Unlocking the Secrets of AISI 4340 Steel

AISI 4340 steel is a medium-carbon low-alloy high-strength steel and a typical representative of low-alloy ultra-high-strength steel. It has good hardenability, well-matched strength and toughness, high fatigue strength and low notch sensitivity, high low-temperature impact toughness, and no obvious temper brittleness.

AISI 4340 steel standard

The execution standard for 4340 steel is ASTM A29/A29M-04

Chemical composition of AISI 4340 steel and equilvalent grade

Standard	Grade	C	Mn	P	S	Si	Ni	Cr	Mo
ASTM A29	4340	0.38-0.43	0.60-0.80	0.035	0.040	0.15-0.35	1.65-2.00	0.70-0.90	0.20-0.30
EN 10250	36CrNiMo4/ 1.6511	0.32-0.40	0.50-0.80	0.035	0.035	≦0.40	0.90-1.20	0.90-1.2	0.15-0.30
BS 970	EN24/ 817M40	0.36-0.44	0.45-0.70	0.035	0.040	0.1-0.40	1.3-1.7	1.00-1.40	0.20-0.35
JIS G4103	SNCM 439/ SNCM8	0.36-0.43	0.60-0.90	0.030	0.030	0.15-0.35	1.60-2.00	0.60-1.00	0.15-0.30

Heat treatmemt

Quenching:AISI 4340 alloy steel heat treat at 830°C (1525°F) followed by quenching in oil.

Forging:AISI 4340 alloy steel is forged at 427 to 1233°C (1800 to 2250°F).

Cold Working:AISI 4340 alloy steel can be cold worked using all conventional methods in the annealed condition. It has high ductility.

Annealing:AISI 4340 alloy steel is annealed at 844°C (1550°F) followed by cooling the furnace.

Tempering:AISI 4340 alloy steel should be in the heat treated or normalized and heat treated condition before tempering. The tempering temperature for depends upon the strength level desired. For strength levels in the 260 – 280 ksi range temper at 232°C (450 F). For strength in the 125 – 200 ksi range temper at 510°C (950 F). And don’t temper the 4340 steels if it is in the 220 – 260 ksi strength range as tempering can result in degradation of impact resistance for this level of strength.

Hardening:AISI 4340 alloy steel can be hardened by cold working or heat treatment.

Physical properties

Properties	Metric	Imperial
Density	7.85 g/cm³	0.284 lb/in³
Melting point	1427°C	2600°F

Mechanical Properties

Mechanical Property Designation	T	*U	V	W	X	Y	Z
Limited Ruling Section (mm)	150	100	63	30	30	30	30
Tensile Strength (Min.)	850	930	1000	1080	1150	1230	1550
Tensile Strength (Max.)	1000	1080	1150	1230	1300	1380
0.2% Proof Stress Mpa (Min.)	665	740	835	925	1005	1080	1125
Elongation on 5.65√S₀ %	13	12	12	11	10	10	5
Izod Impact J (Min.)	54	47	47	41	34	24	10
Charpy Impact J (min.)	50	42	42	35	28	20	9
Brinell Hardness HB (Min.)	248	269	293	311	341	363	444
Brinell Hardness HB (Max.)	302	331	352	375	401	429
*Material stocked generally in condition U. Check test certificate if critical for end use.

AISI 4340 Steel Application

Due to its high strength, toughness, and weldability. It finds applications in various industries, including:

Aerospace: Components such as landing gear, engine parts, and structural components in aircraft.
Automotive: Crankshafts, axle shafts, gears, and other critical components in high-performance engines and transmissions.
Oil and Gas: Drill collars, drill rods, and other downhole drilling equipment require materials with high strength and toughness to withstand harsh drilling conditions.
Marine: Components in marine environments, such as propeller shafts and crankshafts for ship engines.
Defense and Military: Applications include gun barrels, gears, and other components in firearms and military vehicles .
Industrial Machinery: Various industrial machinery components, including gears, shafts, and crankshafts, use 4340 steel due to its high strength and fatigue resistance.
Tool and Die Making: Dies, punches, and other tooling components benefit from the wear resistance and toughness of 4340 steel in applications such as stamping and forming.
Power Generation: Components in power generation equipment, such as turbine shafts and gears, utilize 4340 steel for its strength and resistance to fatigue and wear.
Heavy Equipment: Components in heavy machinery and equipment, such as axles, gears, and crankshafts in construction and mining equipment, rely on the high strength and durability of 4340 steel.
Racing and Motorsports: High-performance racing cars and motorcycles often use 4340 steel for critical components like crankshafts and connecting rods due to its strength and fatigue resistance.