Pieces of steel in cylinder??

There are really only two things that make a seat fall out. Heat and bad machining.

Heat because of slow timing (another reason to get the timing correct). The easiest, most simple way to remove a seat is to heat it up. Once it cools, they fall out.

The other is machining. If the fixture and tooling aren't absolutely rigid, the bore will be out of round. And no matter what you do, the seat will fall out.

No chemicals, no mechanical methods will keep a seat in if either (or both) of those things happen.