If two hammers strike each other, at least one of them will completely shatter with lethal force.
busted
Using a custom rig, the MythBusters repeatedly struck pairs of hammers together, but none shattered. Hammers with wooden handles merely snapped in two and hammers with metal handles bent. The MythBusters then decided to make the steel hammers harder and more brittle by adding more carbon and through heat treatment. In particular, they attempted to case harden the hammers, however it is questionable if this was done correctly. They heated the hammers to high temperatures and then coated the hammer heads in used engine oil. They also decided to have the hammers strike a more sturdy anvil instead of each other. However, during testing, the carbonized hammers merely bent at the handles without shattering. Furthermore, an anvil is generally not made of particularly hard steel, and so that test may have been doomed from the beginning. An anvil with a hardened tool steel insert would have been more appropriate. Though the myth was busted, some hammers come with warnings not to use them to strike another tool or hardened nail with excessive force; although no hammerhead shattered or chipped, high-speed footage showed particle dust flying in all directions, which presents an eye hazard.
and
REVISITED: If two hammers strike each other, at least one hammer will completely shatter with lethal force.
re-busted
First, the Build Team properly heated two modern hammers to the transition point and quenched them, making them as brittle as possible. The two hammers simply snapped at the necks when struck. Then they tested older steel hammers (predating World War II). Though the heads began to chip when struck, one of them ultimately snapped at the neck as well.
