Engine building mailing list?

[Jay Wallace]

At 07:59 PM 11/19/98 -0700, you wrote:
>>At 10:26 AM 11/19/98 -0800, you wrote:
>>>Hi Barrett
>>>
>>>What's hipping?
>>>
>>
>>HIP = Hot Isostatic Press. Put the material in a sealed furnace and
>>apply a gas pressure at elevated temperatures to it in order to
>>remove any closed flaws (pores, etc) in the part. Used for things
>>like turbine blades where creep deformation has resulted in
>>internal cavity formation. The small lab HIP that I use for sinter
>>/ HIP'ing silicon nitride can go to about 2100 C (3800 F) and 200
>>MPa (20,000 psi) in nitrogen or argon.
>
>OK--but is it primarily for sintered (powdered metallurgy) parts), or is it
>also applicable to more conventional billets/forgings, etc.??
>


Greg,
it would seem to be perfectly applicable to conventional
billets/forgings - just about anywhere there are internal flaws
that can be closed up. The trick is to have the flaws _inside_ the
material with no gas path to allow the high pressure gas to the
flaw. A net force is applied by the high pressure gas to the
outside surface of the sample that is not balanced by a similar
force inside the flaw. As a result, the internal flaws are are
forced shut, particularly at at a high temperatures where difusion
can occur, if they are not gas filled. If the internal crack or
void surfaces come into contact, they can even weld themselves
together. Reduced  flaws --> better properties. This, of course,
neglects the effect of the heat treating done by the HIP cycle
(precipitate dissolution or growth, etc) on the properties.

I had better stop here before I spout too much nonsense - it has
been years since I was a metallurgist and I have never investigated
HIP cycles for metals. BTW, I suspect that even very few high
performance racing parts are HIP'ed due to the cost / effort /
gains - but I could very well be wrong here.

Jay