Nitrification is the process of using nitrogen to case harden metal. Today it is usually done with ammonia gas but an ammonium salt bath also works. We have texts telling us that some sword smiths quenched their blades in urine. Stale urine contains a significant amount of ammonium and so it might be possible to use it to nitrify a blade.

My question is whether we have any physical evidence to support the above. Are there any extant swords showing evidence of nitride case-hardening rather than the usual carbide case-hardening?

Hello,
look at Wieland the blacksmith! that's an old germanic legend. He files up a sword that he doesn't think is good enough. Bakes it in bread and feeds it to geese. Then he melts the dung in the oven and makes a new sword out of it. Scientists have repeated the experiment and found that it was a method to add nitrate to the steel. there are also reports that the same methods were used in ancient Turkey.

I know that it works on a theoretical level and have read many swordsmith anecdotes in which the result may have nitrided the blade. What I want is physical evidence. I'm looking for a metallurgical analysis of a sword, which demonstrates that the blade was case hardened with nitrides rather than carbides. Without this it is all speculation.

Due to how shallow the depth of the nitriding process tends to be, it will be very difficult find a surviving example due to corrosion loss and even 'cleaning'.