Some steels also work harden, especially some stainless alloys. You need to take it very slow with light pressure in such cases or you end up snapping off the drill or tap just when you are almost through... ask me how I know. Especially fun when you have invested several hours' work into a complex piece of metal and you now have a hardened tool bit broken off inside a criitical hole. One learns about ways of shattering such bits and picking out the nasty little pieces... really not fun at all.
Often goes well however. I had a stainless block to tap for my scientist brother a couple of years back. Part of a vacuum rig for low pressure cell culture incubation or something. Worth about $4,000, this really precisely made piece of equipment. He needed a larger hole to thread into for a higher rate of air pumping. Had to tap it to 10-32, about 1/2" in. I drilled out the smaller threads then grabbed my lucky 10-32 tap, one I've used for many years and had recently sharpened. After about half an hour and going maybe 1/20th of a turn at a time backing out then oiling then going another 1/20th etc, the hole was finally deep enough for the connecting hardware and I could stop having a slow motion heart attack. The very next time I used that tap it snapped off. On something which didn't matter at all. Guess I got very lucky. I'll stick to newer taps next time something like that comes along.