Low Alloy Steel 4340

AISI 4340 is a heat treatable, low alloy steel containing nickel, chromium and molybdenum. It is known for its toughness and capability of developing high strength in the heat treated condition while retaining good fatigue strength. Used for heavily stressed parts operating under strenuous conditions. AISI 4340 is considered to be a "through hardening" steel such that large section sizes can still be heat treated to high strength.

 

 

Chemical Composition (wt%)

C

Cr

Mo

Fe

Mn

0.38 - 0.43

0.7 - 0.9

0.2 - 0.3

Balance

0.6 - 0.8

 

Ni

P

Si

S

 

1.65 - 2

0.035 max

0.15 -0.3

0.04 max

 

 

 

Physical Properties

Density (g/cm3)

7.7-8.03

Specific Heat (J/kg/C)

475

Melting Point (F)

2600

Elastic Modulus @ R.T. (GPa)

190-210

Electrical Resistivity @ R.T (̦•cm .)

24.8

CTE (21 to 93 ) (m/m)

12.3

Thermal Conductivity (w/mK)

44.5

 

 

Mechanical Properties

Tensile Strength

Ultimate

0.2% Offset

Yield Strength

Elongation

in 2

Reduction of Area

745 MPa

470 MPa

22%

50%

 

Impact Strength (J)

Poissons Ratio

Hardness

Rockwell C

Hardness Rockwell B

51.1

0.27-0.30

17

95

Annealed: heated to 810C, furnace cooled 11C/hr to 355C, air-cooled

 

 

Principal Design Features

4340 alloy steel is an medium carbon alloy steel with a strong nickel addition. It is known for its high strength properties. 4340 is frequently used instead of 4140 and 4150 whenever deeper hardening characteristics, improved ductility, toughness and resistance to abrasion are required. 4340 can be quenched and tempered to a hardness of 50 to 55 Rockwell "C" depending on the size and mass of the part being treated.

 

 

Heat Treatment

Heat treatment for strengthening is done at 1525 F followed by an oil quench. For high strength (over 200 ksi) the alloy should first be normalized at 1650 F prior to heat treatment. In order to avoid decarburization, this grade should be annealed and/or hardened in a controlled neutral atmosphere, vacuum, or neutral salt furnace.

 

 

Annealing

A full anneal may be done at 1550 F followed by controlled (furnace) cooling at a rate not faster than 50 F per hour down to 600 F. From 600 F it may be air cooled

 

 

Aging

Not applicable to this alloy.

 

 

Tempering

The temperature for tempering depends upon the strength level desired. Before tempering the alloy should be in the heat treated or normalized & heat treated condition For strength levels in the 260 - 280 ksi range temper at 450 F. For strength in the 125 - 200 ksi range temper at 950 F. Do NOT temper the alloy if it is in the 220 - 260 ksi strength range as tempering can result in degradation of impact resistance for this level of strength.

 

 

Normalize

Heat slowly and uniformly to 1575 to 1625 F. Soak at temperature until uniformly heated.

Air cool. Resulting hardness should be approximately 330/370 HB.

 

 

Hardening

a. Preheat: Heat to 1200 F. Hold at this temperature until thoroughly soaked.

b. Harden: Heat to 1475 to 1500 F. Soak at heat for 30 minutes for each inch of thickness

(hold sizes under one inch thick for 30 minutes minimum).

c. Quench: Oil quench to 150 to 200 F. Temper immediately.

d. Temper: Double tempering is recommended. Soak at heat for 2 hours per inch of thickness for each temper. Air cool to room temperature between tempers. The normal tempering range for this grade is 400 to 1200 F.

 

Conditions

Tensile Properties

T (C)

Treatment

25

Annealed at 810C

Tensile Strength (MPa)

744.6

Yield Strength (MPa)

472.3

Elongation (%)

22.0

Reduction in Area (%)

49.9

25

Normalized at 870C

Tensile Strength (MPa)

1279.0

Yield Strength (MPa)

861.8

Elongation (%)

12.2

Reduction in Area (%)

36.3

25

Oil quenched, fine grained, tempered at 205C

Tensile Strength (MPa)

1875

Yield Strength (MPa)

1675

Elongation (%)

10

Reduction in Area (%)

38

25

Oil quenched, fine grained, tempered at 315C

Tensile Strength (MPa)

1724

Yield Strength (MPa)

1586

Elongation (%)

10

Reduction in Area (%)

40

25

Oil quenched, fine grained, tempered at 425C

Tensile Strength (MPa)

1469

Yield Strength (MPa)

1365

Elongation (%)

10

Reduction in Area (%)

44

25

Oil quenched, fine grained, tempered at 540C

Tensile Strength (MPa)

1172

Yield Strength (MPa)

1076

Elongation (%)

13

Reduction in Area (%)

51

25

oil quenched, fine grained, tempered at 650C

Tensile Strength (MPa)

965

Yield Strength (MPa)

855

Elongation (%)

19

Reduction in Area (%)

60

 

 

Machinability

Machining is best done with this alloy in the annealed or normalized and tempered condition. It can be machined by all conventional methods. However in the high strength conditions of 200 ksi or greater the machinability is only from 25% to 10% that of the alloy in the annealed condition.

 

 

Forming

4340 has good ductility in the annealed condition and most forming operations are carried out in that condition. It can be bent or formed by spinning or pressing in the annealed state. Bend radii should be 3t or greater.

 

 

Welding

The alloy can be fusion or resistance welded. Preheat and post heat weld procedures should be followed when welding this alloy by established methods.

 

 

Forging

Forging may be done in the range of 2250 F max. down to 1800 F.

 

 

Hot Working

4340 has very good cold forming capability so that hot working should not be needed. Hot working in any but the annealed condition can affect the strength level. Consult the alloy supplier in regard to hot working.

 

 

Cold Working

The 4340 alloy may be cold worked, in the annealed condition, by conventional methods and tooling. It has good ductility.

 

 

Application

Typical applications are for structural use, such as aircraft landing gear, power transmission gears and shafts and other structural parts.