23.613 and normalised 4130 bar stock

[Ng2020]

Issue:
Allowables in MMPDS for 4130 bar stock in the normalised condition have no statiscal basis (Table 2.*.*.* g1), nor is a specification minimum stated in the various bar stock AMS and MIL specifications (only F condition is addressed - I'm guessing the reason for this absence is that 4130 bar is more typcially used in higher strength applications).

Possible Solutions:
To comply with 23.613 (and ASTM equivalent), my options appear to be:
A) heat treat the assembly to f condition per ams2759, with quality control/testing to verify tensile properties. Warping is a risk.
B) have the supplier conduct testing above the requirements of the specs (which as noted above, do not stipulate testing for normalised material) to verify properties in MMPDS. We would then deal with S-basis issues from there.

Neither of these two solutions appear very economical, and will undermine the original motivation for a welded assembly.

Questions:
Is my logic correct?
How has this situation been addressed in the past? Is there another solution path I'm missing?


Background
I am working with a mechanical systems installation for a new part 23 aircraft. The installation includes a 4130 weldment, comprising machined bar stock and tube - welding is favoured over machined parts assemblies in this application, for economy of manufacture.
In this application the critical requirement is stiffness not strength, so the entire weld assembly is normalised condition.
Substantiation is by analysis, with first article proof test.

 

[SWComposites]

Yep, that sounds like your options.

Why don’t you discuss this with your DER or FAA ACO?

 

[Ng2020]

Thanks, would love to but we haven't yet engaged the regulator - no cert plan yet.

 

[rb1957]

I would treat the data in MMPDS as an allowable, at least until the cert folks say "no".

You're doing a full scale test to ultimate, that covers a lot of questions. You could propose going beyond ultimate, applying a "material uncertainty" factor (1.5?). But surely we know the properties of 4130N by now to be confident in what we're doing ??

This is going into serial production (rather than one-of STC) … you could propose ordering material in 10 shipsets and having a coupon made and tested in parallel.

The last thing I'd do is "corrupt" the design with an "unnecessary" heat treat (with all of the real world issues this'd have) just to align to a "better" allowable.

another day in paradise, or is paradise one day closer ?

 

[Ng2020]

Thanks rb1957, agree the heat treat option doesn't make sense.
I don't think we could support a large material factor on test loads so that approach wouldn't adequately address the lack of tensile strength QA provisions in the spec.
We've borrowed the QA provisions for condition F material, combined with the with tensile values for normalised material and put that on the drawing as a requirement.

 

[rb1957]

I think if you're testing each piece (from a coupon or off-cut) that'll be fine.

Depending on how extensive a structure this is, buying a few shipsets at a time and testing a sample, or if you make a few details from one purchased tube then testing the one tube validates all the pieces cut from it.

another day in paradise, or is paradise one day closer ?

 

[tbuelna]

Ng2020,

Did you check sec. 8.2.2.1.1 of MMPDS? Some discussion of allowables in fusion welded alloy steels.

 

[Sparweb]

In addition to the good advice above, I have found that preparing a stress analysis of the part in advance that makes a prediction to the deflection of the part at limit and ultimate load - which you subsequently confirm in these tests - lends a lot of credibility to your statements. Such as the one you make about the part being critical in stiffness not strength. The analysis I'm talking about should be simplified. Back of the envelope. Don't give it to a grad student.

Also, testing to failure while having a stress analysis that predicted the same value and mode of failure sets you up for future growth/modification of the part. Knowing how and when the part actually fails confirms how much margin there actually is in the design, which can get eaten up by later design changes. If you only test to the ultimate load you predict in service today, then the effective margin of safety is "zero". Any slight increase in loading forces you to do another test.

Normalized 4130 steel is well within the "comfort zone" having been used... for a few years, now I'd say...

http://www.sparweb.ca

 

[Ng2020]

Sparweb - yes absolutely agree - I have a complete set of strength check notes for all details. I'm looking forward to the test results.

The concern here was the lack of tensile strength QA in the spec, rather than any uncertainty about 4130 itself. Agree it's been widely used since the dark ages, which is why this issue caught me by surprise... I wonder how often this one gets missed?

Our solution has been to follow the guidance buried in the paragraphs preceding the allowables tables in MMPDS.

Thanks for all the good advice ladies and gents.

 

[WKTaylor]

Ng2... I live confused, please advise, thus...

1. What version MMPDS are You referring-to?

MMPDS-14...
8.2.2.1.1 Design Shear Strengths for Spot and Seam Welds in Uncoated Steels and Nickel
and Cobalt Alloys — The design shear strength for spot welds for these materials are given in Tables 8.2.2.1.1(a) and 8.2.2.1.1(b). The thickness ratio of the thickest sheet to the thinnest outer sheet in the combination should not exceed 4:1.

2. What are the exact specification(s) for 4130 steel [tube, plate, bar/rod, etc] that You are starting with? What is(are) the 'specification [raw-stock] temper'.

