Differences in welding chromoly tubing with mig vs. tig

[737mechanic]

I have read that you do NOT want to weld chromoly tubing with a mig and I have read others say it is fine. I just read the NHRA rules on Roll cages and they make you use Tig ONLY on chromoly but let you use tig OR mig on mild steel. My question is is there that big of a difference in welding chromoly with tig vs mig as far as strength.

When welding with tig do you actually use chromoly filler rod or what.

The reason I want to know is I am about to build a set of upper contol arms for a 1970 GTO and was planing on using chromoly but if I can't or shouldn't use mig then I will just use mild steel and go up a size. If I do use chromoly is it ok to weld it to a mild steel plate or is there a possibility for corrosion to set up.

Any and all suggestions are welcome.

[makoman1860]

Dont take the NHRA too seriously, they really dont have any data to back up their rulings.
Now back to your question, Tig and Mig are both used in aerospace applications on 4130 tubing, typically ER70 series filters are used for their fatigue properties. Also typically the process requires some form of preheat, and a post weld tempering. Most engineering books list 1200F for 1 hour per inch of metal thickness, followed by still air cooling.
You can also arc weld the material, but again there is a process both pre and post weld.
There it is, straight from the engineering books.

[Rojodiablo]

Makoman has it pretty dead on. Mig works fine on 4130. Just let it air cool, don't quench it with water.

[makoman1860]

Originally Posted by Rojodiablo

Makoman has it pretty dead on. Mig works fine on 4130. Just let it air cool, don't quench it with water.

Actually either process by rights should have a post weld operation performed, either a localized tempering, or an oven bake in a jig, or any other common approved method. Just welding and leaving the part is inviting trouble, especially for a suspension part.
Its in the engineering books, not a secret, and not a mystery. And when I say engineering books, I dont mean that drivel sold at Lowes in the welding section.

[Donald Branscom]

When you say post weld treatment are you talking about stress relieving?
There would be no reason for tempering since the tubing is not affected only the actual weld zones. The filler wire is designed to be compatable with the tubing.

[jamlit]

I believe all the Nascar roll bars are mig welded and they seem to stand up pretty good. I am not sure but their could be a proper precess like mentioned above but according to the guy I know that went and seen some of the race shops, the guys were just welding away on the roll bar and chessie.

[makoman1860]

Originally Posted by Donald Branscom

When you say post weld treatment are you talking about stress relieving?
There would be no reason for tempering since the tubing is not affected only the actual weld zones. The filler wire is designed to be compatable with the tubing.

DB,
Nope im talking about tempering. For many years it was called "stress relieving" in the shops, but that is an inaccurate term and has caused much confusion. Actual technical "stress-relieving" takes hours, and "normalizing" the same, but the terms have been used for many different process' over the years....again leading to confusion.
The engineering books have called the local post weld treatment tempering since the late 1940's, and Im trying to do the same as it accurately describes what the process is accomplishing.

[Supe]

For the purposes you're intending, you're never going to heat treat the part. Short of the nitro guys who now have it mandated for certain parts of their cars, nobody does, including the Pro Stockers. Strength isn't a big deal, 4130 is sensitive to heat input, which is why TIG is the preferred (and mandated) process for the NHRA. Without heat treat, you never want to use 4130 filler on 4130 base material. It will be very susceptible to cracking. Mild steel filler rod, either 70s2 or 80d2 are the rods of choice for TIG welding the parts together, with 80d2 being preferred for suspension components.

I'm assuming you mean a set of upper control arms for the rear suspension. Were you planning on using threaded tube adapters? If you're going to be MIG welding these in place, I'd advise using mild steel. Use DOM tubing, not ERW. .095 wall is the common wall thickness for the chrome moly (most adapters you find will come for that size), but you can probably find them for .120 wall as well if you wanted to step it up to the next size. If you can't find the tube adapters in that size from the racing places, check with the offroad guys.

Nascar chassis are MIG welded, but they are also mild steel, not moly.

[makoman1860]

[QUOTE=Supe;200857] Strength isn't a big deal, 4130 is sensitive to heat input, which is why TIG is the preferred (and mandated) process for the NHRA.

