aluminum alloy tensile properties at operating temp.

[Bamend]

The volunteer staff at a local air museum are doing an analysis of recurrring cracking on a 1940s vintage Franklin engine. The A242-T21 cast aluminum heads are threaded onto a cast iron cylinder. We can't find elevated tempertaure tensile properties (Yield and UTS) for the A242 in the T21 temper in our available references. Temperature of interest is up to 400 deg. F. Also, any info available on the change in elastic modulus for that alloy at elevated temperature?

 

[prost]

Are you certain about the A242 designation? Every reference I have says this is Corten steel. Perhaps you mean "24S-T21"? If so, this might be 2024-T21. unfortunately, the usual reference for alloy material properties at elevated temps., Mil-HDBK-5, doesn't list a T21 temper.

 

[Bamend]

The chem analysis results we received from an analytical lab identified it as "A242" but also as "UNS A02420 sand cast aluminum". It is definitely aluminum and definitely not Corten steel. Sorry for the confusion in designation

 

[prost]

My ignorance of cast alumin. designations, sorry. I don't think there is a problem with what you wrote.

I find it curious though that I have found some properties for A242-T7XX in an ASM handbook, but not A242-T2X; I suppose this could be explained away by the age of the alloy. You say that you are investigating 'recurring cracking'; this suggests to me that you are running this engine. If cracking is your problem, what do UTS, YS (yield), E (Young's modulus), all as function of temperature, tell you? UTS, YS, and E are static material properties, which cracking suggests you need the fatigue properties of this material.

 

[Bamend]

Excellent point regarding the fatigue issue. I have fatigue properties for this alloy and candidate replacements at room temp, but I assumed elevated temperature fatigue properties would be really hard to find, given the difficulty in finding static properties at elevated temp. Therefore, I also assumed that I would have to rely upon a comparison of tensile properties to estimate whether the candidate replacement alloys would, in fact, be significant improvements from a fatigue standpoint. However, the other interest in the static properties is concern about potential yielding from thermal stresses in some highly restrained parts of the assumbly that are firmly mated to other materials with much different coefficients of thermal expansion. Some of the failure analysis observations indicate that final dimensions are no longer the same as initial dimensions; therefore concern about static as well as fatigue properties at elevated temp.

 

[berkshire]

Bamend,
Have you talked to Franklin about this problem

http://www.franklinengines.com/

It may be that in the paperwork PZL took over, there was information about this problem and what they did to correct it.
B.E.

 

[Bamend]

Not sure what, if anything other staff learned from Franklin. I stepped into this problem only recently. Part of the challenge of an air museum with 60+ year old aircraft, some of which were experimental, is that we are operating components far longer than originally envisioned by the original manufacturers.

 

[swall]

I found some 400F properties in the Kaiser Aluminum casting book, but only for 242-T21 alloy, not A242. But here it is: UTS-18ksi; TYS-11ksi; 3%elongation. Keep in mind that these are "typical values" not guaranteed minimums and are derived from separately cast test bars. Elastic modulus would not change much with temperature. The A242, due to its chrome addition, would have somewhat higher mechanical properties at elevated temperature, but Kaiser does not have any data.

 

[prost]

Wow! those values of UTS and TYS are terrible! This stuff is as tough as a bubble gum.

 

[kenvlach]

from my earlier response re A356 in thread330-162181:

For the elastic (Young's) modulus of Al alloys at temperature, multiply the 75^oF value* by the following:
98% at 212^oF
95% 300
90% 400
80% 500
From MIL-HDBK-694A, page 50.

*Elastic modulus for Aluminum 242.0-T21, Sand Cast: 10.3 x 10⁶ psi,
from

http://www.matweb.com/search/SpecificMaterial.asp?bassnum=MAC2420T21

So, elastic modulus at 400^oF ~ 9.27 x 10⁶ psi.

FYI-1, The 242 & A242 alloy designations weren't used before ~1969. Previously, 142 & A142. The Matweb 242.0-T21 data are the same as for 142-T21, e.g., page 600 in Casting Kaiser Aluminum, 3rd Edn. Don't they ever remeasure?]

FYI, A242 is ~identical in composition to wrought alloy Al 2218.

 

[Bamend]

Excellent information and thanks for the link to the previous post. The referenced alloy (A356) in the other post is one that is alreadys being considered as a replacement material. I went back and looked at the reported composition of the current alloy. It was reported as A242 but the composiiton actually matches 242. We have since found that the original spec was for AMS 4222E.

 

[prost]

For those who don't understand the distinction, including me, what is the difference between "A242" and "242"?

 

[swall]

A242 has .15-.25 Chrome (if memory serves me correctly). 242 has no chrome.

 

[swall]

C355 is also a good alloy for air cooled cylinder heads.

 

[Bamend]

as noted by SWAL in his message above, there are small differences in Cr content. I believe that

http://www.matweb.com

has composition listings for both versions of the alloy.