3. What specific welding specification are You working per, IE: AWS D17.1 or AWS D1.1 or corporate or... ???

4.1 What specific weld-filler alloy [ERXXXX] are You using... and per what specification [AMSXXXX, AWS AX.XX, or...]?

4. What specific heat-treat specifications [AMS2759/? and/?? and /???] are You working-to [-per, -IAW, etc].

4.1 NOTE. Per MMPDS and AMS2759/1, there is NO 'Condition F' … please advise what You mean.

5. How large/complex are the Assemblies You are discussing?


Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Ng2020]

Hello Wil,
My reference is MMPDS 12. There's a standard table of fusion weld joint allowables in section 8 which dates back to anc-5, not sure what the exact table number is off the top of my head.... Otherwise details are: 4130d4 per mil-s-6758 (which defines cond f tensile qa), Welding per AWS d.17, Ams6457 filler, post weld normalising per ams2759/1.
Can't discuss too much more about the details, it's not a very large or necessarily complex assy, but joints are class a welds.

 

[RoarkS]

Maybe I'm missing something, but that info should come from metal supplier if you ask for certs. Material cert should have actual tested tensile strengths, not to mention alloy composition and such.
I've never had to do down the rabbit hole past that.

I've designed quite a few 4130 tube/machined weldments, TIG welded preheated with 4130 filler and post weld normalized. We had jigs to hold one of them while being normalized too... they liked to sag.

 

[rb1957]

yes, of course … that's another way. And you can specify your allowable in your order (probably have to pay more for it) so you know that the material meets the strength you used in your analysis.

another day in paradise, or is paradise one day closer ?

 

[Ng2020]

There is no requirement in mil-s-6758 for the supplier to furnish tensile test nor hardness data for D4 (N) condition material. Tensile testing only applies to condition F (125ksi).
My recollection bis the AMS specs are similar.

Stating the tensile requirements on the drawing is exactly what we plan to do. If we did not, there would be no basis for using the tensile allowables in MMPDS.

 

[rb1957]

ok,
1) you could add that requirement (testing) to your order (even if it isn't required by spec),
2) putting the strength (used in the analysis) on the drawing does identify the Engineering requirement, but now Inspection will ask … "ok, where's the proof that the material meets this requirement ?" So your note will need some clarification …
a) should each piece (of raw material) be tested ? How ? (iaw ASTM... ?)
b) is it sufficient to test each material batch/melt ? (sounds reasonable)

another day in paradise, or is paradise one day closer ?

 

[Ng2020]

On the drawing I've pointed to the testing/sampling provisions in the spec for condition F, with the exception that I'm calling out ftu and fty for the normalised condition.

 

[rb1957]

ok, so we're saying we're testing the normalised material even though it isn't required in the spec, and we've specified the acceptable value. So the vendor will give you a cert ?

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

Ng...

Please stop using 'condition X'... unless You use '-6758, Condition X'.... or just refer to HT conditions ILO the 'specification condition', in a rational abbreviation system, like this...

[AF] = As-Forged
[AR] = As-Rolled
HT-A = Annealed
HT-N = Normalized
HT-X = Normalized and tempered to 'X'

In Your first post You referred to 'condition F' with no reference specification... and yet used the term normalized in an inconsistent fashion.

NOTE.
AMS-S-6768 replaced MIL-S-6758 [canceled]… and was 'stabilized' by SAE... meaning [usually]… -6758 should only be used for recurring procurement and will eventually 'die' in deference to comparable/better AMSxxxx specifications.

Finally on 10 Nov You coughed-up what you really were discussing... 4130d4 per mil-s-6758 d= Normalized, 4= cold-finished/drawn. OK that made sense in relation to Your issue with normalized material. However on 11 Nov post you stated...

On the drawing I've pointed to the testing/sampling provisions in the spec for condition F, with the exception that I'm calling out ftu and fty for the normalised condition.

In the US, material procurement for aerospace use demands/mandates material processor certificate of compliance be provided to the 'user' for raw material source tracking as part of the finished-part manufacturing process. tedious, but important for QA/QC/compliance. Any specialized tests above/beyond material certification PER SPECIFICATION... are covered in purchase agreements and often done by intermediary laboratories.. of possibly the user... with provision that non-conforming material can be rejected/returned for 'cause'.

BTW for weld assys there are 'newer/better' alloys for welding Assys for higher strength or reliability or fracture/corrosion durability... etc.

-----
I recognize English is your second language, so please try following this advice: the level of precision provided usually defines the quality of answer provided... not just in Eng-Tips... but in our industry.

A few relevant quotes to consider...
…
You deserve to get what you ask for: but if You don’t ask for very much, don’t expect to get very much.” --XXX Engineer [summarizing his experiences with critical aircraft fastener manufacturing].

Alternate versions, by other engineers…

You don't get what you want... You get what you ask for. Ask carefully!” –TheTick, Eng-Tips

You may not get what you asked for... but [for sure!] you never get more than you ask for!” –EdStainless, Eng-Tips
…

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Ng2020]

Honestly, I think more than enough has been said on this topic.

I've provided more than sufficient detail here for an experienced stress engineer to understand the situation.
To me this is evident in the fact that the first few responses confirmed my assumptions.

Thanks all.

 

[WKTaylor]

Ng... OK.

Soooo... please stay with us and ask/present/join-in working tough questions!

NOTE.
I am more of a materials guy... not a stress guy... so deep understanding of the materials/fabrication processes involved is very important to me. There are right and wrong ways to process any material... especially alloy steel.... only gets worse with increased HT-strength level... every detail matters!

In my early years I was criticized for lack of precision... by a VERY senior engineer that I respected... and the criticism aimed a knife at my heart. I was told 'my technical writing must stand-alone and be fully understandable 10-years from now... or I would be fired from a job I love'.


FYI. I lived in Asia for 10-years working as an aero engineer [ROK and JA and all-over]…



Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]