4340 High Tensile Steel

4340 is a 1.8% nickel - chromium - molybdenum high hardenability high tensile steel - generally supplied hardened and tempered in the tensile range of 930 - 1080 Mpa (condition U) - (Rc 28 - 36)..
Characterised by high strength and toughness in relatively large sections.
Pre hardened and tempered 4340 can be further surface hardened by flame or induction hardening and by nitriding.
4340 is used in most industry sectors for applications requiring higher tensile/yield strength than 4140 can provide.
Typical applications are: Heavy Duty Shafts, Gears, Axles, Spindles, Couplings, Pins etc
Colour Code Stocked Sizes
Green & Yellow (Bar End)

Rounds 36 mm to 500 mm Diameter
Bar Finish
Peeled, Turned and Polished, or Hot Rolled.
Related Specifications
Australia AS 1444-1996-4340
Germany W.Nr 1.6565 40NiCrMo6
Great Britain BS970-1955 EN24

BS970 Part 3:1991 - 817M40
Japan JISG 4103 SNCM439


ASTM A29/A29M-1991 4340
ASTM A322 4340
ASTM A331 4340 Cold Finished
SAE 4340
UNS G43400
Chemical Composition (Base Material)
Min. % Max. %
Carbon 0.37 0.44
Silicon 0.10 0.35
Manganese 0.55 0.90
Nickel 1.55 2.00
Chromium 0.65 0.95
Molybdenum 0.20 0.35
Phosphorous 0 0.04
Sulphur 0 0.04
Mechanical Property Requirements for Steels in the Heat-Treated Condition for Black,Turned, Peeled or Ground Finish to AS1444-1996 4340
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.
Heat to 1150 oC - 1200 oC maximum, hold until temperature is uniform throughout the section.

Do not forge below 850 oC.Following forging operation the work piece should be cooled as slowly as possible in sand or dry lime etc...
Heat Treatment
Heat to 800 oC - 850 oC, hold until temperature is uniform throughout the section and cool in furnace.
Flame or Induction Hardening
4340 hardened and tempered bar can be further surface hardened by either the flame or induction hardening methods resulting in a case hardness in excess of Rc 50. Parts should be heated as quickly as possible to the austenitic temperature range (830 oC - 860 oC) and required case depth followed by an immediate oil or water quench, depending upon hardness required, workpiece size/shape and quenching arrangements.

Following quenching to hand warm, most components should be tempered between 150 oC - 200 oC to remove quenching stresses in the case. This will have little effect on case hardness..
Heat to 830 oC - 860 oC, hold until temperature is uniform throughout the section, soak for 10 - 15 minutes per 25 mm section, and quench in oil, water, or polymer as required.*Temper immediately while still hand warm.
4340 hardened and tempered bar can also be successfully nitrided, giving a surface hardness of up to Rc 60. Nitriding is carried out at 490 oC - 530 oC, followed by slow cooling (no quench) reducing the problem of distortion. Parts can therefore be machined to near final size, leaving a grinding allowance only. The tensile strength of the core is usually not affected since the nitriding temperature range is generally below the original tempering temperature employed.

N.B. Nickel is inert to the action of nitrogen and in general resists its diffusion into steel. This can result in lower case hardness or longer nitriding cycle times for steels containing nickel such as 4340.
Stress Relieving
Heat to 600 oC - 650 oC, hold until temperature is uniform throughout the section, soak for 1 hour per 25 mm section, and cool in still air.
Re-heat to 450 oC - 660 oC as required, hold until temperature is uniform throughout the section, soak for 1 hour per 25 mm of section, and cool in still air.

N.B. Tempering should be avoided if possible within the range 250 oC - 450 oC due to temper brittleness.
Notes on Heat Treatment
Heating temperatures, rate of heating and soaking times will vary due to factors such as work piece size/shape also furnace type employed, quenching medium and work piece transfer facilities etc..Please consult your heat treater for best results.
4340 in the hardened and tempered as supplied condition is still regarded as being readily machinable and operations such as sawing, turning, drilling, etc. can be carried out satisfactorily using machine manufacturers recommendations for suitable tool type - feeds and speeds.
Welding of 4340 in the hardened and tempered condition (as normally supplied), is not recommended and should be avoided if at all possible, as the mechanical properties will be altered within the weld heat affected zone. It is preferred that welding be carried out on 4340 while in the annealed condition, and that the work piece, immediately on cooling to hand warm, is then stress relieved at 640 oC - 660 oC prior to hardening and tempering. If welding in the hardened and tempered condition is really necessary, then the work piece, immediately on cooling to hand warm, should be if possible stress relieved at 15 oC below the original tempering temperature (if known).
Welding Procedure
Welding of 4340 in whatever condition should always be carried out using low hydrogen electrodes - please consult your welding consumables supplier.
Suggested pre-heat temperature
Section 25 mm 40 mm 50 mm 75 mm 150 mm 200 mm +
oC 370 400 425 455 510 550
Post Welding
Maximum cooling rate 95 oC per hour down to 95 oC, follow by cooling in still air. N.B. No draught. It is recommended that the work piece if possible is wrapped in an heat resistant blanket or buried in sand etc..
Interlloy believes the information provided is accurate and reliable. However no warranty of accuracy, completeness or reliability is given, nor will any responsibility be taken for errors or omissions.