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PostPosted: May 28, 2008, 8:47 pm 
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Joined: August 13, 2006, 5:14 pm
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Location: Minnesota
anyone use chrome-moly tube for their chassis?

does it only come in round?

can it be mig welded? any thing special about it?


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PostPosted: May 28, 2008, 9:49 pm 
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I see lots of posts on both sides regarding the issue of post heat-treating, some say it's needed, some don't. It's expensive, a lot more than mild steel.

My opinion is that unless you can get the finished chassis heat-treated, it's probably best to stay with ordinary mild steel.

Oh, and the only reason it's "better" is that thinner-section tubing can be used which saves weight. If you can't find the smaller-size tubing, it isn't saving any weight. (Availability of different size tubing is limited, in spite of what the databooks say is out there.)

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PostPosted: May 28, 2008, 10:24 pm 
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The chromoly (4130) alloy is also stronger than mild steel so less is needed to achieve the same strength. However, all steel has the same modulus of elasticity so regardless of alloy, stiffness is dependent upon the size of the tubing. Since most car components are designed for stiffness rather than strength, chromoly is a waste unless you redesign for it. In terms of shape, you can get it in whatever shape you want; they key is finding a supplier.

I've seen people go bonkers about heat treating and doing all sorts of other complicated stuff to 4130 post weld and quite frankly, I've never seen a reason. All of the FSAE frames (and suspension components for that matter) I've done were 4130 and aside from some post welding stress relieving with a torch on the arms, they were used as-is - nothing has failed to date.

Albeit they were all TIG welded; the only reason I wouldn't MIG 4130 would be the available filler wire. ER80S2 should be used on 4130. For some reason (probably lack of demand), I can only find ER70SD6 for the MIG.

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PostPosted: May 29, 2008, 3:56 am 
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Location: Champion, Ohio
This stuff?

http://store.rmweldprod.com/miller/inde ... cts_id=696

http://www.stoodyind.com/Categories/Par ... pooled+MIG)+


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PostPosted: May 29, 2008, 6:44 am 
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Mmmm...yea thats the stuff. :D

I'll have to bookmark those for future reference.

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PostPosted: May 29, 2008, 8:59 am 
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Does the work hardening caused by manufacturing DOM tubing affect the Young's modulas? I am just wondering because as you bend a piece of steel repeatedly, before it breaks, it gets stiffer...

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PostPosted: May 29, 2008, 9:45 am 
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Horizonjob - I had to laugh at your post because even as a degreed mechanical engineer (25yrs ago - forgot alot), I have trouble explaining what I see and what is theoretical correct. Your post is a great example of someone thinking out of the box and coming up with good questions. Unfortunately, without reveiwing, I dont have a positive answer but I think when you are talking about steels having very similiar stiffness - you are also talking about stressing the material below its "elastic limit?". (May not have the right limit - its some kind of limit). When you workharden or change the grain structure in any way (bending, cold forming, heat treating), you can change the elastic limit. When you bend something and it takes a different shape - you did so by going over the elastic limit. It will then take more stress to "unbend" the part not because the metal is stiffer but because the elastic limit is higher.

I hope this helps - This is one subject where I've always been confused between the book and what seems right.


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PostPosted: May 29, 2008, 12:17 pm 
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Here is a part of an article from the Welding Journal of the American Welding Society on the welding of 4130.
Quote:
What about Welding 4130?
In the mid 1970s, while managing an R&D group for a welding filler metals manufacturer, I received a phone call from a dragster chassis builder. The company wanted to weld 4130 tubing and needed a filler metal recommendation. After careful review of the requirements and desired welding practices, the solution was defined. The company was welding 4130 normalized tubing. It would not be heat-treated after welding, and preheat was not desirable. Most of the weld joints were intersecting tubes that required fillet welds.

Filler Metal Choice
The main objective was to produce porosity- and crack-free weld deposits. The best filler material to use was a low-carbon alloy, AWS ER70S-2. This welding alloy has a very low carbon content, nominally 0.06, which can handle dilution into the relatively high (in terms of weld metal) 0.30 carbon in the 4130. The resulting diluted weld deposit has a tensile strength of approximately 590 to 620 MPa (85,000 to 90,000 lb/in.2) The actual strength will depend on the amount of dilution with the 4130, weld bead size, and material thickness. This is usually an under match for the 4130 tubing, which could have 760 to 800 MPa (100,000 to 115,000 lb/in.2) tensile strength, depending on how the material was processed. However, if extra joint strength is required, a slightly larger fillet size or gussets can be employed. In addition, this welding wire contains small amounts of aluminum, titanium, and zirconium. Although these elements were initially added to handle welding over mill scale, they also contribute to a less fluid weld pool. The benefit to the welder is easier out-of-position welding. Note: It is recommended all welding on 4130 be performed on ground surfaces free of oil or grease.

