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welding 4130

K

kpk250

Registered User
Lambertville, NJ
I wanted to get some solid feedback based on experience regarding welding 4130. It seems there are many opinions on the subject. As a mechanical engineer I can appreciate the metallurgy involved. The heat treatment of steel, both intentionally and unintentionally, is a well documented and time proven process. You really can't argue the fact that when 4130 is heated above it's critical temperature and then rapidly cooled, the crystalline structural of the material is changes resulting in a very hard (but brittle) steel. Subsequent tempering by reheating can then relieve the inherent stresses, reduce the hardness and brittleness, and make the material more crack resistant. The questions are:

1. Are we heating the steel tubing above the critical temperature?
- I would certainly say yes; at least locally where the molten weld puddle is.

2. Does the steel tubing in our aircraft cool at a rate which causes this to happen?
- Questionable. Depends on the surrounding structure, tubing thickness, ambient air temperature, and ambient air flow.

3. If brittleness is the result, does it really matter in our fuselages?
- This is a matter of debate. People generally think of most tube fuselages as being over-designed. Engineers typically design in a "factor of safety" which is some multiple of the stress which is actually expected. Then of course some joints may be brittle and others are not depending on the conditions listed above. I can tell you this, if given the choice I would rather fly in an aircraft with welds that won't crack!

So what have all of you experienced? What welding method do you employ (Oxy-Acet or TIG)? Do you heat the weld afterwards?

Here are some links to info on the subject (with conflicting information):
http://www.netwelding.com/Welding 4130.htm

http://home.hiwaay.net/~langford/sportair/

http://www.tigdepot.net/articles.php

http://www.eaa1000.av.org/technicl/4130.htm

http://www.lincolnelectric.com/knowledge/articles/content/chrome-moly.asp
 
Fabman said:
I have a question.

If a weld is strong enough does it need to be stronger?
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and another question.... unless you really ball a fuselage up there are many tubes/bracing that never seem to ever get hurt, and in normal hard use don't get hurt.. so I would think some of them could be smaller, for weight reasons...... experimental 18's of course.....
 
Fair questions on a difficult subject. In a perfect world all welds would be stress relieved and normalized. There are some who maintain that the process of normalizing is so difficult to do that more harm is done trying to do so, rather than just leaving it alone. The best solution would be an enclave large enough to put the entire fuselage in and bake it. Unfortunately it would probably come out like a pretzel.
Air Tractor has been using Tig to weld their fuselages for 15 or 20 years. That is a tough business and they are still doing it. That says a lot.
So I think the answer is stress relieving is not necessary but might make you feel better on high stress areas like motor mounts and landing gear.

Bill Rusk
 
I never heard of laser welding. Here is a description: https://weldguru.com/laser-welding/
It appears to require a very close fit between the parts being welded. This is difficult to do when making a tubular fuselage by hand. Perhaps some of the CNC cut tubing kit suppliers can solve that issue? The above mentions it is suitable for carbon steel.
 
So cooling is one issue, filler rod is another. Even a slight breeze in the area can cause the heat affected zone to harden and become more brittle. Likewise if you use 4130 rod the alloy stays the same and again can become brittle with rapid cooling. If you use a mild steel rod (RG45 for gas or ER70 for TIG) you are reducing the carbon content of the weld metal and reducing the hardness.
 
If you use a mild steel rod (RG45 for gas or ER70 for TIG)
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About 25 yrs ago when I was learning to tig weld 4130, I got info from a very reputable place that tig welded sprint car chassis. They gave me some scrap to practice and said they used RG45 or maybe it was 60. After more practice and reading, I ended up using ER80S-D2 but remember talking to Jim Soares who built Supercub frames at Bozeman and they used ER70-S
 
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I was told waaaay back by an "older" gent that Beechcraft normalized the entire fuselage of the Beech Model 17 after the welding was complete in a giant oven. Anyone know if that's true?
 
mvivion said:
I was told waaaay back by an "older" gent that Beechcraft normalized the entire fuselage of the Beech Model 17 after the welding was complete in a giant oven. Anyone know if that's true?
Click to expand...

I sort of doubt it. I assume that it would move if you actually got it hot enough.
 
I think it's all a trade-off...
Gas welding has a larger heat distribution so less localized stress. But the weld itself may be less consistent.

Stress relieving is likely to result in deformation of some kind, so then cold forming is nessesary to get things back aligned. Cold forming creates its own stresses so not sure we end up further ahead.

I like the Ers80 as it seems to be more tolerant of my shakey slow welding capabilities ie getting a good quality weld with minimum carbonization (if that's a word) is the priority for me.

On more critical welds, some preheat of the surrounding steel can offset the localized heat nature of tig.
S
 
My 2 cents worth...If you are going to weld 4130 with 4130 filler rod, it's probably best to plan on Oxy-Acet and follow prescribed, pre and post heat in schedules with a cherry bomb...To me, you will get a good solid weld, with very little concern for embrittled issues and since the tubes have essentially been annealled, being heated past the transition temper, ductility will not be an issue. If you're planning on welding with mild steel rod, this works as well.

