AISI 4140 Chromoly Steel (UNS G41400): Heat Treatment Guide

ASTM A29/A322 · Published: 2026-06-02 · Updated: July 2026

Quick Reference

AISI 4140 is a chromium-molybdenum (Cr-Mo) low-alloy steel containing 0.38-0.43% C, 0.75-1.00% Mn, 0.80-1.10% Cr, 0.15-0.25% Mo. The chromium provides hardenability and wear resistance; molybdenum resists temper embrittlement and maintains...

AISI 4140 is a chromium-molybdenum (Cr-Mo) low-alloy steel containing 0.38-0.43% C, 0.75-1.00% Mn, 0.80-1.10% Cr, 0.15-0.25% Mo. The chromium provides hardenability and wear resistance; molybdenum resists temper embrittlement and maintains strength at elevated temperatures. 4140 is the most widely used medium-carbon alloy steel for applications requiring high strength, fatigue resistance, and toughness after heat treatment. Typical applications: aircraft landing gear, crankshafts, gears, axles, drill collars, bolt and studs for high-temperature service, and injection mold tooling. Not recommended for: welded structures without PWHT, applications above 400°C (creep strength degrades), or corrosive environments without plating/coating.

Quick Facts

CategoryAlloy Steel
StandardASTM A29/A322
Density7.85 g/cm³
Yield Strength655 MPa (95 ksi) Q&T
Tensile Strength930 MPa (135 ksi) Q&T
Strain (%) Stress (MPa) 0 15 1070 655 MPa 930 MPa 14% Stress-Strain
Engineering stress-strain curve — AISI 4140 Chromoly Steel (UNS G41400): Heat Treatment Guide. Yield (blue ●), UTS (red ▲).

Detailed Mechanical Properties

Elongation25.7% (annealed), 17.7% (quenched & tempered at 540°C)
Hardness197 HB (annealed), 302 HB (Q&T at 540°C), up to 555 HB (Q&T at 205°C)
Charpy V Notch85J at 20°C (Q&T 540°C)
Reduction Of Area56.9% (annealed)

Physical Properties

Melting Point1416°C
Thermal Conductivity42.6 W/m·K at 20°C
Specific Heat473 J/kg·K
Density7.85 g/cm³

Global Equivalents & Cross-Reference

Alternative Standard / GradeAction
EN 42CrMo4 Compare
DIN 1.7225 Compare
SCM440 Compare
42CrMo4 Compare
BS 708M40 Compare

Heat Treatment & Processing

Austenitizing830-860°C, soak 30min per 25mm thickness
QuenchingOil quench (preferred); water quench for sections <6mm (risk of cracking)
Tempering205-650°C depending on desired hardness; temper within 30min of quenching
Normalizing870-900°C, air cool
Annealing790-820°C, furnace cool at 10-20°C/h to 480°C, then air cool
Hardening DepthUp to 25mm in oil (Jominy curve D₁ ≈ 25mm)

Welding & Fabrication

Preheat200-300°C for all thicknesses; maintain throughout welding
Filler MetalE12018 (SMAW), ER120S-G (GMAW) — matching strength; use E9018 for undermatching where stress relief is performed
Interpass Temp200-350°C
PwhtStress relieve at 620-650°C, 1h per 25mm thickness; cool slowly to 300°C before air cooling
Weldability RatingFair — requires preheat and PWHT; avoid welding in hardened condition without annealing first

Related Comparisons

AISI 4140 Chromoly Steel (UNS vs AISI 4340 Alloy Steel — High-S

Compare properties, equivalents & applications

Related Materials

AISI 4340 Ultra-High-Strength Ni-Cr-Mo Alloy Steel

ASTM A29/A322 / AMS 6415

Yield: 1,240 MPa (180 ksi) Q&T

AISI 8620 Case-Hardening Ni-Cr-Mo Alloy Steel

ASTM A29/A322

Yield: 385 MPa (56 ksi) core

EN 34CrNiMo6: High-Strength European Quenched & Tempered Steel

EN 10083-3

Yield: 900 MPa (130 ksi) Q&T

EN 16MnCr5: Case-Hardening Manganese-Chromium Steel

EN 10084

Yield: 490 MPa (71 ksi) core

AISI 52100 Bearing Steel: 1.5% Cr High-Carbon Anti-Friction Grade

ASTM A295/A866

Yield: 1,700 MPa (247 ksi) at 60 HRC

🧮 Material Weight Calculator

Calculate the weight based on this material's density: 7.85 g/cm³

Frequently Asked Questions

What is the difference between 4140 and 4340?

4340 contains 1.65-2.0% nickel in addition to Cr and Mo, dramatically improving low-temperature toughness and through-hardening depth (100+ mm sections). 4340 is used for aircraft landing gear, heavy shafting, and critical fasteners. 4140 is more economical (~30% lower cost) and sufficient for most industrial applications.

References & International Standards

  • ASTM International. Standard Specifications for Steel & Metal Alloys. astm.org
  • International Organization for Standardization (ISO). Metallic Materials — Cross-Reference Database. iso.org
  • American Iron and Steel Institute (AISI). Steel Grade Designations & Equivalents. steel.org
  • European Committee for Standardization (CEN). EN Steel Standards & Numbering System. cencenelec.eu