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AISI 4140 Alloy Steel

Alloy steels are designated by AISI four-digit numbers and comprise different kinds of steels, each with a composition which exceeds the limitations of B, C, Mn, Mo, Ni, Si, Cr, and Va set for carbon steels. AISI 4140 alloy steel is a chromium-, molybdenum-, and manganese-containing low alloy steel. It has high fatigue strength, abrasion and impact resistance, toughness, and torsional strength. The following datasheet gives an overview of AISI 4140 alloy steel.

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Chemical Composition

The following table shows the chemical composition of AISI 4140 alloy steel.

Element Content (%)
Chromium, Cr 0.80 – 1.10
Manganese, Mn 0.75 – 1.0
Carbon, C 0.380 – 0.430
Silicon, Si 0.15 – 0.30
Molybdenum, Mo 0.15 – 0.25
Sulfur, S 0.040
Phosphorous, P 0.035
Iron, Fe Balance

Physical Properties

The physical properties of AISI 4140 alloy steel are highlighted in the following table.

Properties Metric Imperial
Density 7.85 g/cm3 0.284 lb/in³
Melting point 1416°C 2580°F

Mechanical Properties

The following table outlines the mechanical properties of AISI 4140 alloy steel.

Properties Metric Imperial
Tensile strength 655 MPa 95000 psi
Yield strength 415 MPa 60200 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 (in 50 mm) 25.70% 25.70%
Hardness, Brinell 197 197
Hardness, Knoop (converted from Brinell hardness) 219 219
Hardness, Rockwell C (converted from Brinell hardness. Value below normal HRC range, for comparison purposes only) 13 13
Hardness, Vickers (converted from Brinell hardness) 207 207
Machinability (based on AISI 1212 as 100 machinability) 65 65

The thermal properties of AISI 4140 alloy steel are given in the following table.

Thermal Properties

Properties Metric Imperial
Thermal expansion coefficient (@ 0-100°C/32-212°F) 12.2 µm/m°C 6.78 µin/in°F
Thermal conductivity (@ 100°C) 42.6 W/mK 296 BTU in/hr.ft².°F

Other Designations

Other designations equivalent to AISI 4140 alloy steel are listed in the following table.

AMS 6349 ASTM A193 (B7, B7M) ASTM A506 (4140) ASTM A752 (4140)
AMS 6381 ASTM A194 (7, 7M) ASTM A513 ASTM A829
AMS 6382 ASTM A29 (4140) ASTM A513 (4140) SAE J1397 (4140)
AMS 6390 ASTM A320 (L7, L7M, L7D) ASTM A519 (4140) SAE J404 (4140)
AMS 6395 ASTM A322 (4140) ASTM A646 (4140) SAE J412 (4140)

Fabrication and Heat Treatment

Other designations equivalent to AISI 4140 alloy steel are listed in the following table.

Fabrication and Heat Treatment

AISI 4140 alloy steel has good machinability in the annealed condition.

Forming

AISI 4140 alloy steel has high ductility. It can be formed using conventional techniques in the annealed condition. It requires more pressure or force for forming because it is tougher than plain carbon steels.

Welding

AISI 4140 alloy steel can be welded using all conventional techniques. However, the mechanical properties of this steel will be affected if it is welded in the heat-treated condition, and post-weld heat treatment should be performed.

Heat Treatment

AISI 4140 alloy steel is heated at 845°C (1550°F) followed by quenching in oil. Before hardening, it can be normalized by heating it at 913°C (1675°F) for a long period of time, followed by air cooling.

Forging

AISI 4140 alloy steel is forged at 926 to 1205°C (1700 to 2200°F)

Hot Working

AISI 4140 alloy steel can be hot worked at 816 to 1038°C (1500 to 1900°F)

Cold Working

AISI 4140 alloy steel can be cold worked using conventional methods in the annealed condition.

Annealing

AISI 4140 alloy steel is annealed at 872°C (1600°F) followed by slowly cooling in the furnace.

Tempering

AISI 4140 alloy steel can be tempered at 205 to 649°C (400 to 1200°F) depending upon the desired hardness level. The hardness of the steel can be increased if it has a lower tempering temperature. For example, a tensile strength of 225 ksi can be achieved by tempering at 316°C (600°F), and tensile strength of 130 ksi can be achieved by tempering at 538°C (1000°F).

Hardening

AISI 4140 alloy steel can be hardened by cold working, or heating and quenching.

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