I think Dewey is right. It sounds like some kind of rigging hardware swing hook with bolt, although the one pictured is not titanium.
This is called indentation hardness testing. The most common scales for structural metals are the Rockwell B and C hardness scales. The density and color of the hooks described by the o.p. sound like titanium; however, I’ll point out that titanium and its structural alloys, while hard in comparison to untreated steel, are not harder than any but the most mild steel in a heated treated condition. Commercially pure (unalloyed) titanium (Grades 1 through 4) has a nominal hardness of 70 to 100 HRB–about the same range as cartridge brass. Ti-6Al-4V, the most common structural (beta-phase) alloy which is used for most titanium fasteners, has a nominal hardness of 33 HRC (around 107-108 HRB) in the untreated condition, and up to about 40-41 HRC in the treated condition. By comparison, untreated 1095 steel (the most common tool steel used for non-stainless knives and other tool applications) as a pre-heat treat hardness of 100 HRB (12 HRC) untreated but typically 56 HRC or higher and up to 65-66 HRC in the treated condition.
It isn’t clear what the o.p. means by “a steel point”, but I can attest to even treated titanium as readily capable of being scratched and not suitable to highly abrasive applications in tension due to the erosion of the oxide layer making it sensitive to stress corrosion cracking. It is possible that a titanium nitride (TiN) or titanium aluminum nitride (TiAlN) coating could be applied to give it greater surface hardness and abrasion resistance, but that seems unlikely for this application.
My guess is that this is a concrete or masonary anchor (an anchor or lag bolt designed for wood woul have a threat that tapered all the way down to the tip unless designed to go into an insert) used for marine applications where the corrison resistance provided by titanium is important. Although titanium is quite expensive in comparison to common grades of corrosion resistant steel and aluminum (~US$9-10/lb, increased supply from both Russian and East African resources has made it sufficiently available on the market to be used for lightweight equipment, including camping and climbing gear, bicycle frames, watch housings, glasses frames, and golf club heads, as well as in some decorative applications.
Stranger
In my small experience with Ti i found its most distinctive property was its ability to hold permanently colours from heat. It goes vivid peacock blue, shocking red and rather sadly if you carry on heating a dirty brown/black.
They also had steel lag screws for sale and I tried using the sharp point on one of these to scratch it. The object of this test was to see if it was aluminum.
I don’t think I have a clear comparison of the hardness of these objects relative to the range of hardness for steel. I just think they can’t be aluminum or anything else that is much softer than steel, such as zinc or magnesium. Besides, aluminum has more of a whiteness, not brownish.
“Swing hooks” appear to be for children’s swings, with a plastic bushing for a simple bearing to handle the swinging motion. There is no bushing in these. I hunted around for titanium swing hooks and titanium rigging but didn’t find anything similar.
I do suspect the unusual threads may be for masonry. They certainly don’t look like they’re for wood.
I took pictures but can’t authenticate into my imgur account, though I can look at my pictures. I remember it was for some reason difficult to set it up in the first place and took several tries. If anybody recommends a picture sharing means that is less difficult I’ll try it. I’m using an iPad only this week.
Got a can opener? They have magnets.
Found a magnet. It attracts the parts, though very weakly. If I lean the magnet so it is almost falling over, the part can drag it enough to make it fall. I guess this might be 1% of how steel would attract a magnet, easily enough to support the weight of a magnet and more. This isn’t a Lenz effect due to eddy current, it happens in the slow limit - at least that’s how it feels to me.
Does this mean it can’t be titanium? I’m not convinced though I’m surprised. I found a couple things out there that leave the question open:
“Pure titanium metal is mildy paramagnetic, +150.0x10^6 cgs units. It causes substantial MRI artifacts. Titanium metal is commonly contaminated with trace iron. It is much less magnetic than common steels, down to non-magnetic stainless alloys.”
“Titanium isn’t magnetic on its own but usually contains an amount of iron which may alter its magnetic properties.”
Do a spark test - very definitive.
Well, that was dumb.
Back at work, I showed them to the engineer across the hall, who works with titanium frequently. Wow, he says, these are light! Maybe titanium, or perhaps hard plated aluminum.
Then I took dimensions and weighed them, and got a density estimate of 7885 kg/m^3. Stainlesses run around 8000, titanium 4430.
These are just stainless, and for some reason felt way lighter than we thought they should. Something deceptive about the shape, I guess.
False alarm, everybody. Sorry!
Thanks for the follow-up Napier, but did you find out what they are in terms of function as well as material? Just hammock hooks, as originally guessed, or something to do with boat rigging?
I didn’t learn anything new. They look very much like these hammock hooks:
The threads are funny, though - people who look at them say, well, these won’t hold wood, will they? There’s so little thread rising up out of the shank. It’s similar to the third one up from the bottom here:
http://www.chinafastener.biz/gb/send_pic.aspx?path=yp_ifi_90.jpg&width=450
They might just be poorly made. Maybe the threads weren’t supposed to wind up so sparse.
There are some stainless alloys that have must less iron than others, these are used for naval and boat uses.
(post shortened)
Take it home for dinner?
If they are designed for load perpendicular to their axis (i.e. they are driven horizontally into a post and the load is hung straight on them), then the thread doesn’t matter so much - in fact, a sparse thread will probably help in the sense that they are buried in the timber without having torn it up much.
If they are designed for load in line with their axis (i.e. they are driven into the underside of a beam and the load is hung on them), then the thread is really important in preventing them being pulled out.
Another factor: if they are supposed to be driven into end grain, the thread is almost irrelevant - screws into end grain seldom grip properly anyway.
One more potential factor… perhaps they are designed to be driven into living trees - in which case a sparse thread gets them in quick with minimal destruction.
Titanium changes colors only by building up the nitride layer on it. The color comes from the thickness of the layer. To do this you have to immerse it in nitric acid and apply a current. We have heated it up to several hundred F without seeing any change in color. If your metal is changing color by heating I would suspect it was a steel alloy.
I would guess that you had ti64 (90% Titanium, 6% Aluminum, 4% Vanadium) hooks from your description. Pure titanium is a poor choice for a hook, but Ti64 is light and 10 times stronger. Stainless steel would not scratch Ti64. They could also be an aluminum alloy. Some of those alloys are very hard and strong, but I suspect a steel point would scratch them). Both alloys would be useful in an environment where corrosion is encountered. They are usually more expensive than steel due to higher processing, machining, and material costs.
Does it look anything like this ?
If so, you know what they are:) . They are hooks to suspend objects into the electroplating bath.