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I recently had some material that was supplied and certified to AMS 5511 fail 3rd party tensile testing.  AMS 5511 specifies "Tensile Strength, maximum  100 ksi", the 3rd party testing showed 102 ksi. The mill certification shows 98 ksi.  Both test reports showed the material exceeding the ductility requirements. While I think that this is probably in the noise for tensile testing, I am looking for a basis to buy off the material. I don't see a specific orientation listed for the tensiles.

Now the question -   I am wondering why the tensile limit?  Is there a corrosion concern?  I have seen 304L exceed 100 ksi UTS on several occasions and on different product forms. I reviewed similar AMS and other standards and do not see a tensile maximum, so this seems to be unique to AMS 5511. I recognize that sometimes standards made by committees have compromises in them, just trying to understand if there might be an underlying metallurgical concern.

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Peter Ditzel
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I don't know for sure but I have to imagine it's a corrosion concern. High UTS in 304 could signify that maybe there's some residual stress or cold work (from cooling off the solution, solution cycle not being held long enough, etc.) that could impact corrosion resistance. This is also implied by the fact that AMS 5511 is for 304L (with the lower carbon facilitating welding by protecting against sensitization) and it has you do an ASTM A262 test. That said, I would bet that 100k was chosen as a neat round number rather than because it's a true inflection point where all of the sudden the material starts experiencing corrosion failures in service. Unless your application is particularly aggressive where you don't feel this would be prudent or justifiable, I feel like you could accept it on the basis that it's only 2ksi over, which as you mentioned might be explained by normal variation on a tensile test (E8 reports the reproducibility as 1.3% but I would take this as a grain of salt given that it was determined in optimal conditions, plus in your case the MTR reported 98ksi). Even little things like whether the tensile specimen was well-centered in the grips, or whether the technician measures the diameter of the specimen accurately (i.e. .250" vs. .251"), can bias the result by 1-2ksi (yield is generally more variable than UTS because the elastic region isn't always perfectly linear and that can affect the slope of the 0.2% offset line but still). Of course, this variation could just stem from heterogeneity in the material itself, rather than the testing parameters, but suffice it to say it isn't a resounding failure. If you're worried about it, you could re-heat treat the material or just take another 2-3 tensiles from the product and run a retest.

Hello Peter,

If your customer quality system requires that you meet SAE AMS 5511 with UTS of 100 ksi max, you have no choice but reject the lot, unless your customer grants you an exception. Also, quiz the third party testing site on the measurement errors which could be significant.

From the performance standpoint, this is not likely to cause problems. You will need to assess why this material was specified. The 304L strip is typically specified when corrosion is the primary concern. The ductility ensures further cold work in producing the product. If the product meets the ductility target, it will likely not cause issues in the production. 

Also, the UTS and ductility is reported in the rolling direction. If processing requires work in the transverse direction, ductility in the rolling direction may not be a true representation of how a material will behave when deformed.

When I worked at third party labs, we machined and retested 2 additional specimens if one failed any property. Passing would be if both of those specimens passed. I believe that is directly from ASTM E8. You should make sure the lab did such retesting.

This is an interesting topic.  I always wondered the technical necessity of a cap on UTS, which sometimes causes compliance difficulties to users.  Has there been any incident that a structure failed sole because UTS was too high?  When a structure failed and it was also high in UTS, did the material have sufficient ductility and/or toughness?  If appropriate ductility, and if necessary also toughness, are required, is there any benefit/reason of capped UTS?

Shelly Tang

Lead Principal Metallurgical Engineer

Swagelok

Solon, OH 44139

I work in extruded aluminum and have a few customers that are working with automotive crash management systems (bumpers, crash cans, side sill, etc.). They frequently have upper limits on strength to control the order and nature of the crumpling/energy absorption. You want a component that has a lot of strength and ductility in order to maximize energy absorption. However, if the part is too strong, it won't begin to fail/crumple and will instead transfer the forces farther into the system. Reducing the effectiveness of the system.

Thanks, David, for your insights.  That seems to be a reasonable cause.  Then I wonder, in that case shouldn't the cap be on yield strength, instead of UTS?  

I believe that Paul Tibbals is correct when he says "It may be that 100 ksi was set as a limit to guarantee that a certain level of annealing or stress relief had been achieved in a forged / rolled product manufacturing history."  Tensile strength is often used as an "indicator" in specifications. Back in the day, few manufacturers had the ability to measure 0.2% Offset Yield Strength, or even % Reduction in Area... so the specifications included UTS and % Elongation. The subject sheet material could be destined for a cold forming operation, where adequate ductility will be of the utmost importance. 

I don't know AMS 5511 at all, but the possible reason for the upper limit is related to the product being produced from this material. There may be a reason that they don't want the parts to be too strong. Which is to say they want "this" part to fail before another part will fail. 

I work in extruded aluminum and have a few customers that are working with automotive crash management systems (bumpers, crash cans, side sill, etc.). They frequently have upper limits on strength to control the order and nature of the crumpling/energy absorption. You want a component that has a lot of strength and ductility in order to maximize energy absorption. However, if the part is too strong, it won't begin to fail/crumple and will instead transfer the forces farther into the system. Reducing the effectiveness of the system.

I don't know AMS 5511 at all, but the possible reason for the upper limit is related to the product being produced from this material. There may be a reason that they don't want the parts to be too strong. Which is to say they want "this" part to fail before another part will fail. 

I work in extruded aluminum and have a few customers that are working with automotive crash management systems (bumpers, crash cans, side sill, etc.). They frequently have upper limits on strength to control the order and nature of the crumpling/energy absorption. You want a component that has a lot of strength and ductility in order to maximize energy absorption. However, if the part is too strong, it won't begin to fail/crumple and will instead transfer the forces farther into the system. Reducing the effectiveness of the system.

Building off David's comment, I've seen UTS maxima on components that go into missile warheads which I presume was because they wanted it to fail in a prescribed way (obviously in that case, the material is intended to fail, just not too early or too late). I've also seen the same logic applied in tribological systems where, if you have two mating components, you make the more easily replaced one softer so that it wears down first and thereby spares the more expensive component. So yes, that's definitely a thing.

David can correct me if I'm wrong here but I would assume that the automotive aluminum applications he's talking about, that max strength would be very part-specific and therefore controlled at the drawing level, OEM-spec level, etc. rather than at an AMS-spec level, since those are more general use and it would be hard to ensure optimal alignment between what you need in that particular application and what the spec is giving you (unless of course the spec were written with that product in mind and it's a relatively standard product across manufacturers).

I'll also build off of David's comments. Mechanical specifications in AMS 5511 are 100 ksi max UTS and 40% min elongation. The specs did not include yield strength at all or a min UTS (typically 70 ksi for 304L) and hardness. Those minimum YS and UTS limits are common in most 304L specs. Given the product form is "Steel, Corrosion-Resistant, Sheet, Strip, and Plate 19Cr - 9.5Ni (304L) Solution Heat Treated", I think this material is specified in this way to ensure sufficient ductility during subsequent forming operations. Cracking can occur if the material is overworked to the point that the high ductility from the elongation property is exhausted. This ensures material workability.

In my experience, a cap on tensile strength is usually there, to provide extra protection against insufficient ductility, or toughness, even if those properties are measured and recorded separately.