[Lynn Eades]

The only stud extractors that I have had consistent joy with are the spline type. The spiral ones are just a disaster waiting to happen. The worst situation is trying to remove broken brass fittings. (like drain taps in V8 blocks)Don't even try them.
No one has mentioned a die grinder (dremel) or these fine cut off discs. ( I have blown a few to bits by mis- using them as grinding discs)(side loading them) but they are great! With a bit of care you can grind small amounts away.
Heat and quenching works well, but care needs to be execised if the part is heat treated and the intention is to re use it.