stress relieve 4130 tube

[DirtyD]

Ive built a full roll cage in an off-road truck using 4130 1 3/4 x .120 wall tube. This is the first time ive used 4130 as opposed to mild steel. Ive mig welded all joints and everything seems to have come out well. This cage is tied to the frame and will be subjected to a lot of flex.

Do I need to stress relieve these welds?
Will the weld joints be prone to cracking?

If so, what methhod is best to use for stress relieving the joints?

 

[Metalguy]

You'll be safest if you can somehow heat all the weld areas to about 1100 deg F for about 30 min. or more.

The area of concern isn't the weld metal or base metal, but the heat-affected zones(HAZ) right alongside the welds. They will likely be harder and more brittle unless you do the HT.

 

[DirtyD]

Thanks for the reply,Metalguy. How much (weaker)are the HAZ's when post heat treatment is not done? Can I PHT with a torch? What do I need to do this the correct way?

 

[Picone]

check with the local welding supply and get some heat sticks for the proper heat range. If they don't have them, I'm sure they can direct you to the right place. The stick will start to melt when you get it to the correct temp. I've never done it but I've heard that is how it is done. By the way, if you do use a torch, how are you going to do it w/o destroying the cab? I've debated building the cage on my new project w/ DOM or 4130 and I've decided to go w/DOM to avoid the hassle.

If you've got to keep those joints at 1100 for 30 minutes, it's going to take a long time to do a whole cage.

 

[DirtyD]

Hey Picone, thanks for the info. Im trying to find out how critical the post heat treatment is to the strength of the cage. Will this be extremly brittle by not HT the HAZ? It will be a bit difficult to maintain 1100 de. for 30 min. for ea. joint, so I want to make sure its something I need to do. The cab will definetly get a little HT also.

 

[Metalguy]

There is an alternate way to make 4130 welds without fear of hard and brittle HAZ's (I should have written this in my first reply-sorry).

What you can do is use preheat--about 300 deg. F should work.

The aim is to slow down the rate of cooling thru the all-important range of ~1,250-1100 deg F. That's where the hard HAZ are generated.

 

[pipewelder1999]

http://www.lincolnelectric.com/knowledge/articles/content/chrome-moly.asp

contains some information from Lincoln electric that may be of interest to you.

Good day




Gerald Austin
Iuka, Mississippi

http://www.weldinginspectionsvcs.com

 

[Metalguy]

A few basics of welding here.

If the welding process were oxy-fuel, then no preheat or PWHT would be required, because the entire weld area is heated very high, thus greatly reducing the quench/cooling rate.

TIG doesn't heat the surrounding base metal as high, but I think the cooling rate is slower than with MIG, the process that was used. For a structure loaded with cyclic stresses, *I* would do something other than MIG weld it cold and then leave it.

 

[DirtyD]

Thanks to all for the help! MetalGuy, What would you suggest I do from this point. Just how brittle is this cage with no HT?

 

[Picone]

brittle enought to where you better HT the tubes all around your body (especially the upper joints around the roof and top corners)! Hopefully you cut holes in the floor board so you can drop the whole cage down and hit it w/ the torch. It's a lot of work, but it could save your life or the guy riding shotgun.

 

[DirtyD]

Thanks Picone! Will do. I appreciate everyones help.

 

[Metalguy]

Since you have already built this cage, here's what I'd do. This isn't real scientific but it would work. Find someone with or rent a few thermocouples and the instrument for reading temp. Then find some kind of high-temp. wrapping material and wrap each joint with about a 1/4" thick layer-depending on what you use. You might ask a local HT company for assistance here. Wrap the T/C under the wrapping, making sure it touches the tubing.

Then use a torch to *slowly* heat the area up to ~1,100 deg F. I know I said 30 min. before, but even a few seconds at 1,100 will help a lot. But DO NOT heat it over 1,300 deg!

You'll need someone to take the temp. readings while you torch it.

If you can find some nichrome, etc. wire you could also wrap it around the joints and use your welding machine to supply the heat.