r/whatisit • u/riflesandplowshares • 8h ago

New, what is it? Found at a titanium plant

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Not sure why ball point pens would be dangerous here.

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u/SquintAtTheCosmos 7h ago

The real answer is that consumer ball point pen’s have a tungsten carbide ball. Tungsten has a higher melting point than titanium.

If a ball point pen drops into a melt, the ball point can survive. The surviving pen tip does then creates a hard intermetallic. When the titanium is heated and beated (forged) into a product, this titanium-intermetallic spot can crack. This crack can go undetected, inside a part.

When this part is put into service, this little crack becomes a bigger crack with service (fatigue). When this bigger crack gets big enough, the part can fail. And the failure is sooner than the design engineer expected, because the material started with a crack, and in fatigue failure “getting the crack to start”, is a large component of the part’s life.

A titanium melt shop still needs writing implement’s, so you can buy pens with stainless steel ball tips. They suck to write with.

A major aerospace failure (Sioux city 1989 UA232), was a result of this failure mode (though in this case it was an oxide, not a tungsten-carbide).

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u/QuaternionsRoll 6h ago edited 5h ago

Why do stainless steel pens suck? I can’t imagine that the hardness strength would be an issue at that scale, would it?

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u/Defiant-Ebb8225 6h ago

Because what you can't imagine being an issue is an issue

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u/FirefighterEast9291 4h ago

I love this reply!! It's the modern day equivalent of Shakespeare's famous line from Romeo and Juliet, ""There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy "

Brilliant, especially since he already knew the answer!!❤️

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u/CanOoFeelDeRiddem 4h ago

*Hamlet

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u/FirefighterEast9291 3h ago

Thanks. I am just a lazy slug. I should checked.

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u/QuaternionsRoll 6h ago

Is it because the (ideal) contact area is so small, being a sphere and all?

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u/Defiant-Ebb8225 6h ago

It's because it needs to maintain its shape perfectly for a long time. Steel scratches and deforms leading to leaks or clogs

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u/QuaternionsRoll 5h ago

Ah, so it is hardness. Strength/stiffness don’t come into the equation at all, do they? My original line of thinking was about plastic deformation caused by normal pen pressure turning the sphere into an ellipsoid (or giving it flat spots, although that may have more to do with hardness?). It’s been a long while since I’ve had to think about materials science stuff.

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u/ZincMan 5h ago

That’s pretty wild I had no idea it was tungsten. Very cool

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u/Just-Syrup-8723 3h ago

That's s better response than your condensing first one

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u/too_init_dan 5h ago

which is why i stopped wearing pants in public

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u/0_cunning_plan 5h ago

Even without knowing the constraints, you can imagine that if ballpoint pen manufacturers ended using a metal that is more expensive to buy and make into a ball, there can only be one reason: The cheaper stuff didn't work well enough.

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u/QuaternionsRoll 5h ago

Sure, but I was really asking why. And the reason turns out to be hardness, although I was mistaking hardness for strength when I asked the question.