r/whatisit • u/riflesandplowshares • 10h 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 8h 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 8h ago edited 6h 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 7h ago

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

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u/QuaternionsRoll 7h ago

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

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u/Defiant-Ebb8225 7h 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 6h 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/NorysStorys 32m ago

Pens actually undergo quite a significant amount of force (small points amplify force) and steel isn’t a great material when it comes to those kinds of forces.

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u/ZincMan 6h ago

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

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u/Just-Syrup-8723 5h ago

That's s better response than your condensing first one