HP 9-4-20 is a low-alloy, high-strength steel that features high cobalt and nickel additions which provide toughness. HP 9-4-20 is known for its outstanding fracture toughness, in addition to its good hardenability and weldability.
The alloy is produced by the consumable electrode vacuum arc remelting process in order to ensure optimum cleanliness.
Similar machinability practices as those used for machining 4320 steel to comparable hardness levels are recommended for HP 9-4-20.
Typical applications for HP 9-4-20 include rocket motor cases, aircraft structural components, pressure vessels, armor plate, and gears.
| AMS Spec | Application |
| AMS 6468 | AMS 6468 is a low-alloy, high strength steel with incredible fracture toughness thanks to its high concentration of nickel and cobalt. |
| AMS 6523 | AMS 6523 features excellent hardenability and weldability but cannot be forged below 1700ºF (925ºC). |
| AMS 6525 | AMS 6525 is characterized by great toughness and can be tempered at 400-1100ºF (205-595ºC) depending upon desired properties. |