AISI / Alloy Steel 4320

Chemical Analysis
C% Carbon
0.17 – 0.22
Mn% Manganese
0.45 – 0.65
P% Phosphorus
0.040 max
S% Sulfur
0.040 max
Si% Silicon
0.20 – 0.35
Ni% Nickel
1.62 – 2.00
Cr% Chromium
0.40 – 0.60
Mo% Molybdenum
0.20 – 0.30

General characteristics of Alloy Steel 4320

AISI 4320 is a nickel-chromium – molybdenum case – hardening steel that displays good strength, hardenability and toughness.


This alloy is used in the manufacture of forged gears and pinions and other heavy-duty machinery components.


Heat treatment

Annealing: This alloy might be processed through case hardening and subsequent heat treatment operations without the need for annealing. Its microstructure as forged will be suitable for such treatments, but in the case of small sections where a very fine structure may result from forging, a normalizing treatment would be recommended prior to further heat treatment.


This alloy is normalized at a temperature of around 1700ºF (925ºC.) Air cooling follows heat treatment. It should be noted that experience alone will determine the normalizing temperature for a specific part following given forging conditions.


This alloy should be austenitized following carburizing at a temperature in the region of 1525 -1550ºF (830 – 845ºC) followed by oil quenching.


The alloy will be tempered at a temperature that will most probably be based on past experience, according to required mechanical properties.


The alloy is readily machinable in the forged – depending upon section size – condition or following normalizing. Finish machining following harden and temper is applicable.


AISI 4320 is readily weldable by all normal methods. The alloy should however be welded in the as-forged or normalized conditions only, and not following harden and temper.

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