AISI / Alloy Steel 4640

Chemical Analysis
C% Carbon
0.38 – 0.43
Mn% Manganese
0.45 – 0.65
P% Phosphorus
0.040 max
S% Sulfur
0.040 max
Si% Silicon
0.20 – 0.35
Ni% Nickel
1.65 – 2.00
Mo% Molybdenum
0.2 – 0.3

General characteristics of Alloy Steel 4640

This alloy is a nickel-bearing steel showing good hardenability, strength and toughness. The alloy may be heat treated to high strength levels.


The alloy is used in applications where combinations of strength and toughness are required, but where somewhat higher strength than that obtained from 4630 grade is required. Forged parts are made from this alloy for the oil and gas industry, including components for offshore drilling rigs.


The alloy would be forged between 2150 and 1700ºF (1175 and 925ºC.) Parts would preferably be slow cooled or transferred to an equalization furnace.

Heat treatment

Annealing: For optimum machinability, this alloy would be austenitized at 1575ºF (860ºC) cooled to 1320ºF(715°C) then to 1110ºF (600ºC) at 15ºF per hr. This will give a coarse pearlitic structure. An alternative is an iso-anneal at 1150ºF (620ºC) for 8 hours.

Normalizing: 1600-1650ºF (870-900ºC.) followed by air cool.

Hardening:1500 – 1600ºF (815 – 870ºC), followed by water or oil quench depending upon size and intricacy of part.

Tempering : At temperatures based on past experience.

Machinability: The grade is readily machinable providing correctly heat treated.

Weldability:The alloy is readily welded by all fusion processes, but should be welded prior to any heat treatment. Pre and post weld heat are recommended.

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