Alloy steel piping with AISI 4140 classification has high fatigue strength, toughness, torsional strength, and impact and abrasion resistance. Products under this standard are highly ductile, increasing after annealing. 

Piping under this specification is designated by a four-digit AISI number, i.e. 4140, which represents the following.

  • AISI for American Iron and Steel Institute
  • 4, indicating that the pipe possesses higher amounts of molybdenum than other steel 
  • 1 designating that 4140 steel piping contains chromium
  • 40, which differentiates 4140 Steel from other steels in the series.

Pipe production for AISI 4140 piping requires more pressure as alloy piping under this specification is tougher than most carbon steels. Moreover, AISI 4140 piping, though easily welded, requires post-weld heat treatment to be welded sufficiently once hardened.

AISI 4140: Different from Carbon Steel Piping

These alloy steel products 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 standard carbon steels. 

Thus, AISI 4140 alloy steel piping contains low amounts of chromium, molybdenum, and manganese. This piping also has a high fatigue strength, abrasion and impact resistance, toughness, and torsional strength. 


Relevant Specifications

This specification outlines the requirements for standard AISI 4140 alloy steel piping with the following counterpart designations; 

France Germany Italy  Japan Sweden United Kingdom   United States:AFNOR 40 CD 4 , AFNOR 42 CD 4 DIN 1.7225 UNI 38 CrMo 4 KB , UNI 40 CrMo 4 , UNI G 4O CrMo 4 JIS SCM 4 , JIS SCM 4 H SS 2244 B.S. 708 A 42 , B.S. 708 M 40 , B.S. 709 M 40 AMS 6378 , AMS 6379 , AMS 6381B , AMS 6382 , AMS 6382G , AMS 6390 , AMS 6390A , AMS 6395 , ASTM A322 , ASTM A331 , ASTM A505 , ASTM A519 , ASTM A547 , ASTM A646 ,  MIL SPEC MIL-S-16974 ,  SAE J404 , SAE J412 , SAE J770 , UNS G41400 

Uses of AISI 4140 Alloy Steel Pipes 

AISI steel piping is typically used in aerospace, oil, gas, automotive, agricultural, and defence industries.

AISI 4140 Alloy Pipe Production Processes 

AISI 4140 piping typically uses iron, carbon, and other alloying elements produced using an electric or oxygen furnace. The principal alloying elements used in AISI 4140 alloy steel pipes are Chromium, Manganese, and Molybdenum.

The steel used may be annealed, possibly several times. The steel for AISI pipe production may have been heated to a molten phase to be poured into the desired form. 

Typically, alloy steel piping can either be hot-worked or cold-worked through rollers or other tools to reach the desired thickness. Other special operations can also be added to this process to reduce mill scale or improve mechanical properties.


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


AISI 4140 alloy steel piping has high ductility as the production requires more pressure or force for forming. This process is necessary because AISI alloy CrMnMo steel is tougher than plain carbon steel.


AISI 4140 alloy steel pipes can be welded using all conventional techniques. However, the product's mechanical properties will be affected if the alloy steel is welded in the heat-treated condition. Post-weld heat treatment is thus preferred for welding AISI alloy steel piping.

Heat Treatment

The alloy steel used in AISI 4140 piping is typically heated at 845°C (1550°F), followed by quenching in oil. Before hardening, it is normalized by heating it at 913°C (1675°F) for an extended period, followed by air cooling.


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

Hot Working

AISI 4140 alloy steel pipes 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.


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


AISI 4140 alloy steel piping 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 the alloy steel used in pipe production has a lower tempering temperature. 



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

  •  EAF: Electric Arc Furnace
  •  EAF + LF + VD: Refined-smelting and vacuum degassing


Forming Processes

  • Hot Rolling 
  • Hot Forging: Electro-hydraulic; High-speed-hydraulic; Oil-hydraulic; Precision-forging


Types of Heat Treated AISI 4140 Alloy Steel Pipes 

  • A: Annealed (full/soft/spheroidizing)
  • N: Normalized
  • NT: Normalized and tempered
  • QT: Quenched and tempered (water/oil)

Chemical Composition

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

ElementWeight in Percentage (%)
C (Carbon)0.38-0.43
Mn (Manganese)0.75
P (Phosphorous)0.035 (maximum)
S (Sulphur)0.04 (maximum)
Si (Silicon)0.15 - 0.30
Cr (Chromium)0.80 - 1.10
Mo (Molybdenum)0.15 - 0.25

Mechanical Properties

AISI 4140 alloy piping uses a low alloy steel that relies on elements other than just iron and carbon to enhance their mechanical properties. In this case, elements like chromium, molybdenum, and manganese are used to increase the strength and hardenability of the steel, making it a “chromoly” steel.

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

Density (X 1000 kg/m³)7.7 to 8.03 (at 25 Degree Celsius)
Poisson's Ratio0.27 to 0.30 (at 25 Degree Celsius) 
Elastic Modulus (GPa/ksi)190 to 210/ 27557 to 30458 (at 25 Degree Celsius) 
Tensile Strength ( MPa/psi)655/ 95000 
Yield Strength(MPa/psi)417/ 60200 (at 25 Degree Celsius) 
Reduction in Area (%)56.9
Hardness (HB)197 (at 25 Degree Celsius) 
Impact Strength (J)54.5 (at 25 Degree Celsius) 

Thermal Properties

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

Thermal Expansion Coefficient (@ 0-100°C or 32-212°F)12.2 µm/m°C or 6.78 µin/in°F 
Thermal Conductivity (W/m-K/(@ 100°C))42.6 W/mK or 296 BTU in/hr.ft².°F 
Specific Heat (J/kg-K)473

Physical Properties

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

PropertiesValues (Metric and Imperial)
Density7.85 g/cm3 or 0.284 lb/in³
Melting Point1416°C or 2580°F