welding chromoly to mild.....any issues?

[Harold Bettes]

Howdy Willie, 

Thanks for all the effort and sharing the info with the rest of us!

I look forward to meeting you on the salt someday and will hopefully be able to click a glass with you or break some bread.

Best Regards from the high country where it has finally rolled into spring.

HB2

[isiahstites]

Interesting, so after looking at the pictures the weld itself is actually sstronger than the piece of tubing? Do you guys agree? I have always thought that the weld would be the weak link.

This is assuming that the pieces of tubing that were welded together were properly prepared.

Scott

[willieworld]

thanks guys  just fired sheris bike up yesturday sounds like a record breaker --i have until monday to get mine to the paint shop albert the painter said he will have it painted buy wednesday that gives me 15 days to assemble plumb and wire before el mirage see you there   willie buchta

[Peter Jack]

Scott:

If the weld is done correctly that will not be where the fracture occurs. Depending on how the weld is blended into what is being welded and the quality of the material being welded the break will occur right at the junction of the weld to the material or in the material itself.

Pete

[Sumner]

Scott:

If the weld is done correctly that will not be where the fracture occurs. Depending on how the weld is blended into what is being welded and the quality of the material being welded the break will occur right at the junction of the weld to the material or in the material itself.

Pete

I guess the moral to this story is when things go to hell and break at least you can say "my weld held"  .

Willie thanks for taking the time to do all the testing  ,

Sum

[SteveM]

Great stuff here guys.
    I'm new to the LSR scence, but I am a professional Metallurgical Engineer with a decent amount of welding and practical industrial/manufacturing experience.  All of the welds in the photos look very good and have failed in an expected manner.  As was stated above, if the weld is made properly, the strength of the weld will exceed the material immediately adjacent to the weld.
    Welding rods/wire for steel are typically rated based on their tensile strength after welding.  Let's say 70,000 psi tensile strength.  In its as-delivered condition, the tubing will probably be slightly above this strength level for mild steel, and well above it (possibly double) for 4130 type materials.
    However, when the welding occurs, there is a Heat Affected Zone (HAZ) which forms around the weld itself.  This HAZ will extend into the base material differing amounts depending on how much heat in introduced during the welding or post-weld heat treatment.  The base material becomes softer (and weaker) in this HAZ.  For 4130 type materials, the difference between the base strength and the HAZ is greater than the difference between mild steel and its HAZ.  In either case, the strength of the welded joint is limited by the strength of the material in the HAZ.  That is why all "good" welds will fail near the weld seam, but not in the actual weld bead itself.
     I agree that for our purposes, mild steel is the way to go.  The only possible benefit to 4130 tubing would be to reduce the weight.  There is no strength benefit to 4130 unless the entire frame is post-weld heat treated to restore the proper hardness and strength.

Again, what a great thread.

SteveM.

[willieworld]

thanks steve  i am trying to get two bikes ready for el mirage but will do some filler rod --minimal weld(low heat ) and some butt weld testing after may 18 th     willie buchta   greenfield mo.

[isiahstites]

Curious, since we are talking about the welds and the strength of the welds. When welding tubing together like this and with a good tight fit is it recommended to use filler? I don't know but I am assuming adding filler to the joint/weld would make it stronger. Is this true?

Scott

[Sumner]

Curious, since we are talking about the welds and the strength of the welds. When welding tubing together like this and with a good tight fit is it recommended to use filler? I don't know but I am assuming adding filler to the joint/weld would make it stronger. Is this true?

Scott

I would say definitely use filler rod.  I know with a tig it is easy to flow two pieces together on a tight fit and I do it on something that is non-structural where I want a flat weld area that I don't have to grind much.  The problem is with a joint no matter how tight material is going to have to flow to fill the joint and create the weld.  If you don't add filler where is that going to come from?  The surrounding metal, so you have now taken some away from there and it is weaker.  The fill also spreads out the surface area of the welded joint which is evident by the fact that it is breaking past the filler in a thinner section of the pieces that were joined.

At least that is my take on it.

Another thing to consider that I didn't when making the tongue on my teardrop is the effect of welding in gussets and such and just moving the weak point to a new point.  If anyone is interested I posted my screw-up on my site.....

http://purplesagetradingpost.com/sumner/teardrop/teardrop14.html


........... I have been laughed at for the failure of the tongue, but felt by posting this info it might save someone else from the same problem or worst.  What happened with the tongue can happen many places on our race cars or other projects.  Hope no one else has the same problem,

Sum

[isiahstites]

Sumner,

             Thanks for the reply and posting the trailer write-up, it takes a man to admit his mistakes.
I believe in most things in life you must first fail prior to heading down the path of success, it is how we learn!

Scott

[Chicane]

If I may offer a simple suggestion...

I highly suggest that everyone pick up a copy of Carroll Smith's "Engineer To Win", Motorbooks Workshop, ISBN-13:978-0-87938-186-8. It should be manditory reading in my opinion. It basically a guide to materials technology and how to build a chassis that doesnt break. Actually... I would recommend his whole book series.

I believe that the biggest issue with CM, especially for structural and/or roll cages, is that it needs to be heat treated. If you dont heat treat CM, you will end up with an expensive product with pretty much the same strength as 1020... but with brittle weldments. At the very least... if the structure is not fully heat treated... it should be at the very minimum, normalized... as it is critical that the weldments be stress relieved.

Always use filler rod. As it has been suggested by others during the topic of discussion, I too have found that welding with OXWELD 32 CMS has shown the best results with CM.

[willieworld]

Here's some good information that you might want to check out.
http://www.lincolnelectric.com/knowledge/articles/content/chrome-moly.asp


Willie Buchta

[John Burk]

My experience with chrome-moly is the front engine dragster chassis I built in 1965 and tortured for 5 years (still have it) . The cage is mild steel which I had tig welded and the rest is 1 1/4" .049" wall 4130 which I gas welded . We raced on lots of rough strips in the north east , got it sideways on rough tracks and bent the rails sideways and had to heat and straighten them . None of my gas welds ever cracked . From my experience chrome-moly tubing is very user friendly . With welded triangulated tubing the ones in compression normally buckle before the tubes or welds fail in tension . The softened metal around the welds protects the welds as in Willie's tests . Compression (buckling) failures are much better than tension failures .
John

[willieworld]

john im looking at my nhra rule book and it says all 4130 chromoly must be tig welded --mild steel must be welded with mig or tig only  ---i think that gas welding chromoly isnt a bad idea if you know what you are doing and follow some aircraft industry standards but tig welding is the answer  its easy its fast its clean just dont try it in the wind as far as your car is concerned hang it from the ceiling it would cost more to get it updated than what you could build a new one for as far as nhra is concerned  ---i would like to thank every one who has posted here i hope we have learned something to make the sport safer now how about faster   thanks  willie buchta

[38Chevy454]

Well, here is another metallurgist voice.  A chance for this newbie to contribute technical content.  I agree with what SteveM said.  The welded assy should always fail in the HAZ.  It is the weakest point.  Cr-Mo has to be TIG or gas welded because if you try MIG or SMAW (stick), the weld cools too fast and you get brittle martensite.  This is where the slow-cooling is beneficial in TIG or gas welding.  You get a weld with normalized or annealed like properties.  Meaning ductile.  Using a more ductile filler just helps to keep the weld from being brittle.

I do agree with the general consensus of many prior replies that Cr-Mo is spending money that is not an issue for land speed racing.  Weight is not the enemy like other forms of racing.  So just use mild steel tubing and have easier fabrication or welding.