Check the Machinerys Handbook tables on what treating. You will see that for many common alloys, the temperature that its tempered at to reach the typical hardness you will find in those applications is higher than the temperature needed to braze. For fusion welding it's likely a different situation.

Check the Machinerys Handbook tables on heat treating. You will see that for many common alloys, the temperature that its tempered at to reach the typical hardness you will find in those applications is higher than the temperature needed to braze. For fusion welding it's likely a different situation.