HP9 – 4 – 20 Alloy Steel

Chemical Analysis
C Carbon
0.16 – 0.23
Mn Manganese
0.25 – 0.40
Si Silicon
Cr Chromium
V Vanadium
0.06 – 0.12
Mo Molybdemun
0.90 – 1.10
Ni Nickel
8.50 – 9.05
Co Cobalt
4.25 – 4.75

General characteristics of HP9 – 4 – 20 Alloy Steel

This is a low-alloy, high-strength steel with high nickel and cobalt additions to promote toughness. The alloy has good hardenability and weldability and excellent fracture toughness. For optimum cleanliness, the alloy is produced by the consumable electrode vacuum arc remelting process.


Aircraft structural components, rocket motor cases, armor plate, pressure vessels, gears.

Forging HP9 – 4 – 20 Alloy Steel

For ingot breakdown heat to 2250ºF (1230ºC) and to 1950ºF (1065ºC) for finish forging. Do not forge below 1700ºF (925ºC). Parts may be transferred to an annealing furnace after forging – see heat treatment.

Heat treatment


1300ºF (705ºC) for 4 hours, air cool to room temp, plus 1150ºF (620ºC) for 4-8 hours and air cool. Alternative is a sub-critical anneal at 1150ºF (620ºC) for 12-24 hours and air cool.


Would normally be carried out prior to hardening and tempering, at 1650-1750ºF (900-955ºC) for 1 hour followed by air cooling.


Austenitize at 1550-1600ºF (845-870ºC) for 1 hour and oil quench.


At 400-1100ºF (205-595ºC) depending upon desired properties. Double tempering is recommended.


This treatment would be carried out to remove any retained austenite, and might be performed at –100ºF (-73ºC) for 2 hours followed by air warm back to room temperature.


Those practices used for machining 4320 steel at similar hardness levels are recommended for HP 9-4-20 grade.


Helium-shielded TIG welding is the preferred method, using HP 9-4-20 filler wire.