That's kinda old school and only reason I don't like it, is the slightly uglier fillets vs TIG. and how long it takes to properly do a cluster...(think 45 minutes to an hour, all-in)

I have taken to TIG welding 4130 normalized tubing, with Mild Steel rod and skipping the pre-heat, instead prepping the joint, tack and weld...but I have found over time that Oxy-Acetlyene post heating with the cherry bomb, for 10-15 minutes, ramping down temperature allows for a very nice cooling cycle. The beads look nice, with reduced pitting and suck-back and if you overlap your start and finish a bit, you can almost make them dis-appear entirely.

As for physicals, we have tested many many fillets, butts, tube joints...and have never had a finding showing big heat affected zones, due to being able to control the weld puddle (heat) so finitely with TIG...we've had thousands of joints of all kinds pass Dye-Penetrant and Magnetic Particle and X-Ray non-destructive testing.

I'm very comfortable with this method and consider it preferred, as it is based on multiple approved aircraft/aerospace manufacturers processes.

Cub Crafters however, MIG welds 4130. Pretty sure they use mild steel filler. MIG is obviously wire feed, where the filler rod, also acts as the heat source. In our business (aerospace for Boeing and Military) this is not approved and I have never viewed the practice as sound. And would not advocate for it.

I have seen many failed MIG joints over the years and almost all of them are appurtenant to start/stop regions of the puddle. We see large variations in depth of penetration of the beads and in general, I don't feel comfortable with it for aircraft use.

As far as annealling the 4130 post weld, it's just a practice I like. If there were a way to anneal the entire structure after welding, that would be great, but in reality, not practical and you'd probably never get rid of some of the stress relief warpage.

If it's me...99% of weld beads follow the simple guideline that if they "Look Right" they are most likely adequate. I've seen a lot of crap weld jobs that would shock people as to just how strong they are.

Since our fuselages, tails, gear etc...are mostly captive structures with limited physical requirements on the actual joints and most of the presented load paths are pretty well spread over multiple joints, I don't tend to get to worried which method...I just like to see good workmanship and as close as possible to an "approved process".

No sense re-inventing the wheel..

Steve
 
Something to remember here, when Piper was building these things about 90% of the structure was 1025, not 4130. Piper used mild steel because it was cheaper and still had positive margins for strength. The only reason we use 4130 today is availability. Steel mills stopped making 1025 when airframe manufacturers stopped building steel tube airplanes in quantity. It didn’t make economic sense to stock two different alloys in the same sizes when one alloy satisfied the market. Worrying about the strength of the weld filler metal is going way overkill when the tubing being used already has tensile strength much higher than what is required for the application.
 
Although....when you consider the resulting known strengths of the welds using 1020 low carbon steels, vs. 4130, The new material itself which resulted in the problem of needing attention to not have the heat affected zone / cooling / crack propagation issues that came along with it....sort of is an unintended consequences thing. We do need to be slightly mindful, but as I said previously, your eye is a pretty good judge of a good weld, vs. bad.

I don't disagree with you though. It's all a bit of overkill.

S.
 
Damn Straight....a couple coat hangers, roll of duct tape, some ratchet straps, a come-along and few pieces of bamboo will get you home from about anywhere.

(I was just trying to provide a legitimate answer as well...) I shoulda ended with...DBLS (don't be like steve)

S.
 
dgapilot said:
Something to remember here, when Piper was building these things about 90% of the structure was 1025, not 4130. Piper used mild steel because it was cheaper and still had positive margins for strength. The only reason we use 4130 today is availability. Steel mills stopped making 1025 when airframe manufacturers stopped building steel tube airplanes in quantity. It didn’t make economic sense to stock two different alloys in the same sizes when one alloy satisfied the market. Worrying about the strength of the weld filler metal is going way overkill when the tubing being used already has tensile strength much higher than what is required for the application.
Click to expand...
McMaster-Carr sells 1026 tubing and I’m sure other suppliers do as well. The chemistry is the same as 1025, especially with the alloy element percentage range. Why is it 1026 and not 1025 (both are low-carbon steels) is any one’s guess. Sometimes it’s even sold as A36. Confusing for sure…
 
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Is the laser welding we are talking about the ones I see on Fakebook or Instascam that look too good to be true?
OT but MIG was mentioned above - all Maule aircraft are and have always been MIG welded and FAA approved correct?

I find TIG and Gas welding easier than getting a good MIG bead - that could be operator error. For crappy (mine) farm welds, MIG is my go to. TIG on my airplane stuff.

EDIT: I don't think there is a documented case of a weld failure causing an accident in experimental aircraft (but I heard that statement long ago, I did not do current research) and their are some really, really ugly welds that are flying.
 
I agree, lots of ugly welds work just great.

The laser welds seem to be all over the place in terms of what they are viewed as in the aircraft business.

