Martial artist who punches metal?

[Douglas]

I heard of a fighter (I believe he is a kung fu master) who has really hard knuckles due to him toughening them up on steel a thousand times every day. He started out by just lightly tapping them, then over the years, his fists steadily became harder until now anything less than steel will shatter. A guy at work told me that he has to be very careful not to punch too hard when sparring or in a fight, otherwise he will kill his opponent.

(I saw him on TV a while back, but I've tried googleing him but can't find any info. I don't know his name and if anyone knows anything about him, please tell. Thanks. )