Metal changes in strength when heated and cooled in different manners.
If you heat and cool metals, the properties of the metal change in hardness and strength.
How high of temp is it brought to and how is it cooled? You most likely want the complete piece to be heated and cooled to the same temp and rate for consistancy. Different methods of cooling are air cooling, oil quenching, water quenching. There are so many factors with metals, it would be hard to tell what you have metalurgically when you are done unless you really know what you are doing.
That being said, I know that these modifications have been done for years and many with no ill effects. But rest assured, I would be willing to bet that there are many failures and near failures out there because of it.
When doing this type of modifications, you are "at your own risk".
