Why the hardest material available isn't automatically the strongest — or the best choice for the job.
It's an easy mental shortcut: harder means stronger means tougher, so just pick the hardest option and move on. Real materials don't work that way. Strength, hardness, and toughness are three separate measurements, taken with three different tests, describing three different physical behaviors — and within a single alloy, pushing one of them up through heat treatment routinely pulls another one down. A file needs to be hard enough to scratch other metals. A hammer head needs to survive thousands of impacts without shattering. A structural bolt needs to carry load without stretching permanently. None of those is the same requirement, and "as hard as possible" is the wrong answer to at least two of them.