn can lead to a hard and brittle HAZ.

That is what I'm talking about. I've always thought that MIG puts the heat into the filler so the base tends to act as a heat sink. Tig puts more  heat into the base metal. All other things being equal I think the MIG should be more brittle due to a faster cooling rate, in any metal. 

[John Burk]

Before about '62 NHRA wouldn't allow tig welded chassis , it was stick or gas . Kent Fuller gas welded his dragster chassis , preheated with a rose bud and welded with a 00 tip . Dragmaster stick welded their first 4130 chassis and had the whole thing heat treated in an oven .

[Rex Schimmer]

Back in the 70s when I was much younger and could not afford a TIG machine, I built several Husky based dirt bikes that I raced both motocross and desert with, did all of the welds with a torch and never had a weld failure. There are some "tricks" to it but it worked well. I now have a TIG and would not use anything else. I have used several stainless alloy filler rods on some projects with good results but prefer the ER70S2 filler in .045 diameter for almost everything I do now regardless of thickness. I do not weld anything thicker than 1/4 in 4130 because if you need to use 1/4 plate in a structure you need to think about your design.  (And the SCTA requirement to use 1/4 base plates welded into unit body cars is very questionable engineering in my mind.)



Regarding post welding heat treatment, I think it is a lot of work for not much return IF (and that is a big IF!) you have designed the weld joint correctly. Things like control arms and small parts are one thing but trying to do a full chassis is asking for potential disaster. You will need a tool to retain the structure in its designed shape and this makes the tool design a big challenge. The best approach, especially for land speed racing is to stick with mild steel tubing and TIG welding.

Rex

[johnneilson]

Back in the 70s when I was much younger and could not afford a TIG machine, I built several Husky based dirt bikes that I raced both motocross and desert with, did all of the welds with a torch and never had a weld failure. There are some "tricks" to it but it worked well. I now have a TIG and would not use anything else. I have used several stainless alloy filler rods on some projects with good results but prefer the ER70S2 filler in .045 diameter for almost everything I do now regardless of thickness. I do not weld anything thicker than 1/4 in 4130 because if you need to use 1/4 plate in a structure you need to think about your design.  (And the SCTA requirement to use 1/4 base plates welded into unit body cars is very questionable engineering in my mind.)



Regarding post welding heat treatment, I think it is a lot of work for not much return IF (and that is a big IF!) you have designed the weld joint correctly. Things like control arms and small parts are one thing but trying to do a full chassis is asking for potential disaster. You will need a tool to retain the structure in its designed shape and this makes the tool design a big challenge. The best approach, especially for land speed racing is to stick with mild steel tubing and TIG welding.

Rex

Rex, agreed on the mild tubing

In SCCA, the preferred footing material dropped from 1/4" to a min  spec of .080 thk. It looks like the thicker material would slice the unibody and using thinner material allows for deformation but still stays attached. I saw a video of a Mustang flipping over and landing on roof. As it did, you saw the down tubes punch through the floor and protrude up into the air.

A couple more days of rain like this and I am building and ARK!!!

John

[Gwillard]

 The problem with using Mig is that the lower heat input results in a narrow HAZ and faster cooling rates, which in turn can lead to a hard and brittle HAZ.

That is what I'm talking about. I've always thought that MIG puts the heat into the filler so the base tends to act as a heat sink. Tig puts more  heat into the base metal. All other things being equal I think the MIG should be more brittle due to a faster cooling rate, in any metal. 

Mig works well on mild steel because the low carbon equivalency, a product of carbon and other elements, doesn not promote the formation of martensite even at extremely rapid cooling rates. 1020 steel can be water quenched and it will remain ductile.

[saltfever]

Name brand welding rod (or wire) usually has a traceable pedigree resulting in high, consistent quality, that you can stake you life on. However, about any piece of tubing you will buy today is most likely imported. It is an extremely competitive industry and you should be very careful about buying on price alone. I have machined tubing with hard and brittle spots that must have been re-melt with ball bearings or axels thrown in. Yes, low grade material!  When buying mild steel tubing for a roll-bar, I suggest you ask for traceability to the heat and the actual metallurgical analysis. A cert usually costs a little more. 4130 by nature, is a high quality product but you shouldn’t let your guard down. Know your supplier and know exactly what you are getting. I’m interested in other ideas on how to guarantee a good quality product.