A-arm construction???

[Wes Allen]

Anyone familiar with the construction of chrome moly aero tube A-arm fabrication. Do you use chrome moly filler rod or regular steel filler rod? Do you make the bushings out of 4130 steel or can you use 1018? I know you have to build a jig and I would only consider Tig welding. I realize the bushings need to be bored or honed after welding. I've looked at the Chassis shop website, are there any others?
Thanks Wes

[Rick Kirchner]

The Jerry Bickel catalog has a lot of fab supplies, and Coleman has some. Most of the dirt track catalogs carry stuff like that because they break it a lot.

[J Leonard]

aedmotorsport.com

[Phil Green]

The Chassis Shop has very nice 4130 threaded bushings that fit round tube. They also sell 4130 tube.

I made a set of control arms for my F500 a few years ago. I just used mild steel filler (TIG), but had them stress relieved(?) after welding by a local heat treat company. No sign of cracks or issues yet.

No connection with Chassis Shop, but it's close to me and they have good stuff.

[Rich]

4130 is a high carbon steel. Makes it strong but also brittle. When you weld it you need a proper filler. You can use 310 or 312 stainless filler or er80s-d2. If that is not available $$$ then you can use er70-s2. er 70 s6 will also work but offer less strength. The idea is to dilute the carbon content that builds up in the weld that makes it prone to cracking. Thin walled tubing does not need to be pre heated and you should never try to normalize it with a torch.

[Dano]

Wag-Aero has the tubing and it is almost half of the cost then Aircraft Spruce. Although AS state the thickness is .049 vs. WA at .035.

4130-MIL-T-6736, 1.349" x .571", .035

[J Leonard]

Joining the leading/larger radius edge to the trailing/smaller edge can be tricky. And the gussets are not that fun either

[DrJim]

4130 really has to be classified as a low alloy steel with medium carbon at best. It welds extremely well with TIG and OX/ACC tourch. For TIG applicaton ER70S-2, which is a carbon steel rod, can be used with 4130 with good results. The actual strength of this rod will increase when welding 4130 because the chrome and moly in the 4130 will alloy with the weld metal making it stronger. Welding 4130 with ER80SB-2 would be a better approach because the weld and base 4130 have smilar strength. Using any form of 300 series stainless steel rod with steel where high strength and fatigue life is required can be poblematic. The welds are typically of lower stength and the interface between the base and weld metal is subject to cracking due to chrome carbide formation during welding. Yet, may people swear by it. Lawyers just love this one.

[Rich]

You are grouping too much when you say 300 series stainless. Whyle what you say is ture for 308 and 316, it is not true for 310 and 312. Both of those will not cause cracking when used correctly. As for the ER70s-2, the weld strength will incerase but tube strength will drop. The er 80 is a better aproach, but the 310-312 and the ER80 fillers are quite expensive and not always available.

[DrJim]

Rick,
Good point. 310 has about twice the nickle as the other 300's noted. Nickle tends to yield a lot making it good for welding applictions where cracks induced from welding are a problem. 312 is a harder sell. It has a little more nickle than 308/316, but also much more chrome which is the problem element. I would be interested in some of your experiences of welding with the various 300 alloys.

On the ER70s-2 decreasing the parent metal (tube) strength, I do not understand how this can happen. The tube outside the weld will be the same with either the 70 or 80 filler - there is no significant migration one way or the other between the molten weld and the unmelted tube. Both filler rods will have the same heat treating effect on the tube. During the weld, the filler rod alloys with the tube being melted and this alloy becomes the solidified weld. Alloying during the welding strengthens the solidified weld and the tube stays the same? Am I missing something here?

The concerning issue with welding a thermal hardening alloy such a 4130 is that the parent metal is heat treated and becomes hard, right next to the weld. Ideally a stress relieving would be used to decrease this weld induced hardness. Experience with 4130 tubing has shown that such is not required. However, it is good practice to not weld on a cold piece of 4130 - a mild pre-heat reduces the hardness by allowing the weld to cool slower.

Your input is appreciated. Thanks.

[Rich]

I have only ever used the 310 on 4130. Actually 4130 is pretty much all I weld. I am no expert on these things, but as I make frames that take a beating I try to learn as much as I can. The ER70 will produce a 70,000psi weld where 4130 is generally around 100,000 to 160,000. This makes the weld weaker than the parent metal but also specificaly the tube around the weld. This is not a large bit of the tube by any means, but most cracks form in the heat affected zone and travel into the weld. With the frames that I have seen crack most are within 1cm of the weld. I would say of those, most are due to abuse in some form or another and even age. As for the 312, I have never used it. I have no need as I get my 310 for free (untill the guy I get it off runs out). The guy I get my 310 off is a very educated welder, and the guy that uses the 312. He uses it for repairing tool steel. I don't know what els he uses for what though he has filler wire worth more than I am

[jerry freeman]

I'm not an engineer and I won't give a bunch of numbers to support this except the approximate speed of impact. I weld roll cages and frames for drag cars. I put a cage on an alcohol funny car that got crashed three years ago. The guy turned left past half track at about 210 mph. He hit the concrete wall at approximately 180 almost with a direct shot at the cage by the time the car turned around sideways and half upside down. The only thing that happened to the cage was a half collapsed 1.625X.083 wall chromemoly tube. I used ER70s rods and didn't pre-heat anything or stress releive anything. No cracked or broken welds. If you can TIG weld ok and use ER70s rod you should be fine doing anything you want on these cars. The filler rod is easy to use and easy and cheap to get anywhere.

By the way, the guy was racing 10 days later also. Thank God.

Jerry

[Rich]

Well pre heating thin walled 4130 tubing is not necessary. I know guys that swear by not heat treating. Their argument is it makes the tubes more brittle where not treated tubes will flex. Maybe they are right.....

[Dennis McCarthy]

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

[Wes Allen]

Thanks guys for all your help. We might try some of the simple stuff and see how it works out.