Selection summary
Overview
SAE 4140 is a chromium-molybdenum alloy-steel chemistry with greater hardenability than plain medium-carbon steel. Final properties depend on section size, austenitizing, quench severity, tempering, cleanliness, and product specification.
This page separates standard-oriented limits from typical reference data. Purchase acceptance comes from the ordered specification and certified material test report.
Names and systems
Identity & standards
Similar designations across ASTM, EN, JIS, and other systems are not automatically interchangeable. Product form and ordered standard control chemistry, properties, dimensions, condition, and testing.
Reference chemistry
Composition
| Element | Reference range | Why it matters |
|---|---|---|
| Carbon | 0.38–0.43% | Strength and heat-treatment response |
| Chromium | 0.80–1.10% | Hardenability and wear response |
| Molybdenum | 0.15–0.25% | Hardenability and tempering resistance |
Concise grade-family reference only. Confirm every element limit against the current standard for the exact product ordered.
Condition matters
Reference properties
Annealed, normalized, and quenched-tempered conditions differ substantially
Specify section, heat treatment, hardness or strength, and test location
Typical Cr-Mo steel reference value
Environment first
Corrosion behavior
Not a corrosion-resistant alloy; protect surfaces for wet or corrosive service.
Hydrogen embrittlement and plating bake requirements can matter at high strength.
Temperature, concentration, deposits, crevices, stress, surface finish, cleaning, and fabrication quality can change service performance.
Service limits
Temperature & service
Tempering temperature and subsequent thermal exposure determine retained strength and toughness.
Delivery condition
Heat treatment & condition
4140 can be ordered annealed, normalized, quenched and tempered, cold finished, or in a specified prehardened condition; mechanical properties must name section size and heat-treatment condition.
Austenitizing, quench severity, tempering temperature, section size, and cleanliness determine hardness, toughness, residual stress, and distortion; post-machining treatment needs its own controls.
Process route
Fabrication
Machining and forming are normally planned around annealed or prehardened condition.
Welding can require preheat, low-hydrogen practice, controlled cooling, and postweld heat treatment.
Magnetic response: Ferromagnetic.
Decision boundaries
Comparisons & substitution
Compared with 1045, chromium-molybdenum 4140 generally provides greater hardenability and strength consistency through thicker sections, at higher material and processing complexity.
4130, 4340, and European 42CrMo4 are commonly compared for certain applications but are not automatically interchangeable across chemistry, hardenability, cleanliness, dimensions, and product standards.
Where it is used
Applications
Ordering scope
Product forms & standards
Hot-rolled or cold-finished alloy-steel bar under ASTM A322/A29 or another explicitly ordered product specification and condition.
Forgings, tubing, plate, gears, shafts, and pressure components require product- and service-specific standards; the 4140 chemistry name alone is incomplete.
Order heat treatment, hardness/strength range, section size, hardenability, grain size, cleanliness, ultrasonic testing, decarburization, straightness, and traceability as required.
Decision support
Selection guidance
Choose this grade when
- Through-section hardenability and quenched-tempered strength exceed plain-carbon capability.
- The supplier can control heat treatment, testing location, cleanliness, and ultrasonic quality.
Review another grade when
- Weldability, corrosion resistance, or very low-temperature toughness is the primary requirement.
- 4340-class toughness or a lower-carbon case-hardening grade is more suitable.
Before ordering
Procurement checklist
State the exact material and product specification, edition, grade, dimensions, tolerances, and delivery condition.
Require heat identity and a material test report showing chemistry, mechanical tests, and any ordered supplementary tests.
Confirm fabrication route, welding or heat treatment, inspection, marking, corrosion protection, export packing, and destination requirements.
State section size, heat-treatment route, hardness/strength range, impact or fatigue needs, grain size, cleanliness, and ultrasonic class.
Traceable evidence