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.
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.
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 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.
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:
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 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.
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 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...