AISI / Alloy Steel 4720

Chemical Analysis
C% Carbon
0.17 – 0.22
Mn% Manganese
0.50 – 0.70
P% Phosphorus
0.040 max
S% Sulfur
0.040 max
Si% Silicon
0.20 – 0.35
Cr% Chromium
0.35 – 0.55
Ni% Nickel
0.90 – 1.20
Mo% Molybdenum
0.15 – 0.25

General characteristics of Alloy Steel 4720

This is a nickel-chromium-molybdenum case-hardening alloy designed for good strength, wear-resistance and toughness properties.


The alloy is used in the production of forged gears and shafting where full advantage may be taken of its case-hardened and heat-treated condition.


The alloy is forged at a temperature in the region of 2200ºF (1205ºC) and subsequently air cooled. It may be forged down to a temperature of approximately 1700ºF (925ºC.)

Heat treatment

Annealing: The alloy is rarely annealed as optimum machinability, prior to case hardening and subsequent harden and temper, is obtained in a forged or a normalized structure.

Normalizing: This alloy would be normalized prior to case hardening or hardening and tempering. This operation is particularly recommended in cases where certain areas of the forged part show differing microstructures. The normalizing operation results in all the various structural components being re-dissolved in austenite, thus ensuring a uniform structure upon air cooling from the normalizing temperature, which in this case would be in the region of 1700ºF (925ºC.)

Hardening: The austenitizing temperature for this alloy prior to quenching is 1500-1550ºF (815-845ºC), followed by quenching in oil or water depending upon section size and complexity of part.

Tempering: Parts would be tempered at a temperature that would normally be based upon past experience, a temperature that would give the required mechanical properties.


The alloy is readily machinable, providing the appropriate heat treatments are employed.


This alloy is readily weldable by normal fusion methods, but should always be welded prior to carburizing and other heat treatments.

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