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Commonly used Steel Alloys in Lugs?

Commonly used Steel Alloys in Lugs?

Commonly used Steel Alloys in Lugs?

In high-load lugs like fuselage-wing root applications, I was made aware that high-strength steel is commonly used. Anybody knows which specific steel alloys these lugs are made of?

In MMPDS, I can see different category of steels: Carbon Steels, Low-Aloy, Intermediate Alloy, High-alloy etc.

Don't have much experience with steel alloys; hence the question.


RE: Commonly used Steel Alloys in Lugs?

I would bet you'd see a lot of 41xx and 43xx steels (low alloy) and occasionally some intermediates (ES-1).

Low alloys will probably dominate though. 41xx and 43xx see a lot of use because they offer a reasonable balance of machineability, ultimate strength, cost, etc.

RE: Commonly used Steel Alloys in Lugs?

there's also 15-5 and 13-8.

the thing to remember with steel is it's the heat treat that determines the strength. Be careful with more than 1/2 hard HTs.

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

RE: Commonly used Steel Alloys in Lugs?

My 2cents...

Strength is only one element to designing adequate lug-joints. Following elements are also critical...

Melting for maximum purity and processing for optimum grain to improve ST, LT and L properties Ftu/Fty, Fbru/Fbry [etc], Stress Corrosion Cracking resistance, highest mechanical properties and best fracture toughness.

Section thickness for full HT is critical... especially for thick-section parts. Some steel alloys cannot be HT processed in thicknesses useful for lugs.

Typical low alloy steels with good fracture toughness and thick-section HT capability, that I'm familiar with are as follows...

4340 [sometimes listed as 4340M] AMS6454 and AMS6414 [Vacuum-Arc-Remelted version] Typically HT 125-up-to-220-KSI, 260-to-280-KSI

4330V [sometimes listed as 4330M], AMS6411 and AMS6340 [Vacuum-Arc-Remelted version] Typically HT 220-to-240-KSI

4335V [sometimes listed as 4335M], AMS6435 and AMS6429 [Vacuum-Arc-Remelted version] Typically HT 220-to-240-KSI

9Ni-4Co-0.30C AMS6526 Typically HT 220-to-240-KSI

300M [ultra-high strength variant of 4340M] AMS6417, Typically HT 270-to-290-KSI and 280-to-305-KSI

PH13-8Mo AMS5934 [extra-high fracture toughness] HT 135-up-to-220-KSI

CAUTION... above 180-KSI, steel manufacturing processes become VERY process critical.

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]
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RE: Commonly used Steel Alloys in Lugs?

Folks, thanks for the responses. Will be of great help!

RE: Commonly used Steel Alloys in Lugs?


The type of structural component you describe would seem to be single load path, and thus would likely be considered "fracture critical traceable". This would require special processes/procedures to be followed at every step including design, analysis, procurement of raw material, manufacturing/processing, final quality assurance, and any inspections/monitoring required over its service life. As WKTaylor noted, the quality of the raw material (vacuum-melt quality wrought bar or forgings), careful attention to design details (like fillet profiles and section thickness transitions), and manufacturing processes used to produce the part (careful heat treat procedures and machining steps to first remove hot worked surface scale and later any decarb surfaces, and shot peen and roller burnishing of lug/fastener holes to improve fatigue performance).

With a fracture critical aircraft component, raw material is usually not a significant part of the total cost to produce it. So it makes sense to use a high quality material, and benefit from reduced weight and improved reliability.

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