AISI 4340 Alloy Steel (UNS G43400)

Topics Covered

Introduction
Chemical Composition
Physical Properties
Mechanical Properties
Thermal Properties
Other Designations
     Fabrication and Heat Treatment
     Machinability
     Forming
     Welding
     Heat Treatment
     Forging
     Cold Working
     Annealing
     Tempering
     Hardening
Applications


Introduction

Alloy steels are designated by AISI four-digit numbers. They comprise different kinds of steels having composition exceeding the limitations of B, C, Mn, Mo, Ni, Si, Cr, and Va set for carbon steels.

AISI 4340 alloy steel is a heat treatable and low alloy steel containing chromium, nickel and molybdenum. It has high toughness and strength in the heat treated condition.

Chemical Composition

The following table shows the chemical composition of AISI 4340 alloy steel.

Element Content (%)
Iron, Fe 95.195 - 96.33
Nickel, Ni 1.65 - 2.00
Chromium, Cr 0.700 - 0.900
Manganese, Mn 0.600 - 0.800
Carbon, C 0.370 - 0.430
Molybdenum, Mo 0.200 - 0.300
Silicon, Si 0.150 - 0.300
Sulfur, S  0.0400
Phosphorous, P  0.0350

Physical Properties

The physical properties of AISI 4340 alloy steel are given in the following table.

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

Mechanical Properties

The mechanical properties of annealed AISI 4340 alloy steel are displayed in the following table.

Properties Metric Imperial
Tensile strength 745 MPa 108000 psi
Yield strength 470 MPa 68200 psi
Bulk modulus (typical for steel) 140 GPa 20300 ksi
Shear modulus (typical for steel) 80 GPa 11600 ksi
Elastic modulus 190-210 GPa 27557-30458 ksi
Poisson's ratio 0.27-0.30 0.27-0.30
Elongation at break 22% 22%
Reduction of area 50% 50%
Hardness, Brinell 217 217
Hardness, Knoop (converted from Brinell hardness) 240 240
Hardness, Rockwell B (converted from Brinell hardness) 95 95
Hardness, Rockwell C (converted from Brinell hardness. Value below normal HRC range, for comparison purposes only) 17 17
Hardness, Vickers (converted from Brinell hardness) 228 228
Machinability (annealed and cold drawn. Based on 100 machinability for AISI 1212 steel.) 50 50

Thermal Properties

The following table outlines the thermal properties of AISI 4340 alloy steel.

Properties Metric Imperial
Thermal expansion co-efficient (20°C/68°F, specimen oil hardened, 600°C (1110°F) temper 12.3 µm/m°C 6.83 µin/in°F
Thermal conductivity (typical steel) 44.5 W/mK 309 BTU in/hr.ft².°F

Other Designations

Other designations that are equivalent to AISI 4340 alloy steel include the following:

AMS 6359 AMS 6454 ASTM A331 (4340) ASTM A711 SAE J1397 (4340)
AMS 6409 ASTM A29 (4340) ASTM A506 (4340) ASTM A752 (4340) SAE J404 (4340)
AMS 6414 ASTM A320 (L43) ASTM A519 (4340) ASTM A829 SAE J412 (4340)
AMS 6415 ASTM A322 (4340) ASTM A646 (4340-7) MIL S-5000
         

Fabrication and Heat Treatment

Machinability

AISI 4340 alloy steel can be machined using all conventional techniques. Machining process can be performed in the annealed or normalized and tempered conditions.

Forming

AISI 4340 alloy steel has good ductility and formability in the annealed condition. It can be bent or formed by pressing or spinning in the annealed state.

Welding

AISI 4340 alloy steel can be welded using fusion or resistance welding methods. Preheat and post heat weld procedures are followed while welding this steel by established methods.

Heat Treatment

AISI 4340 alloy steel is heat treated 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. Tempering temperatures depend upon the desired strength level.

Hardening

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

Applications

AISI 4340 alloy steel is mainly used in power transmission gears and shafts, aircraft landing gear, and other structural parts.

Date Added: Sep 7, 2012 | Updated: Jul 11, 2013
Comments
  1. Akhtar Aziz Akhtar Aziz Netherlands says:

    Thanks for all the information. I have a question for you:

    We applied cadmium cyanide plating to avoid rusting of this material but after certain time, the material started to rust. We need to know what type of coating is compatible with the Cadmium Cyanide coating for a stronger rust protection system. Not Electro plating.

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