kaliti

1. (of metal) to harden; to quench-harden
   Kovač kali čelik u vodi. — The blacksmith hardens the steel by quenching it in water.
2. (of character) to toughen; to steel
   Teški dani kale karakter. — Hard times toughen your character.
   Sportisti se kale trenirajući po hladnom vremenu. — Athletes toughen themselves up by training in cold weather.

The participle prekaljen "seasoned, battle-hardened" is very common.