Alloy Steel AISI 8640

 General Characteristics Alloy Steel AISI 8640
Alloy steel 8640 is a low-alloy steel containing nickel, chromium and molybdenum as alloying elements. The alloy has good hardenability, and good strength and toughness properties.

Chemical Analysis

C% Mn% P% S% Si% Ni%  Cr% Mo%
0.38/0.43 0.75/1.00 0.040 max 0.040 max 0.20/0.35 0.40/0.70 0.40/0.60 0.15/0.25

Applications
This alloy finds its uses in applications where strength, toughness and wear resistance are of prime importance. It is widely used as high-strength forged fasteners.

 Forging
This alloy is forged at a start temperature of around 2200ºF (1205ºC) and could be forged down to approximately 1700ºF(925ºC.) The alloy is best slow cooled after forging, but may also be transferred to a furnace held at a temperature approximating the forging finishing temperature, soaked then air cooled.

Heat Treatment
Annealing: For optimum machinabilty of this alloy, a microstructure consisting of coarse lamellar pearlite to coarse spheroidite is considered to be optimum. Such a structure can be obtained by austenitizing at 1525ºF (830ºC), furnace cooling to 1340ºF (725ºC) then to 1180ºF (640º) at 20ºF/hr. An alternative is an iso-anneal at 1225ºF (660ºC)

Normalizing: 1600ºF (870ºC) and air cool.
Hardening: The alloy will be austenitized at around 1525 – 1575ºF (830 – 855ºC) and oil or water quenched depending upon section size and intricacy.

Tempering: to desired hardness, using temperatures based on past experience, following water or oil quench.

Machinability: This grade of alloy is readily machined following suitable heat treatment.

Weldability: This steel may be welded by normal fusion methods, but prior to hardening and tempering. Pre heat and post heat are recommended.