Hardening drill rod - how to avoid dimensional changes?

No, it's soft. That way you can machine it however it's needed, then harden it afterwards.

Invar, IIRC, is a special cast-iron alloy that has a near-zero coefficient of thermal expansion. Moore Special Tool used it for the heads of their jig grinders.

I wasn't aware that it had special hardening properties. Are you sure you aren't thinking of its freedom from thermal expansion, in use?

-- Ed Huntress

No, I'm not that sure. But I know, that there do exist special steels that do not change their sizes (or _very_ little) if hardened. They are used for stamp and die making. Roechling has them (among others). And I _think_ they are called invar, but have no source at hand to verify that.

OK, I did a quick Google. Invar is not cast. But also I didn't find that property of not changing it's size when hardened. But still it doesn't change it's size over a big temerature range. It's mostly used for gauges.

Anyone has a name at hand?

Nick

This effect/fact (Carbon in steel) shows, that steels having very litte C-content should be better. They will have Cr, Mo, Mb etc. There is a C-equivalent that says that you can replace C with other elements (metals). If you want, I can dig for that formula.

Nick

This will be a big surprise to the precision machine tool builders, especially Moore, who used Invar castings for over 30 years.

It's probably one of those metals that you cast or not, depending on what you need. I've never read about its mechanical properties so it may not matter. The zero-temperature-expansion coefficient is its reason for existing. It is a b*tch to machine, however.

-- Ed Huntress

I've machined Invar 36 with no difficulties. It's not like titanium, for example. What is a bitch is the price. I had bid on the tool in question, but had the good sense to bid with the Invar provided by the buyer, which they did. I needed only a small piece, about four square inches, material thickness of

1/2". I was quoted $1,000 for a piece 12" square, and couldn't buy it any smaller. Buyer be ware!

Harold

I'd agree with several other posters and polish to size. But, you didn't say anything about tempering after hardening. It sure would be a shame to go to all that trouble and then have the edges chip off or worse, due to brittleness. I can tell you from personal experience that your piece of hardened W1 won't even survive a fall to the floor in the fully hardened state.

Pete Stanaitis

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