When I made the rivet snaps for my traction engine wheel spokes I used silver steel. Hardening was done by soaking at 800°C and quenching in brine. It is critical to vigorously agitate the part when quenching. Otherwise you don't get anything like full hardness. I tempered at around 240°C. The snaps have been used to close 3/16" steel rivets using a flypress, so they have a fairly hard life:

After 150+ rivets the upper snap has a slightly mushroomed head, but it should last the remaining 100 or so rivets. If I was making the snaps again I'd reduce the tempering temperature by ten degrees or so. The flypress is probably less of a shock load, but the forces are much higher, than using a hammer. My initial riveting trials with a lump hammer failed to close the snap head properly, whereas a few good swings on the flypress did the job:

Andrew