Drawing No. EH–MT–002 // Engineering Materials
AISI 4140 Alloy Steel Properties
Reviewed September 2026
AISI 4140 is a chromium-molybdenum low-alloy steel. This reference is tied to one explicit oil-quench and temper condition so the strength values are not confused with annealed, normalized or differently tempered 4140.
Representative properties
The stated basis, temperature and direction are part of the value. Switch units without changing the physical reference.
What changes these properties?
Engineering variability
- Yield and tensile strength vary strongly with tempering temperature and section size.
- The displayed elastic modulus is a typical steel value in the selected dataset; it is much less heat-treatment-sensitive than strength.
- Specific heat in the source is tabulated at 150–200°C rather than room temperature and is labeled accordingly.
- Final design should use the actual product specification, heat treatment, hardness and mill test data.
Selection note
Widely used for shafts, gears, bolts, tooling and machinery parts needing higher hardenability and strength than plain-carbon steel.
Where a property is shown as qualitative rather than numeric, that is intentional: the selected source or the material physics does not justify a single comparable scalar.
AISI 4140 in practice
The general-purpose chromium-molybdenum engineering steel — the grade most machine parts default to once plain carbon steel is not enough.
Where it is specified
4140 (EN 42CrMo4, 1.7225) is the workhorse for shafts, spindles, axles, gears, couplings, high-strength bolting, tooling holders, and oilfield components such as drill collars and wellhead parts. It is specified when a part needs 850–1000 MPa tensile strength with reasonable toughness, in sections where plain carbon steel could not be hardened through.
It is normally bought pre-hardened and tempered (often 28–32 HRC) and machined in that condition, which avoids the distortion and scale of heat treating a finished part.
Chosen over what
Against 1045 carbon steel: 4140's chromium and molybdenum give far better hardenability, so a 75 mm shaft can be through-hardened rather than merely case-hardened. Molybdenum also suppresses temper embrittlement.
Against 4340: 4140 is cheaper and easier to machine, but 4340's nickel gives greater hardenability and better toughness at high strength. Cross over to 4340 when the section exceeds roughly 100 mm or when the duty is genuinely impact-loaded.
Against stainless: 4140 is far stronger than austenitic grades and cheaper, but has no corrosion resistance whatsoever and must be plated, nitrided or painted.
How it fails
Hydrogen embrittlement is the characteristic risk. Above about 1000 MPa tensile strength, 4140 becomes sensitive to hydrogen from electroplating, pickling or cathodic protection. Plated high-strength parts must be baked within hours of plating, and delayed failure hours or days after assembly is the classic symptom.
Temper embrittlement if tempered in or slow-cooled through the 375–575 °C range, which drops impact toughness sharply without changing hardness — so a hardness test will not detect it.
Fatigue from grinding damage. Abusive grinding after hardening produces surface re-austenitising and tensile residual stress; grinding burn is a frequent root cause in shaft failures.
Quench cracking in parts with sharp section changes, since 4140 is oil-quenched and thermal gradients are severe.
Fabrication notes
Machinability around 65% of B1112 in the annealed condition, falling as hardness rises. At 30 HRC it machines well with carbide; above 40 HRC it needs CBN or grinding.
Weldability is poor and this is the grade's main limitation. Carbon equivalent is about 0.80, so welding demands preheat of 300–400 °C, low-hydrogen consumables, controlled interpass temperature and post-weld tempering. Welding a hardened 4140 part without post-weld heat treatment reliably produces a brittle, crack-prone heat-affected zone.
Surface engineering is common: nitriding gives a 600–700 HV case with minimal distortion because it is done below the tempering temperature, and it also improves fatigue strength through compressive residual stress. Induction hardening suits journals and bearing seats.
Specifications and standards
ASTM A29/A29M and SAE J404 for chemistry; ASTM A193 B7 for bolting; EN 10083-3 for 42CrMo4; AMS 6382 and 6349 for aerospace bar; NACE MR0175/ISO 15156 restricts hardness to 22 HRC for sour service.
References
ASM Handbook Vol. 4, Heat Treating, for hardenability and Jominy data; Totten, Steel Heat Treatment Handbook; SAE J1268 for hardenability bands.
How to interpret this material
Stiffness
Young’s modulus describes elastic stiffness. For crystals and composites, orientation can make one scalar modulus only a directional reference.
Strength / failure
Ductile metals can use yield and tensile strength. Brittle ceramics/semiconductors and anisotropic composites require fracture, flexural or directional strength descriptions instead.
Thermal response
Conductivity, heat capacity and expansion are temperature dependent. The last column is a melting or transition descriptor only where that physical concept is appropriate.
Technical sources
The database favors manufacturer, industry, government or academic technical references and labels secondary condition-specific datasets when a primary source does not expose a complete property set.