QUOTE]

Actually heat input is not a factor except in distortion, it doesnt change the process if using an arc. If that were the case OA welding would be horrible.....and we know that isnt the case at all. And the NHRA mandates it, because they were told to. Ask for their test data.......

[737mechanic]

Thanks guys for all your info. I am not worried about mig welding chromoly now.

Supe Actually these are going to be front upper control arms for a 1970 GTO. They are like these.

I yhave called everyone in the dallas area and I can not find chromoly so DOM is probably what I will be using now.

[Donald Branscom]

You can order small amounts of chromoly tubing from aircraft supply houses. like aircraft spruce. http://www.aircraftspruce.com/menus/me/
Also try Ebay. I ordered some tubing on ebay and got a very good deal.
UPS will deleiver up to 9 feet lengths. Sometimes 12 feet.

Mig welding has a higher heat input than TIG, that is why TIG is prefered. Less damage in the heat affected zone.

[Donald Branscom]

Originally Posted by makoman1860

DB,
Nope im talking about tempering. For many years it was called "stress relieving" in the shops, but that is an inaccurate term and has caused much confusion. Actual technical "stress-relieving" takes hours, and "normalizing" the same, but the terms have been used for many different process' over the years....again leading to confusion.
The engineering books have called the local post weld treatment tempering since the late 1940's, and Im trying to do the same as it accurately describes what the process is accomplishing.

Each one of thos words TEMPERING, NORMALIZING, and STRESS RELIEVING, have very exact meanings.
TEMPERING is explained on page 2134 of "Machinery's Handbook",twentieth edition.
I would not see a reason to temper the 4130 after welding.
To temper the 4130( with welded portions), first would have to be annealed so that the tubing and welded metal are consistant. This whole process gets out of my range of knowledge and I would refer the question to an engineering website. But to put the weldment thru all of that seems a little overkill.
They do not do that on aircraft parts i have welded. Only stress relieving by ultrasound or vibratory process .Anyway like I say you can look up each one of those processes in Machinery's handbook. as far as the complete series of treatments that would take an engineering metalurgist to set all that up.

[kbnit]

Don,

"To temper the 4130( with welded portions), first would have to be annealed so that the tubing and welded metal are consistant."

This is not possible, the tubing is a wrought structure, a weldment is a cast structure. You can't change the cast structure to a wrought structure by any form of heat treatment. You can provide the opportunity for the alloying constituents to return to solid solution.

"Only stress relieving by ultrasound or vibratory process"

Stress relieving involves taking a part with multiple stored energies across different boundary planes (stress), taking them all to a higher energy level, and then bringing them back to a lower energy level together. There is no phase transformation involved, so no effect on the martensite.
Tempering involves a phase change in the material that changes the martensite to bainite. The martensite forms adjacent to the weld in the HAZ, (if i'm wrong here, makoman, step in), because the HAZ is effectively quenched by the lower temperature of the adjacent base metal, and is extremely brittle. It is a function of carbon segregation during the cooling of the weld puddle. Tempering, or phase transformation, allows the carbon to migrate somewhat back into solid solution, and decreases the brittleness.

I build hot zone componenets for jet engines, ram air turbine struts, etc., I have not seen a lot of 4130 in a stressed application in an aircraft, but if it is indeed a low stress application, then stress relief may suit you well.

I love my Machinery's Handbook, it's great for a great deal of issues, but for things like this I refer to the AWS Handbook "Metals and Their Weldability".

I'm with Makoman, I suggest tempering the weld area, particularly for a high stress application like this

[makoman1860]

Originally Posted by kbnit

Don,

"To temper the 4130( with welded portions), first would have to be annealed so that the tubing and welded metal are consistant."

This is not possible, the tubing is a wrought structure, a weldment is a cast structure. You can't change the cast structure to a wrought structure by any form of heat treatment. You can provide the opportunity for the alloying constituents to return to solid solution.

