AISI / Alloy Steel 4120

Chemical Analysis
C% Carbon
0.18 – 0.23
Mn% Manganese
0.90 – 1.20 max
P% Phosphorus
0.040 max
S% Sulfur
0.040 max
Si% Silicon
0.20 – 0.35
Cr% Chromium
0.40 – 0.60
Mo% Molybdenum
0.08 – 0.15

General characteristics of Alloy Steel 4120

This is a chromium-molybdenum alloy steel used in the case-hardened condition and displaying good strength and toughness properties.


This low-alloy steel finds applications as forgings in the aerospace and oil and gas industries – as forged valve bodies and pumps – as well as in the automotive, agricultural and defense industries.


This alloy may be forged at around 2200ºF (1205ºC) down to a temperature in the region of 1700ºF (925ºC.) Forged parts may be air cooled.

Heat treatment

Annealing: This alloy would not normally be annealed and in fact it may be machined in the as-forged condition. In the event of parts being of complicated shape and hence there being variations in structure throughout the forged parts, a normalizing treatment may be carried out prior to case hardening.


This grade of steel would be normalized at around 1700ºF (925ºC) and subsequently air cooled.

Hardening:Austenitizing of this alloy after carburizing and prior to quenching would be carried out at a temperature in the region of 1525-1575ºF (830-860ºC). Simple shapes might be water quenched, all others oil quenched.

Tempering:Parts would be tempered at temperatures, based on past experience, that would result in the required mechanical properties.


This grade will readily machine providing appropriate heat treatments are carried out.


This grade will weld using all fusion methods, but should be welded prior to carburizing and subsequent heat treatments.

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