I dug this hard drive spindle out of a dead harddrive. Its the thing that
sits right on top of the spindle with like 3 little Torox screws on it. When
I got it off this thing looks shiny grey, like some grade of stainless steel
but they are totally non magnetic... I mean I stuck the strong magnet from
the same harddrive on it and it acted like it was nothing. The metal is hard
too, a file just remove a little bit of the metal and seems to damage the
file rather than the file doing anything to it. It weights very little,
almost like aluminum but its NOT aluminum. The color isn't the same (the
whole harddrive spindle is aluminum and this metal is darker)
I am suspecting Titanium, what do you think?
heres a picture...
The last picture is the thing under a strong magnet, note it did not even
react to the magnetic force. It's not the same material as the magnet base
plate because it sticks to magnet like crazy.
"tai fu" wrote in
SOME stainless steels are non-magnetic,some are magnetic.
If it's light it could be magnesium or titanium. Probably magnesium.
(it's castable and easily machined compared to Ti.)
Yes, it is. Your point being? Stainless steel containing nickel is
"...when nickel is added, for instance, the austenite structure of iron
is stabilized. This crystal structure makes such steels
But that's irrelevent. As wikipedia says,
There are different types of stainless steels: when nickel is added,
for instance, the austenite structure of iron is stabilized. This
crystal structure makes such steels non-magnetic and less brittle at
So it's what the nickel-steel alloy does for the crystal structure,
not the properties of either metal.
Non magnetic stainless steels are called "Austenitic" stainless steels. They
have a crystal structure phase called austenite. Magnetic stainless steels
are called :Martensitic" stainless steels. They have a crystal structure
called martensite.The difference between the two is the level of nickel.
Nickel is an austenite forming element.
As an EE, I didn't have to learn much about materials. Other than Si, I
only got a small thimble full of material science. But if it hadn't
been for that, I'd be thinking Austenite was somebody from central Texas
Seriously, Doc, what about heating / cooling / quenching the stainless?
Would the various forms of heat treating, since it affects the
properties, have any effect on the magnetism?
Not really. What happens is this:
When you heat an iron alloy, the iron crystal changes. The crystal is shaped
by the arrangement of the atoms. At "room temperature", the atoms are
arranged in one shape, and at high temperature, the atoms rearrange them
selves into another. This new shape arrangement at high temperature is
The austenite (atomic arrangement) changes back when it's cooled. This is
called an "allotropic phase change". When you start to add other elements,
like nickel, the temperature at which the austenite changes back is lower.
Generally, the more nickel you add, the lower the temperature at which the
phase change happens. If you add enough nickel, the temperature is lower
than room temperature. So you have austenite, or austenitic steel.
So... What does all this have to do with magnetism? Simple; austenite is non
magnetic. That's why iron magnets loose their magnetism at high
temperatures, and why hot steel (above the austenite phase change
temperature) is non magnetic.
Also see "Currie point".
Quenching austenitic stainless steels wont really do much. Actually, heating
them to a low temperature (like 800F) will harden them. That's called aging
and that's a completely different thing called diffusion.
But we digress.
If there is little or no nickel in the iron alloy, but a quantity of carbon
or chromium (lots of chromium in stainless), the austenite with undergo a
phase change directly to a phase called martensite. Thus, martensitic
stainless steel. Martensite is magnetic. The phase of martensite is also
known as a super-saturated interstitial solid solution.
Wasn't that fun!
So far as magnetism is affected by crystal structure, quite a bit. The
cornerstone of the steel industry is not the production, but the
post-processing: the controlled (accelerated or not) cooling and
subsequent reheating "recipe" of the different alloys.
It takes time for this rearrangement to take place. Quenching (in
water, oil, salt, or lead baths) is a key process in stopping this
recrystallization from taking place and 'freezing' the desired
microstructure in place.
An allotropic phase change is simply a solid-solid phase change, as
opposed to solid-liquid, solid-gas, etc. While the addition of alloying
elements will affect various transition temperatures and corrosion and
physical characteristics, they will not affect what is controlled by
energy and time. Solid-state phase changes, for most nearly all
materials systems, require the energy (heat) and time to make their
transitions. If you cool it very quickly (and don't reheat it much), it
will lock in the structure you had at the high temperature. In the
steel industry, these types of processes are mapped on
Time-Temperature-Transformation (TTT) diagrams. An example of a simple
and a good diagram with microstructures is here:
Actually, the "Stainless Steels" link on the first website has a good
overview of this stuff.
I think a couple concepts are getting mixed or over-simplified here, but
we are already into to much metallurgical detail for a RMR thread.
Tai, send me an email, I'll give you my address, and you can send me a
part of the piece. I'll put it in the SEM and we'll get an elemental
analysis. :-) Frankly, I'm a little surprised it's not aluminum since
all the spindle cap needs to do is hold the top platter down and the
screws in place. And since precision (it's a machined part) is critical
here, a hardened steel or other metal is quite unusual.
Quenching dosent freez the the microstructure, it transforms it from face
centered cubic to body centered tetragonal.
While thats true, the most important aspect is that it is reversable and the
material can have two phases at the same time.
While the addition of alloying
This is not exactly true. Our example uses nickel as an alloying element.
Nickel forms a substitutional solid soulition. No amount of time will change
Solid-state phase changes, for most nearly all
This is not true. The structure at high temperature is face centered cubic
austenite. If cooled slower than a TTT diagram sugests, it will form body
centered cubic ferrite. If cooled rapidly (quenching) it will form body
centered tretragonal martensite.
That's why I over simplified it. (Being a chief metallurgist at a 26.6
billion dollar sales per year company.) I didn't want to get technical.
It's fun where knowledge comes from. I'm a computer geek, but what
metallurgy I know comes from photography and keeping marine aquariums -
where you learn about stainless steel for obvious reasons.
These days, I keep exotic parrots, and a real concern is toys and
accessories that contain toxic metals, especially zinc. Therefore an
interest in using stainless, but the parrot people tend to be a lot
Almost EVERY SINGLE MONTH someone says you can "check for stainless
steel" with a magnet. (If it should be magnetic or not varies from
person to person) And I have to go through the whole explanation