Overheating & Aluminium Heads

[66FAIRLANE]

How can you tell or test if an overheating incident has softened aluminium heads?

[Fordman]

JMO but I believe adding excessive heat to any metal only hardens it. It may cause some warpage to the deck surface but I cannot see how it would soften it. Annealing is a completely different process but aluminum heads are too thick for this IMHO. I form this opinion from my experience as a bodyman.....John

[stroked67]

I agree with fordman, excessive overheating would only warp the heads, coolant in oil is the only way to tell without pulling heads and using machinists straight edge with a set of feeler gauges I believe.

[Falcon67]

X3 - as noted.  Current AL head tend to have really thick decks, so it may be harder to hurt them.

[66FAIRLANE]

Just as an FYI here is a link to my original post that prompted this question.
http://fepower.net/simplemachinesforum/index.php?topic=5697.0

The gauge didn't even show hot so I am pretty sure it didn't have a lot of coolant in it. And I was flogging it pretty hard. It must have got super hot. The black anodised fittings on my oil cooler have discoloured!

[Katz427]

I think you will find that getting aluminum too hot will soften it. We would check the Rockwell hardness on pistons of an overheated diesel. If she got too hot you would find the Rockwell numbers had dropped and if out of spec the pistons got tossed into the scrap bin.
That said as I remember you, have too get aluminum over 400-450 F for a period of time. If you got the motor hot like 250F then I would agree with the wisdom you may need to take a cut on the deck surface of the heads.  At least check them IMO.

[jayb]

Cylinder heads are heat treated to a T-6 temper, which involves heating the castings to over 1000 degrees and then quenching in water.  When properly heat treated, heads or other aluminum parts will not have the heat treat affected by a simple overheating event in the vehicle.  However, just like any other process at a foundry, sometimes heat treating is not done correctly, and this can lead to the heads being more prone to softening up with an overheating event.  Also, welding of course can cause the heads to go soft.

Having said that, I've seen information from some manufacturers claiming that their heads should not be exposed to temperatures over 250F.  Either they are being extremely conservative, or the heads are not properly heat treated; 350F should be no problem with a properly heat treated part.

[Cyclone Joe]

The aging temperature for A356 family of aluminum is ~320F or there abouts.  Most aluminum heads are cast from this alloy or something within the family.  So getting that temperature, or hotter, will begin to overage the metal, which in affect will lower its strength, both yield and ultimate, and reduce its hardness.  Also, the preload from the bolts can locally yield the material as the material is weakened, and as a result, you now can loose some or most of your fastener preload and head gasket compression.

Here's some data on the subject showing if you age too long this alloy will soften (granted its for a wheel, but it gives you an idea).
https://www.newarctech.com/cm/dpl/downloads/content/69/A356_Aluminum_Wheel_Facts.pdf

Joe

[FB]

The aging temperature for A356 family of aluminum is ~320F or there abouts.  Most aluminum heads are cast from this alloy or something within the family.  So getting that temperature, or hotter, will begin to overage the metal, which in affect will lower its strength, both yield and ultimate, and reduce its hardness.  Also, the preload from the bolts can locally yield the material as the material is weakened, and as a result, you now can loose some or most of your fastener preload and head gasket compression.

Here's some data on the subject showing if you age too long this alloy will soften (granted its for a wheel, but it gives you an idea).
https://www.newarctech.com/cm/dpl/downloads/content/69/A356_Aluminum_Wheel_Facts.pdf

Joe

agreed, we heat treat and age aluminum at work all the time, however the aging at 320 -350 degrees is a 24 hour process i doubt the overheating of your heads is going on for 24 HR periods.
the over aging won't soften but will harden the head, not good either as it brittle-izes the material, a simple test will tell you where your temper is on a piece of material or your head.
check it.....it'll put your mind at ease
   the 43.00 is the conductivity, the 4.01 is resistance in OHMs                       
356.0-T51, sand 43.00 4.010E-08 ALASM
356 Sand Cond. T51 43.00 2.494E+07 4.010E-08 CSNDT
356.0-T6, sand 39.00 4.420E-08 ALASM
356 Sand Cond. T6 39.00 2.262E+07 4.421E-08 CSNDT
356.0-T7,

[Cyclone Joe]

Totally agree with ya Fred, assuming you started with an annealed head.  But if it was already at a temper, and then went to 370-400F, that 24 hour process can be done in an hour, or less.  Those curves show how the hardness dissipates after you reach proper temper.

Whats the likelihood it got that hot everywhere on the head, its hard to say. 

[TomP]

I would think the exhaust ports see way more heat than any coolant overheated areas.

[Cyclone Joe]

Yup, totally, which is why steel inserts are installed.  They're also designed to have thicker walls to operate in the near annealed condition whereas the rest of the head isnt.  In reality the head is probably fine, unless it warped.

[66FAIRLANE]

Thanks guys. Thought I would check as I have heard of it before. Maybe just urban myth. Engine came out today so hope I can find a smoking gun.

[TomP]

Where'd you get those cool headers?

The guck on the rocker arms looks like a concern, coolant in the oil?

[66FAIRLANE]

The coolant in the oil is why its coming out. My own stupid fault though.

Made the headers about 20 years ago. That was the easy side.