AISI 4340 Mould Steel Bar


AISI 4340 is a low alloy steel with chemical composition of 0.4% C, 0.8% Cr, 0.25% Mo and 1.8% Ni, strength levels up to 150 KSI achieved by suitable heat treatment. Often used instead of AISI 4140 at the higher strength levels because of its better hardenability and improved CVN impact toughness.


4340 alloy steel is generally hardened and tempered in the tensile range of 930 – 1080 Mpa. Pre hardened and tempered 4340 steels can be surface hardened futher by flame or induction hardening and by nitriding. The 4340 steel has good shock and impact resistance as well as wear and abrasion resistance in the hardened condition. AISI 4340 steel properties offer good ductility in the annealed condition, allowing it to be bent or formed. 


Chemical Composition:


0.38~0.430.15~0.300.60~0.80≤0.035≤0.0400.70~0.901.65 ~ 2.000.20~0.30


Mechanical Propoerties:


Tensile Strength
Impact Energy
Elongation at Fracture
Reduction in Cross Section on Fracture
Impact toughness value αkv (J/cm2) Hardness(Brinell)
≥ 862(85) ≥ 1282(100) ≥ 78

≥ 12

 ≥ 55

≥ 98

≤ 363


Equivalent Grade:


StandardASTM A29 EN 10250BS 970 JIS G4103
EN24/817M40SNCM 439/SNCM8


Heat Treatment:


Heat treatment after forging is carried out to render the steel suitable for machining, and to meet the mechanical property limits specified for the steel’s particular applications. There are several ways to heat treat a given steel part and it is only through practice that the optimum temperature and conditions may be defined. The following information should be taken as a guide only.



To obtain a pearlitic structure on certain parts for machining, 4340 should be annealed at a nominal temperature of 1525 º F (830 º C,) cooling to 1350 º F (730 º C) and furnace cooling to 1130 º F (610 º C) at a rate of 20 º F (11 º C) per hour, then air cooling. This process, known as full annealing, is very time consuming as it involves slow cooling over the entire temperature range from the austenitizing temperature to a temperature well below that at which transformation is complete.


To obtain a spheroidized structure in 4340 grade involves austenitizing at 1380 º F (750 º C,) furnace cooling to 1300 º F (705 º C) then to 1050 º F (565 º C) at a rate of 5 º F (3 º C) per hour. This structure will probably result in better machinability than will the coarse lamellar pearlite structure obtained by the full anneal process.



This process is defined as heating a steel to a temperature above the ferrite to austenite transformation temperature, then cooling in air to a temperature well below this transformation temperature. The treatment may be carried out on forged products as a conditioning treatment prior to final heat treatment. Normalizing also serves to refine the structure of forgings that might have cooled non-uniformly from their forging operation. The nominal normalizing temperature for 4340 grade is 1500 º F (815 º C,) but production experience may necessitate a temperature either 50 º F (10 º C) above or below this temperature. . As a rule of thumb, when forgings are normalized before, say, carburizing or hardening and tempering, the upper range of normalizing temperatures is used. When normalizing is the final heat treatment, the lower temperature range is used.



This heat treatment results in the formation of martensite after quenching, resulting in increased hardness and tensile strength. A temperature range of 1500 – 1550 º F (815 – 845 º C) is the normal austenitizing temperature for direct hardening of 4340 grade. Oil quenching is normally used for 4340 as the alloy’s hardenability is suited to this quenching method.



Tempering is carried out to relieve stresses from the hardening process, but primarily to obtain mechanical properties required for the final application. The actual tempering temperature will be chosen to meet the required properties, and in many cases will be a matter of trial and error.


This grade 4340 may also be hardened by nitriding or flame or induction hardening.



This grade is readily machinable, with either a coarse lamellar pearlitic structure or a spheroidized structure being best advised, depending upon section size and complexity and amount of machining to be carried out. If there is doubt about the suitability of any other structure, then a spheroidizrd structure should be aimed for in heat treatment.



This grade is readily welded in the annealed condition, but welding in the hardened and tempered condition should be avoided where possible because of the effect on mechanical properties. Welding in the nitrided or flame or induction hardened conditions is not recommended.


Guidance details for welding include the use of low-hydrogen electrodes and a preheat from 400 – 570 º F (200 – 300 º C,) if possible to be maintained during welding. Parts should be slow cooled after welding in ashes or sand and stress relieved if possible.



It is used as important quenched and tempered parts that require good toughness, high strength, and large size, such as heavy machinery high-load shafts, turbine shafts with a diameter greater than 250mm, helicopter rotor shafts, turbojet turbine shafts, blades, and high Load transmission parts, crankshaft fasteners. Gears, etc.; can also be used for rotor shafts and blades whose operating temperature exceeds 400℃; after nitriding, it can be used to manufacture important parts with special performance requirements. It can be used for ultra-high temperature after low temperature tempering or austempering. Strength steel is used.

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