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validating Al 2024-T351 with hardness testingHelpful Member! 

JacobK (Mechanical) (OP)
27 Sep 06 13:38
I have a 2024-T351 machined part that, since it's manufacure, was found to be made with questionable processes (not enough thermocouples in the heat treating process). The part is a prototype and has been tested and I need to know if the testing was valid in light of the discrepant material.

To validate that the part was not stronger than spec I have had it hardness tested with a result of Rockwell B: 71. This is lower than the MatWeb value of 75 so I tested a sample of clean material with a result of 77- which is higher than the MatWeb value.

The assebly process of the prototype part required heating at 350F for 4 hours. When the sample of test material was heated similarly and then re-tested the Rockwell B went to 83.

Now it looks like the sample became (much) harder after heating and the prototype part is still pretty low after heating.

Is corrolating hardness to strenth a valid method of testing this material?

Does heating this material as described anneal, age or over age?

Thank you!
Helpful Member!  swall (Materials)
27 Sep 06 14:53
First of all,heating 2024-T351 for 4 hours at 350F will definitely increase the hardness, due to artificial ageing. You will end up with something closer to the T6 heat treat. Hardness would increase, as you observed with the one sample--although your part/sample descriptions are not quite clear as to which was the part in question and what you did to the part vs what you did to the "sample". Hardness testing should show roughly a 10pt Rb or 25 Brinell points difference between a T351 temper and a T6 or T7 temper. Electrical conductivity would also show this--2024-T3 material conductivity runs 28.5-34% vs 34-44% for T6.
JacobK (Mechanical) (OP)
28 Sep 06 11:39
Now I'm a little confused.

Mil Hanbook 5 has a plot that shows the "effect of exposure at elevated temperatures on the room temperature tensile ultimate strength". The plot shows a reduction in strength for any heating 200-700F for times .5-10,000 hours and then allowed to cool to room temp.

It is my understanding that a reduction in strength corresponds to a reduction in hardness (and an increase in strength corresponds to an increase in hardness).

Is this not the case for 2024-T351 aluminum plate? Is there some reference that show the correlation?

Thanks.
swall (Materials)
28 Sep 06 13:45
What is the suspected heat treat anomally on the part? Poor temperature control in the solution treatment? What is the thickness of the plate you are working with?
JacobK (Mechanical) (OP)
28 Sep 06 13:50
The anomally is quesionable temperature control in the heat treating equipment. There were not enough thermocouples to satisfy AMS 2772 and AMS 2750.

The plate is 4.5".
swall (Materials)
28 Sep 06 14:49
Do you have any scraps of the plate that did not see the 350F for 4 hours that the component saw?
JacobK (Mechanical) (OP)
28 Sep 06 20:27
Not really. We have access to other 2024-T351 4" plate, but it is not from the same sheet as the part in question.

I have found a reference that has answered my question regarding suitability of hardness testing to determine strenghth of aluminum. FAA AC 43.13-1B, 'Acceptable Methods Techniques, and Practices of Aircraft Inspection and Repair' states that hardness testing is suitable, within limits, the tensile strength of steel. Furthermore, it notes that hardness testing of aluminum should be limited to distinguishing between annealed and heat-treated aluminum of the same alloy.
swall (Materials)
29 Sep 06 7:44
Electrical conductivity is somewhat better than hardness testing. Not as much overlap between T351, T62 and T851 values as there is with hardness testing. If this component is for a critical application, I would consider bringing an NDT tech to do conductivity testing.
kenvlach (Materials)
29 Sep 06 14:14
Hi,
I agree pretty much with swall's comments. Hardness testing won't tell you much unless starting from the same piece of material and treating differently.
Heating 2024-T351 for 4 hours at 350oF produces about 1/4 of the transformation to T851, cf. 12 hours at 375oF in ASTM B597.
But, your material has some ambiguity in the amounts of both initial aging and artificial aging.

Comparing the MatWeb pages for 2024-T351 and 2024-T851, the biggest property difference seems to be
Elongation at Break: 19% for T351 vs. 5% for T851.
Maybe worth testing, as probably an important property.
wktaylor (Aeronautics)
29 Sep 06 17:33
Guys...

In aerospace we use both hardness and conductivity [%IACS] as a nondestructive quanitification of proper heat treatment.

Refer to AMS2658B for required testing processes and values.

Note for purposes of this spec, -T3 = -T351 = T3511 [etc]

I suspect that conductivity readings will be in the ballpark above -T3... up-to as high as the values for -T6. Believe it-or-not, hardness values for these heat-treat states are about the same.

Regards, Wil Taylor

JacobK (Mechanical) (OP)
4 Oct 06 13:07
I'm going to re-test my part with new material. There doesn't seem to be a conservative way to be sure that my materials were correct during the original testing.

Thanks guys for all of your help!

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