General Characteristics of Alloy Steel AISI 8740
Alloy 8740 is a nickel-chromium-molybdenum low-alloy steel with 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 | 0.20/0.30 |
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.