Here’s a clip of an aggressive heat treat/quenching process designed to allow maximum strength and hardness from this 4340 Nickel Alloy. This type of 4340 is also known for it’s elongation and fatigue properties post heat treating. This makes machining significantly more difficult, because of the material’s hardness most machining inserts or tooling can’t machine it.
To complicate even further, this treating is performed before and after machining while still meeting finish grind tolerances up to .0004 +/- .00002 (four ten thousands of an inch). To give an idea of this alloy’s advanced properties…this material is used for the wing hinge pins on the F-22 Raptor.