Heat treating changes metal microstructure (you'll need to cut & paste the link)
https://jfheattreatinginc.com/2023/08/the-science-behind-metal-heat-treating-understanding-microstructures-and-material-properties/#:~:text=Heat%20treating%20involves%20inducing%20phase,austenite%2C%20and%20pearlite%20to%20martensite.
Metal can be re-heat treated
https://astforgedwheels.com/heat-treating-metals-full-overview-of-the-process-types/External hammers
https://www.practicalmachinist.com/forum/threads/gun-hammer.254427/ I would go with bushveld's advice, and that of an expert machinist & gunsmith