Several years after making this recommendation, when looking at a catalog from the dragster chassis manufacturer, it was interesting to note it advertising its use of the ER70S-2 filler metal for their 4130 welding. In fact, offering it for sale for those customers purchasing frame parts and doing their own welding!

The Internet was searched to see what current recommendations were being made for joining 4130 tubing. Several hundred sites were found that recommend the ER70S-2 welding wire alloy. It was the predominant recommendation. Typical of the Internet, however, there were many improper descriptions of why this alloy should be used and several incorrect recommendations.

Go for Higher Strength
If a higher strength weld is required for perhaps a butt-joint weld that cannot be reinforced, strengthened with a gusset, or put in a less critically stressed area, there are several possible solutions. The use of AWS ER80S-D2, which contains 0.50 moly, will provide a weld deposit with higher strength. When diluted into the 4130 base material, a weld tensile level of 760 to 800 MPa (110,000 to 115,000 lb/in.2) can be achieved. If this higher strength welding wire is employed, a minimum preheat of 65¡C (150¡F) is recommended. It is also possible to use an AWS ER312 stainless steel welding wire. Weld strength can increase to a level slightly higher than with AWS ER80S-D2.

Generally, the use of this high chrome stainless alloy is only needed when welding stainless to steel. Do not use an austenitic stainless steel such as an ER308L, which is, unfortunately, sometimes recommended. Diluting this or similar austenitic stainless alloys with 4130 can lead to cracks. Also, consider that providing a higher strength weld deposit cannot compensate for the reduction in strength that will most likely occur in the base metal immediately next to the weld deposit. To achieve the higher strength, the base metal was heat-treated, reducing the weld heat-affected zone area hardness.

If the part is heat-treated after welding to achieve very high strength, a matching chemistry filler metal to the 4130 must be employed. Because of the relatively high carbon content, a minimum of 200¡C, (400¡F) preheat and very slow cooling after welding should be used to avoid cracking. After welding, the part can be heated to 870¡C (1600¡F), quenched in oil or water then tempered back to 370¡C (700¡F). This might be considered a complex cycle, but it will result in a tensile strength of approximately 1380 MPa (200,000 lb/in.2). Since the weld is the same chemistry as the base metal, it and the heat-affected zone will have properties similar to the base metal when heat-treated. All critical welds of this type should be inspected for internal soundness to assure they are free from cracks.

Closing Advice
When welding 4130 chrome moly in the normalized condition, AWS ER70S-2 filler metal, with its low carbon content; is the proper choice. If the part is to be heat-treated after welding, then a filler metal matching the 4130 chemistry should be employed. This requires preheat and special precautions to avoid cracking.

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PostPosted: May 29, 2008, 1:01 pm 
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The voice of reason
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Thanks PPAM20, that was a good explanation and makes complete sense. I did a lot of fabrication with coat hanger wire when I was a kid, must be tough growing up nowadays...

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PostPosted: June 5, 2008, 11:49 pm 
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From my somewhat limited experience with aircraft, here are a few comments.

4130 was originally developed for aircraft fuselage welding with gas welding techniques.

The excess heat from the torch provides a larger and more gradual heat effected zone. With TIG welding, this zone is very small. Typically, TIG welded thin tubing stuctures are stress relieved if they are to be highly stressed. Typical homebuilt aircraft are 4130 normalized tubing, gas welded, with no post stress relief.

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PostPosted: July 24, 2008, 2:49 am 
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On the 4130 parts I have welded (gas) I was told to use a standard mild steel filler rod. The only thing that I was told to make sure I did was to have more filler material in a joint than the parent material. The extra filler material makes up for the strength differences. The fellow who showed/taught me is a certified aircraft welder so I tend to believe him that it works. That's what they use in the shop to produce replacement parts unless they specify a filler. There are 4130 aircraft frames that are mig'd without pre/post heat treatment. Other than making sure they don't
have any wind/breezes/hurricanes blowing around the cooling frame they also just use a mild steel filler wire. One of those designs is aerobatic so these aren't always babied.

There is of course special filler rods or wires for doing 4130 but tends to be pricey or hard to get.

Dave


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PostPosted: July 25, 2008, 4:04 pm 
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There are airplanes flying with MIG welded 4130 and no reports yet of planes crashing due to the welds not being stress relieved (heat treated).

But it really comes down to WHY BOTHER? 4130 is so expensive, and for what? You might be better off spending that money on extra go-fast goodies for your power plant. There are a lot of guys racing their Locosts and I haven't heard a single one yet say "If only I had used 4130...."

Just my 2 cents,

John


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PostPosted: July 25, 2008, 4:56 pm 
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Well there isn't any good argument about it being cheap cuz it defiantly ain't.

Really the only things it has going for itself is that you can get it with a much thinner wall thickness and that it is very good quality. It would cost about $300 to do the basic book frame but you could save 50 lbs over a standard book frame. Thing is even going from 16g sqr to 16g round you could save 20ish pounds over a book.

Dave


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