You can also view previously answered questions in the HSW Question Archive . I have seen this on lots of things made of metal. Answer Steel is an amazing substance. By mixing in different elements and by heating and cooling it in different ways you can create surprisingly different properties.
Some steels are easily bent while others are so brittle they shatter.
Some rust and others don't, etc. The idea behind case hardening is to have two different types of steel at two different points in time. During manufacturing what you would like is a relatively soft steel that is easy to bend and machine. For a lock's shank, however, a soft steel is not good because it is easy to cut with a metal saw. After the piece is formed, therefore, you harden it to make it very difficult to cut.