Welding Tips and Tricks

My son and I are building our own suspension parts for a Pantera using Corvette uprights. We built our own tubing bender and made the upper and lower "A" arms out of 4130 tubing welded to some mild steel components (the ball joint mounting at the end, some gussets, the coil over shock mount, and the heim joint connectors at the inner end.) We then read an article that warned of welding 4130 to mild steel would result in cracking at the welds. Is this a problem, and is there any way around it?

We talked to a local sprint car chassis builder who says he never heard of the problem, and does it all the time.

I hate thinking about starting all over again, either using mild steel tubing, or all 4130 parts.

Opinions, please.

just use your typical mild filler rod and all is well we do this to weld the moly tubing to plates or boxes that weld into the floor of the car for roll cages. passes tech and not a single issue in rolled cars

What type of filler metal are you using? Use an er80s-D2 filler and that will help with the admixture of the 4130 and mild steel. You will have no problem.

Hi Darth,
have you got any pictures/plans of your homemade tube bender? I would be very pleased if you could post them up...
Thank you!

Darth, without seeing it I would guess the warning or caution that you read about is in regard to the differing tensile strengths of 4130 and mild steel. They have different elongation characteristics under stress. 4130 is stronger but stiffer, or more ridgid if you will. 4130 can become brittle if welded with excess heat and not stress relieved (heat treated). There is a lot of factors all happening at once that determine where a part may fail under severe loads or stress. Geometry or shape and design, how and where the stress is applied etc. The welds should not just automatically crack unless welded poorly without regard for the heat sensitivity of the chromoly. But again, understand that you are joining two different metals and they will behave differently under stress. Your parts design may have a percentage of "overbuild" to it that will result in you never seeing a problem, but on the other hand, driven to it's limits any part will fail somewhere. Understanding where and how, is part of the whole design engineering process. When you are joining mild steel and 4130 and subject the part to failing loads, it will typically fail at the junction of the two metals due to the mild steel wanting to elongate (flex, bend, etc.) more than the stiffer 4130. Ideally the whole part would absorb the stress until failure finally occurs due to design overload, but with the welds intact, ie something else breaks first.

4130 is often welded with mild steel rod to provide some flex at the weld seams and preventing failure at the weld while having the benifit of the strength and rigidity of the overall structure. The exception to that would be where the ultimate strength is required and as such the part would be likely welded with vacuum melt 4130 rod, and the whole structure heat treated. So you can see that there are many factors to consider when designing parts. Knowing how the metals will react and where to use them in your design is the key. Many folks get away with things simply because there is an abundance of "overbuild" to something. Where it becomes critical is when you become very very wheight concience, and the part has to be light AND strong. Also, .... 4130 has a very porous micro structure and when you weld it with the usual copper coated rod from your local supply, the copper that does not boil off during welding imbeds into this micro structure and can cause cracking in the HAZ. Copper will not fuse into the metal mixture and acts as an impurity, so get a hold of some high quality uncoated rod. - Best luck!

Great post trackmaster! Lots of good readin there.

Oh hey, thanx man! Somehow way back when, i managed to log literally 3 or 4 thousand hours of under the hood time on chromoly chassis, frames, aircraft engine mounts, Harley flat track frames, and all of the Red Line BMX stuff back in the late 70's. Fun times for sure! Happy to tell ya anything I know, ....... which sadly won't take very long .... ha! LOL!