So i've been brazing a rack or two. The other day i was working with a piece of 4130 tube from Aircraft Spruce. I wanted to pinch the end of the tube to form a tab. I heated it with the torch (hot, like yellow hot) then pinched it in the vice. Then i put it in water to cool it.
Later, when it came time to drill a hole in the tab, i drilled through the first wall of the tube (in drill press) and by the time it was punching through the first wall and gettting to the second, it just seemed to stop making any progress. I thought my bit was bad so i threw another one in the press, same thing. Well, they were both old bits. I got a brand new one and still was having a really hard time making any progress! Finally i realized, i must have hardened the steel.
I re-heated it, then let it cool naturally. I tried drilling again and while not buttery soft like mild steel, i did get the hole finished. The other side i pinched in the vise without heating and that side drilled normally (much easier).
So, am i right in assuming that i hardened that steel and that is why i couldn't drill it? It's easier to flatten in the vise when heated. I wonder if i could still drill it alright if i hadn't quenched it in water afterword. I may have to try that next time.
You should never heat this steel to yellow hot to form it. It forms easily when cold. You might encounter a situation where the tooling to cold form it is too much trouble, but you do not need to heat it above a dull red (dim ambient light) to form it. Yellow heat is a good deal for iron, or steel when smithing heavy heats. The main problem with tubing is that being thin walled it can simply burn up enough that one looses integrity. With large smithing projects the scale formed comes off in a shower of spars, but since there is plenty of bulk to redistribute, it can't harm anything that it is loosing a little.
It is not normal to get enough hardness in a steel with 30 points of carbon for it to resist a serious drill bit. The bit is hardened to a much higher level than one can ever achieve with 30 points, and carbide will drill even hardened tool steel. But you got what you got in there, and the evidence was before you as others have said. One thing about drilling metal is that it requires slow speed and steady heavy pressure. When using a drill driver set it at the screw driving speed, not the drilling speed. A brace and bit with a drill bit, actually does an amazing job of drilling simple jobs since one can develop a lot of piercing pressure in that configuration. Of course a drill press with a vise is great, and in many places one can be picked up for 30 bucks.
If i had a press or even a serious vise cold-forming would work better. My vise is pretty wimpy though. I'll try it again not quite as hot and without quenching. That will probably give better results than yellow hot or totally cold.
Thanks for the tip on drill speed. I definitely run large bits at low speed, but smaller bits i usually don't bother to move belts around and just run at whatever speed was last used (within reason).
Thought for others to comment on; If one wanted to put a tab with a hole in it on the end of a stay or whatever, it seems easier to take a piece of bar stock over to the grinder and flatten both sides in a bit, drill it, and then trim it to fit into the end of the tube followed by some brass and a bit of file work to pretty it up? Just a thought