How the heck to you stamp serial numbers in cast bottom brackets!

It seemed it should be as simple as, put the stamp in place, and whack it with a hammer.

This approach seemed to not do much of anything, so I lined up the punch again and whacked it again... Ok, a little more of an indent, but this approach over an over eventually ended up a stamp that shifted over multiple whacks, and was still not very deep.

For the next try, I heated up the BB with a torch. While this got me a better stamp, it also deformed the BB... No, it was not glowing red when I did this.

- Do I heat or not?
- Is it my punches? They are a set from the hardware store, but not showing any deformation from my attempts so far.

It is seeming harder than it should be, and I suspect I am missing something obvious.

Thanks!

Mike