Re: [ihc] Front Spindles Heat treated?

[bearbvd@xxxxxxxxxxxxxx (Greg Hermann)]

At 10:01 PM 3/14/04, Ryan Moore wrote:
>----- Original Message -----
>> >
>> > So, Jim, these are the temps for annealing steal.
>> Not really;  True annealing is done by heating to the austenitic
>condition,
>> 1350 F to 1700 F, at that temp. the carbon goes into solution with the
>iron
>> and the steel is unaffected by a magnet.  Then you slow cool it to about
>200
>> degrees F taking about 1 to 6 hours depending on the alloy content.  The
>> higher the carbon equivalency the longer it takes.  Cooling to quickly is
>> quenching which causes brittlness or what is called, "file hard" or ,"dead
>> hard".  Normalizing heating to the same austenitic temp, but letting it
>cool
>> natural.
>>
>
>IIRC from my materials classes, there are more processes too, one of which
>is to actually slow the cooling process down... then there's forging vs
>casting and all that great stuff.
>
>-Ryan
>
Too long or too hot at the austenitic temp and you get grain growth--which
reduces strength (particularly in fatigue). Forging--or hammering (which is
really synonomous with forging)--and rolling (really pretty similar, too)
while hot are the only ways to reduce grain size.

Which explains why liberal use of a chipping hammer improves the strength
of a weld !

The quench from austenitic temp is what gives martensite, as Jim said. A
variable called 'hardenability' is what determines how slowly you can cool
a steel from the austenitic state and still get martensite. Higher
hardenability means a steel will get hard (go martensitic) at a SLOWER
cooling rate. Higher alloy content (things like vanadium, chromium,
molybdenum, nickel, and _YES_ , uranium) will increase a steel's
hardenability. Higher carbon content will ALSO increase the hardenability.
This is pretty significant when working with thick sectioned parts. Don't
much matter WHAT you quench them in, you can only cool off the inside SO
fast. This is why thick sectioned parts need to be made of high alloy steel
if the intent is to harden them for higher strength.

Another trade off is that higher carbon content tends to increase the
brittleness of steel in the martensitic state--SO, one doesn't want the
carbon TOO high.

What you get via the tempering or drawing process is 'tempered martensite'.
After a full anneal, you get either bainite or pearlite--which are the
softer, lower strength states of steel.

So far, talking about conventional plain carbon and alloy steels. There are
some other pretty interesting critters out there, known as 'precipitation
hardening' steels. They would almost precisely BACKWARDS from conventional
steels. To anneal them, you quench them from a high temp. To harden them,
you HOLD them at a high temp for a period of time and then cool them
slowly. So called 'maraging' steels--as in 'martensite by aging'--are among
the PH type steels, and are among the VERY strongest alloys available--as
in from 250 to 350 thousand psi tensile strength when heat treated ! These
steels are used for RATHER critical parts--such as main rotor shafts on
helicopters.

One of the special things about maraging steels is that you can get full
martensite ALL the way through a heavy section part--without any risk of
'quench cracking'--which can happen in conventional alloy steels when you
use too harsh (fast) a quench in an effort to get full hardness to a
greater depth. Another special quality of these alloys is that --- EVEN at
full martensite hardness---they are not in the least brittle, as Jim
described with conventional steels.  The reason this is so is because the
maraging steels are VERY low in carbon content--like 0.02% , as compared
to, say, 0.40% in 4340, which is a conventional high alloy steel. The lower
the carbon content in martensite, the less brittle it is---but the lack of
carbon does NOT affect the strength. The pickle with conventional steels is
that lower carbon content reduces hardenability, to such an extent that
it's simply NOT POSSIBLE to get an ultra low carbon steel to harden (turn
to martensite) at all. This problem simply goes away in the PH type steels.

Drawbacks to the PH type steels ?? COST -- they are a MULTIPLE more
expensive than conventional high alloy steels--maybe 3 times the per pound
price of 4340e (in which the 'e' signifies 'electric melt') !! And,
also--they are extremely sensitive to 'stress-corrosion cracking'. Use them
in a nasty, corrosive--say salt water-- environment, and you can get
unpredictable failures.

Sorry--didn't start out with any intent to write a partial outline for a
materials science text !! :-)

Greg