MD helicopter used a very sophisticated laser style weld on their mast assemblies. I’m always dubious of new things that make it look simple.

I’ve never owned a Maule and couldn’t comment on how they do it.


To me, it all has its place and if one method was available and I needed to get home, I’d use it and then fix it better later.


S
 
Stinson and Maule are MIG welded.

The Laser welding I have seen is quite impressive. Does not heat up the surrounding area.
PXL_20240528_174133630.webp

PXL_20240528_174054430.webp
 
Steve Pierce said:
The Laser welding I have seen is quite impressive. Does not heat up the surrounding area.

View attachment 110200
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That IS impressive. Being an old school welder, I have to ask if there has been any testing to compare the strength of the joint when compared to that which has had welding rod building up the bead? I would assume we would need some sort of documentation before we used this process on an aircraft fuselage? We old schoolers will need some proof before we would feel comfortable with this new process. It sure is pretty though.
 
skywagon8a said:
That IS impressive. Being an old school welder, I have to ask if there has been any testing to compare the strength of the joint when compared to that which has had welding rod building up the bead? I would assume we would need some sort of documentation before we used this process on an aircraft fuselage? We old schoolers will need some proof before we would feel comfortable with this new process. It sure is pretty though.
Click to expand...
This was in an engine shop I visited that is getting this process certified for repairing crankcases.
 
https://www.xometry.com/resources/sheet/disadvantages-of-laser-welding/ ( interesting short read)

My opinion, is that for certain alloys mainly in production manufacturing, replacing things like resistance welding, we will see this readily implemented utilizing robotics mainly, for automotive and other industries. I think it's unlikely we will see any broad acceptance any time soon for aircraft and specifically GA aircraft, given the alloys we most frequently see...(2000, 6000 and 7000 series aluminum) and low carbon steels and/or precipitation hardening stainless (4130, 4340, 13-8ph, 15-5, 17-7).

As far as filler, Laser can do both autogenous welding and added filler welds. Most of the current problems seem to surround the issues of cracking from such rapid heating and cooling of the joint, whether filler metal is used, or autogenous.

It may all get worked out in time with methodologies and specific processes, but right now the adaptations into products seem centered on auto, hardware related, construction and other directions.

Time will tell I guess as more skilled welders are lost each year and industry looks to replace skill with technology.


Steve.
 
Piper built 25,000 Cubs and didn't normalize anything, they just sent the fuselage down the line.
Air Tractor, on the other hand, welds the engine/landing gear/wing attachment section first, sends it to the oven in Dallas then welds the rest of the fuselage afterwards.
Don't overthink it. TIG away,
 
guanaco said:
Piper built 25,000 Cubs and didn't normalize anything, they just sent the fuselage down the line.
Air Tractor, on the other hand, welds the engine/landing gear/wing attachment section first, sends it to the oven in Dallas then welds the rest of the fuselage afterwards.
Don't overthink it. TIG away,
Click to expand...
Up into the 1970s, the fuselages were primarily 1020 or 1025 material, a non-heat treatable material. That said, unless you are working in an area where you have drafts blowing on your welds, a normal air cool should be fine. Using mild steel rod (RG45 for gas, ER70SX for Tig) also adds to the ductility of the joint reducing the need for normalizing the joint after welding. There are very few applications where the tensile strength of 4130 is necessary in these fuselages.
 
dgapilot said:
Up into the 1970s, the fuselages were primarily 1020 or 1025 material, a non-heat treatable material. That said, unless you are working in an area where you have drafts blowing on your welds, a normal air cool should be fine. Using mild steel rod (RG45 for gas, ER70SX for Tig) also adds to the ductility of the joint reducing the need for normalizing the joint after welding. There are very few applications where the tensile strength of 4130 is necessary in these fuselages.
Click to expand...
It's not that 1020 or 1025 isn't heat treat-able...in fact it is...1500-1600 degrees and water quenched...it just doesnt get much for the effort, as you state correctly, 4130 annealled is overkill, strength-wise. Really, you are Carburizing the 1020(adding Carbon) whereas for 4130 you have quite a few selections of what you'd like to heat treat to, for the physicals you wanted.

The point is you'd have a ton of warpage if you heated an entire fuselage to 1550 and dunked it...which would never make sense and wasn't at all necessary when you gas welded 1020. The Pre-Heat, Weld and stress relieve (post heat) was all done, one cluster at a time. One worker with a cherry bomb for pre and post heat and another doing the actual welding. 4130 is mostly the same. post heating is cheap insurance to slow the bead shrinkage by cooling it more slowly.

Pretty efficient back then.

I believe they did away with 1020 simply for the fact that the steel mills quit producing in the thin walls and 4130 is still produced in thin wall tubing today, so it was just a simple, stability of supply issue.
 
In 1970 Piper started TIG welding, the fuselages stretched 9/16" when TIG welded.

Looked at a wrecked Kitfox, lots of broken tubes just outside of the heat effected zone, MIG welded. Don't see that in Super Cubs, oxy acetylene or TIG.
 
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