How to tell high speed steel from carbon steel

The spark test? My reference is in a book (Tubal Cain, model engineers handbook 3rd ed page 7.4) but I'm sure I have seen it on the web

In my experience, hardware store grade(Vermont American for example) carbon steel taps often have black oxided threads while industrial grade HSS taps are usually bright all over. The cheap carbon steel taps seem to have rolled threads with the flutes and tapered entrance threads ground after rolling and hardening. HSS taps are ground all over and the difference is readily apparent to the eye. Most HSS taps I have seen are marked as such. Spark testing the shank should produce bright white sparks for carbon steel while HSS makes fewer yellow to orange sparks.

Randy

If you had a handy spark spectrometer, that'd work. I've never seen any Hanson-made taps that weren't carbon steel, usually the cheapest- made stuff on the rack at the hardware store. If it was HSS, they'd say so. You could heat one up to red hot and let it cool slowly and see if a file would cut it afterwards. Nothing wrong with using carbon steel taps for one-offs, just don't expect production duty out of one.

Stan

These are bright all over.

I will check and post a followup, I think that these are ground all over.

Sounds easy to try. I will do so as soon as I can.

Thanks Randy (and thanks to the other poster who suggested a spark test).

BTW, there is a reason to use carbon steel taps. They tend to be tougher than the HSS taps, so they don't break as easily. Also, you can touch dull taps up with a Dremel tool (or equivalent die grinder) so don't give up on your Ace taps yet. I have tapped a buncha holes with Ace taps.

GWE

That's nice to know. Mine are in sets of three (taper, plug and bottoming). All my other taps that I had before, are HSS. I will give them a try.

And -

when you do bust 'em, you can heat'em up and they'll be soft when they cool down, so you can drill them out.

That's funny, my experience is the exact opposite. If you break off a small (#6, #8, #10) carbon steel tap in a open ended hole, it can usually be removed by driving it out with a punch. The first good hit will usually shear off the threads and a few more will push the core on through. Often the threads in the hole will be undamaged.

If you try it with a HSS tap you are SOL. Now, maybe larger carbon taps have softer cores which give some flexibility, but the smaller ones have always been much more brittle than HSS in my experience.

Randy