Current 4130 Tubing Specifications

[John Massaro]

Ok, I know I've probably gone off on more about the metals business in another thread than most folks here would have probably liked to have been exposed to.

But it got my curiosity aroused, since I've been out of that realm for over 25 years, as to what are the current spec's for 4130 tubing used in air frame applications.

In doing some searching on the internet I am confused, and concerned, about something I'm finding.

What was used years ago, and apparently still called off by many, is the specification MIL-T-6736, 4130 alloy, seamless or welded tubing, in it's various heat treatments. According to what I am finding online this spec was canceled a few years back (1998), with no successive revision, revision B was the last apparently, indicated.

Here are a couple of links that I found to indicate that this is the case:

http://www.techstreet.com/products/1498058

http://www.everyspec.com/MIL-SPECS/MIL-SPECS-MIL-T/MIL-T-6736B_11548/

So, to me at least, this brings up the question of, if the spec was canceled in 1998 (and when I do a search for a "6736c I get nothing) , what spec being sold today on what basis? I'd think that the manufacturers would have dropped compliance with it once it was canceled. Or are they selling goods supposedly made to an obsolete specification that is no longer enforceable?

Now, I'm not trying to split hairs, as it would seem on the surface, but, why are seemingly ALL of the suppliers still using this defunct specification and not the current ones? This goes back to what I spoke about on another thread about requesting the "original mill test reports".

And yes, there are "suggested" replacement specifications under the jurisdiction of SAE and AMS, but not the military..

Not trying to get something started, or folks upset, just a "buyer beware" is all. If the only specification a supplier can provide material to what appears to be an obsolete MIL-T specification I'd ask questions as to its age, and what other specifications appear on the original mill test reports and stenciled on the material, if any.

[Robert440]

John,
I see what your on to. Looks like the military dropped their specs in favor of the civilian specs. Funny because I know I've seen tubing on the internet described as being Milspec!
Robert.

[alaskabearhawk]

This seems to sum it up:

"In the U.S. during the 1980s and early 1990s, it was argued that the large number of standards, nearly 30,000 by 1990, imposed unnecessary restrictions, increased cost to contractors (and hence the DOD, since the costs in the end pass along to the customer), and impeded the incorporation of the latest technology. Responding to increasing criticism, Secretary of Defense William Perry issued a memorandum in 1994 that prohibited the use of most defense standards without a waiver.This has become known as the "Perry memo". Many defense standards were canceled. In their place, the DOD encouraged the use of industry standards, such as ISO 9000 series for quality assurance, SAE standards such as the AS and AMS series (e.g., AS9100, AMS 2404), and others. Weapon systems were required to use "performance specifications" that described the desired features and performance of the weapon, as opposed to how those goals would be reached (that is, exactly which technology and which materials would be used). In 2005 the DOD issued a new memorandum which eliminated the requirement to obtain a waiver in order to use defense standards. The 2005 memo did not reinstate any canceled defense standards."

[John Massaro]

This is good info Paul, thanks.

What remains is, why some of the experimental aircraft materials supply companies, in particular a very large one, explains all of the flavors that alloy tubing can come in, but no direct specifics regarding its own inventory other than size and alloy, at least not from what one can see on its online offering.
Another of the widely recognized suppliers does make mention of MIL-T-6736 when you read the description of 4130 and its applications but does not directly say that its inventory was made to that or any other specification for that matter.

Is anyone else seeing what I'm seeing here?

Neither of these, on their web sites at least, directly claims that their 4130 is made to any spec at all, but rather it seems that by very careful wording, they lead one to believe that it is.

Point being, if their 4130 is made to a spec why don't they just say so?

TW states on their 4130 tubing web page "Our tubing is manufactured to MIL-T-6736B, AMS-T-6736 , ASTM-A519 and AMS 6360 specifications."

1. MIL-T-6736 canceled in 1998
2. AMS-T-6736 canceled in 2008
3. ASTM-A519 is a non-aircraft quality specification. ie. "commercial quality" as opposed to "aircraft quality"..
4. AMS 6360 is still active, in effect, latest revision "M"

So, in spite of their information being hosed, at least they make the claim directly.

In the end, for me personally, when the time comes to purchase the 4130 tubing it will be either US or German made, be certified to AMS 6360 rev M and have original mill test reports and matching stenciling on the tube.

[alaskabearhawk]

Hey John...

I think this is a case of paralysis by analysis. I am guilty as the next guy for this. I ran this by my son who is an engineer at Boeing. He said that because a standard has been cancelled doesn't mean that it's not valid. The material can still be manufactured to meet the older spec. Boeing has drawings from older designs they still refer to and they still have the older specs. The manufacture of parts can still use the older spec. We read over the spec for tubing and the AMS6360, 6361 and 6362 refers to a heat treat spec that may not be valid for the design. Read the attached MIL spec and look at the second page. The strength for the normalized tubing is much less that the HT specs. The AMS series is a HT spec for tubing. In other words, the 6360 tubing may be too brittle or too strong for the application in the BH. I'm sure the tubing you get will be just fine from any supplier, just as long as it isn't Chinese...and that is just from a QC point of view. YMMV.

http://standards.sae.org/ams6360m/

http://standards.sae.org/ams6361f/

http://standards.sae.org/ams6362g/

[John Massaro]

I was thinking that although the spec might be canceled by it's jurisdictional body, it could still be adhered to by any party that chooses to, so we agree on that aspect.

I do see the variations in the physical properties due to heat treatment, with chemical properties, SAE 4130, identical in each:

AMS 6360 - 160, 000 psi minimum tensile strength

AMS 6361 - 125, 000 psi " " "

AMS 6362 - 150, 000 psi " " "

MIL-T-6736 cond N - 95, 000 psi minimum tensile strength (up to and including .187 wall thickness)

With all of these stated as "minimums" it's getting clearer to me, at least where on the basis of tensile strength alone, one spec could cover several. I wish I had the resources to compare yield and elongation requirements in each spec as these are very important physical attributes to consider as well, especially when one starts thinking about brittleness or ductility properties.

yield strength = how much force it takes before the sample begins to elongate or stretch from its original size.

tensile strength = how much force it takes to pull apart a given sample to the breaking point. (there are specifications that state what the sample's physical size must be)

elongation = typically measured from the point at which the sample begins to stretch up to that point where it breaks apart. typically stated as a percentage in a 2" long sample

Today there is mention in the specs that they are applicable to 4130 with "thin wall thickness sections" what ever that means. When you think about it, it's a relative term with what is thin to you could be thick to me. A .095 wall could be considered thin on a 2" OD in one case, and be considered thick on a tube with a 3/8" OD on another. So much for clarity on the part of the engineers who develop these specs.
.
Thank you to your son the engineer for adding some genuine expertise to this thread.

So very true as you state it Paul, I too can confess to "analysis paralysis" as this might be the case on this subject.

But additionally, as I am not an engineer of any formal training, it just goes to show that a little bit of knowledge can be dangerous!

[spinningwrench]

I was at American Champion Aircraft this morning and their welder said their 4130 is now all German sourced. Didn't know if this was a performance related issue or just price. I do know 4130 has taken a drastic price hike recently.

[svyolo]

The Chinese have gotten really good at learning the rules and gaming the system for free trade agreements. I wouldn't be surprised if the "German 4130" came from Germany but was originally made in China. The Germans might just resell it at a higher mark up.