"The Enlightenment" wrote in message
...


The SR71 achieves a temperature of between 300C and 450C in localised
areas that Janes listed in 1974 while many web sites speak of 1100F as
the Blackbirds skin temperature. This is well withing the heat
treatment range though short of the full annealing I claimed.


Having bothered to look up the values in the company materials database
I find that the maximum permissible stress for Titanium alloys complying
with
ASTM-B3777-7 reaches zero at a temperature of 346 C. While this
is doubtless conservative it does graphically show the loss in structural
strength of the material at relatively low temperatures.

Even the most heat resistant of titanium alloys such as 6l-4V is only good
for around 800 deg F (430 C) and at that point its
load bearing properties are gravely compromised with a
maximum life expectancy of 1000 hrs requiring a stress
level of no more than 25% of that available at room temperature.

In short were the airframe hot enough for annealing to occur
it would fall apart under the stresses of supersonic flight.

Keith