Post #4 of the Smithing FAQ contains a famous post about MIM from Smith & Wesson engineer Herb Belin.

FAQ's

In that post, Mr Belin states that the hammer and sear are case hardened. It is possible that he was wrong (unlikely), or that methods have changed.

Recently, I have seen posts from people I consider highly knowledgeable stating that the hammer and trigger are hard all the way through. If anyone has more detailed information, I will be very interested.