It’s harder to force the lock on chests or large boxes made of metal, and increases the chance your weapon breaks. Additionally, the material of the weapon used to force the lock affects the result, if it’s used to pry rather than bash:

• Iron and metal behave as normal.
• Mithril is stronger than iron, and breaks less.
• Copper is slightly weaker than iron, and breaks a little more often.
• Softer metals, such as silver and gold, break less often but tend to bend. A “bent” weapon doesn’t actually become bent internally; it just loses a lot of enchantment.
• Wood is about as good as copper.
• Glass almost always breaks, and can break even if used to bash open a lock.