Krieghoff Fireblue Magazine Release Button

[CJS57]

Perhaps the fire blue hardened the metal a bit as it is a part that could wear with the magazine constantly going in and out?

[DonVoigt]

Originally Posted by CJS57

Perhaps the fire blue hardened the metal a bit as it is a part that could wear with the magazine constantly going in and out?

No,
re-heating the hardened and polished part results in the color change- from straw to fire blue. This treatment is intended(usually) to remove some of the hardness=brittleness of the part. It also helps to eliminate internal stresses.

If one continues to heat the color continues to change until it gets to grey and then becomes hot enough to glow orange. A chart of the color changes has been posted here several times.

The incidental color- which can be very pleasing- is not the main reason for "fire blue", except perhaps in custom finishng.

To the other question or comment- for sure - the temperature of the oven or bath used for the heat treatment will result in edges or thin or other spots being blue, while others are still straw.

Kind of like you see in these tools; where you can see the transition from straw to darker straw, blue, purplish blue, and purple brown.