Forging Alloy Steel 4120

Chemical Analysis
C Carbon
.18 – .23
Si Silicon
0.90 – 1.20
P Phosphorus
.040 max
S Sulfur
.040 max
Mo Molybdenum
0.20 – 0.35

General Characteristics of AISI 4120

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

Forging AISI 4120

AISI 4120 may be forged at around 2200ºF (1205ºC) down to a temperature in the region of 1700ºF (925ºC.) Forged parts may be air cooled.

Forging Applications

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.

Heat Treatment

Annealing: AISI 4120 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.

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

Hardening: Austenitizing of AISI 4120 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.

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

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

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