OT- can someone tell me what this metal is??

Stainless steel is non-magnetic.

this thing is too light to be stainless steel.... also stainless steel looks white compared to this one. I had a peice of titanium before and its exactly this color.

Take a grinder too it, if you get white sparks she be Ti!

I'd duno what to say it is! What HD did it come off?

a Hitachi Deskstar... 60gb

"tai fu" wrote in news:ei6nu8$ifu$ snipped-for-privacy@geraldo.cc.utexas.edu:

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.)

Common misconception. SOME varieties of stainless steel are non-magnetic; nickel is sometimes added (to make it shiny) and that kills the magnetism. Other formulations are magnetic.

If this thing goes right on top of a harddrive platter it would make sense to use something that is totally non magnetic....

Nickel is magnetic.

Yes, it is. Your point being? Stainless steel containing nickel is NON-magnetic.

"...when nickel is added, for instance, the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic.... "

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 low temperatures.

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.

Doc

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?

Doug

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 called austenite.

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!

Doc

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 process is:

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.

Justin

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 that.

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.

In the

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.

I took a drill bit and drilled a small sample... turns out its chrome plated aluminum. at least the drill shaving burns like aluminum (not at all)

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 less technical.

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 again...

Isn't that what the Six Million Dollar Man's legs and right arm were made from?

I thought they were made with titanium (all the more reason to charge 10,000 dollars extra)