0.38 – 0.43
0.75 – 1.00 max
0.20 – 0.35
0.40 – 0.70
0.40 – 0.60
0.15 – 0.25
General characteristics of Alloy Steel 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.
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.
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.
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.
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