"Only stress relieving by ultrasound or vibratory process"

Stress relieving involves taking a part with multiple stored energies across different boundary planes (stress), taking them all to a higher energy level, and then bringing them back to a lower energy level together. There is no phase transformation involved, so no effect on the martensite.
Tempering involves a phase change in the material that changes the martensite to bainite. The martensite forms adjacent to the weld in the HAZ, (if i'm wrong here, makoman, step in), because the HAZ is effectively quenched by the lower temperature of the adjacent base metal, and is extremely brittle. It is a function of carbon segregation during the cooling of the weld puddle. Tempering, or phase transformation, allows the carbon to migrate somewhat back into solid solution, and decreases the brittleness.

I build hot zone componenets for jet engines, ram air turbine struts, etc., I have not seen a lot of 4130 in a stressed application in an aircraft, but if it is indeed a low stress application, then stress relief may suit you well.

I love my Machinery's Handbook, it's great for a great deal of issues, but for things like this I refer to the AWS Handbook "Metals and Their Weldability".

I'm with Makoman, I suggest tempering the weld area, particularly for a high stress application like this

You got it 99%!
Except the tempering simply tempers the martensite structures themselves, it doesnt change them to bainite. That would require heating to the austenizing temperature and using a slower cooling rate. Tempered martensite has excellent metalurgical properties.
Just a tiny detail but you got it. Oh and getting back to DB, post weld treatments of various types in 4130 are very common on aerospace applications, and have been for 75 years or more.

[kbnit]

Makoman

Thanks for the clarifying that.

Regards, KB

[JOHN1]

In short, when can you mig weld chrome moly and just let it air cool, or can you?

I think that is what most of us would do!

John

[makoman1860]

Originally Posted by JOHN1

In short, when can you mig weld chrome moly and just let it air cool, or can you?

I think that is what most of us would do!

John

John,
It depends how much you rely on the weld to not fail. The big question is, why would anyone shortcut a welding process? I know of structures that are mig welded from 4130, and only the critical joints treated after welding....but they have done the engineering to determine that, and have the field experience with the same design for 30 years. I would hope nobody welding suspension parts has the "shortcut" mindset.

[JOHN1]

Originally Posted by makoman1860

John,
It depends how much you rely on the weld to not fail. The big question is, why would anyone shortcut a welding process? I know of structures that are mig welded from 4130, and only the critical joints treated after welding....but they have done the engineering to determine that, and have the field experience with the same design for 30 years. I would hope nobody welding suspension parts has the "shortcut" mindset.

Your very right, I should had said "non-critical welds".

For a few years I ran NHRA (Bikes) and the frames had to be TIG.

Please the excuse the " short cut" demeanor I displayed.

I am very safety conscience.

John

[makoman1860]

Originally Posted by JOHN1

Your very right, I should had said "non-critical welds".

For a few years I ran NHRA (Bikes) and the frames had to be TIG.

Please the excuse the " short cut" demeanor I displayed.

I am very safety conscience.

John

John,
Not a problem. I guess I really dont see many non-critical parts made from 4130. Usually if the joint is that under stressed or overbuilt, 4130 is just a waste of money. But that being said, if a guy is in his shop, without knowing exactly the loads and fatigue cycles placed on a part, he would be best to do everything possible to ensure a lasting weld.
When in doubt...build it stout! but that doesnt have to mean heavy, just using best practice. So in the short...I cant see any well designed application of 4130 that would not be best welded using an proven ENTIRE process, be it stick, mig, tig, or OA.

[Rojodiablo]

One thing many fail to touch on when they discuss welding 4130, but it applies to everything welded, is the use of gussets in the design. Many times, in fact truthfully, MOST times, truck, bike, etc. parts are welded, painted, and then cut loose on the world. I see many junctions, especially in tube structures that have no gusseting in the weld area. And the issue winds up being not enough material to 'grab' onto for the part to not tear right next to the weld. Nearly all the offroad truck and buggy parts are simply welded and done. And with that much abuse, many will break. Some hold out a loong time, and some don't last. But all the smarter ones I see, they use a few well placed gussets to help spread the weld load, because they know there is no post heat treatment being applied to the chassis.