AISI 4140 Chromoly Steel (UNS G41400): Heat Treatment Guide vs AISI 4340 Alloy Steel — High-Strength Nickel-Chromium-Molybdenum Properties
Side-by-side engineering comparison of AISI 4140 Chromoly Steel (UNS G41400): Heat Treatment Guide (ASTM A29/A322) and AISI 4340 Alloy Steel — High-Strength Nickel-Chromium-Molybdenum Properties (ASTM A322). Compare mechanical properties, chemical composition, density, yield strength, international equivalents, and typical applications to select the right material for your project.
Quick Comparison
| Property | AISI 4140 Chromoly Steel | AISI 4340 Alloy Steel — H |
|---|---|---|
| Standard | ASTM A29/A322 | ASTM A322 |
| Category | Alloy Steel | |
| Density | 7.85 g/cm³ | 7850 |
| Yield Strength | 655 MPa (95 ksi) Q&T | 860 MPa |
| Tensile Strength | 930 MPa (135 ksi) Q&T | 1030 MPa |
| Key Applications | 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... | <p>AISI 4340 is an ultra-high-strength nickel-chromium-molybdenum alloy steel used in aircraft landing gear, crankshafts... |
International Equivalents
| AISI 4140 Chromoly S Equivalents | AISI 4340 Alloy Stee Equivalents |
|---|---|
| EN 42CrMo4 | 40NiCrMo7 |
| DIN 1.7225 | SNCM439 |
| SCM440 | 817M40 |
| 42CrMo4 | — |
| BS 708M40 | — |
How to Choose
| Choose AISI 4140 Chromoly Steel when... | 4140 is the workhorse Cr-Mo alloy steel—sufficient for most industrial shafts and bolts. |
| Choose AISI 4340 Alloy Steel — H when... | 4340 adds 1.65-2.0% nickel, tripling through-hardening depth and dramatically improving toughness below -40°C. |
Selection Guide
Choose 4140 for general industrial applications requiring high strength and good toughness through moderate section sizes (up to 60mm). Choose 4340 when deep through-hardening (>100mm), extreme toughness, or sub-zero service temperature is required. The 1.65-2.0% nickel in 4340 triples hardenability depth and dramatically improves low-temperature toughness — at roughly double the material cost.
Key Decision Factors
- Section thickness and hardenability — 4340 through-hardens sections 3× thicker than 4140 due to nickel content
- Low-temperature toughness — 4340 remains ductile at -40°C; 4140 becomes brittle below approximately -20°C depending on temper
- Cost differential — 4340 costs approximately 2× 4140 due to 1.65-2.0% nickel content
- Hydrogen embrittlement risk — both grades are susceptible after electroplating; 4340's higher strength makes it more prone to HE failure
When to Use Each
Use AISI 4140 Chromoly S for:
4140 is the workhorse Cr-Mo alloy steel for shafts, bolts, studs, and gears in general machinery — providing 850-1000 MPa tensile strength after quench and temper.
4140's combination of strength, toughness, and chrome-plating suitability makes it standard for hydraulic piston rods and cylinder barrels.
4140 costs roughly half of 4340 while providing adequate properties for the majority of industrial applications with section sizes under 60mm.
Use AISI 4340 Alloy Stee for:
4340's exceptional toughness and deep hardenability make it the standard material for landing gear components that must withstand extreme impact loads and fatigue cycling.
4340's nickel content provides 3× the hardenability of 4140, ensuring through-hardening of heavy cross-sections that would develop a soft core in 4140.
4340 maintains useful toughness at -40°C and below, while 4140 undergoes a ductile-to-brittle transition that limits its use in sub-zero environments.
Frequently Asked Questions
What is the main difference between AISI 4140 Chromoly Steel (UNS and AISI 4340 Alloy Steel — High-S?
AISI 4140 Chromoly Steel (UNS G41400): Heat Treatment Guide (ASTM A29/A322) provides 655 MPa (95 ksi) Q&T yield strength at 7.85 g/cm³ density, while AISI 4340 Alloy Steel — High-Strength Nickel-Chromium-Molybdenum Properties (ASTM A322) delivers 860 MPa at 7850. The choice depends on whether your application prioritizes section thickness under 60mm and lower cost or deep hardenability (100+mm) and low-temperature toughness.
Can AISI 4140 Chromoly Steel (UNS be substituted for AISI 4340 Alloy Steel — High-S?
Direct substitution is generally not recommended as these materials belong to different categories (Alloy Steel vs ) with fundamentally different properties. Consult a materials engineer for application-specific guidance.
Can I substitute 4140 for 4340 to save cost?
Only if your application does not require 4340's specific advantages. If section thickness is under 60mm, service temperature stays above -20°C, and impact loading is moderate, 4140 is adequate and costs half as much. Never substitute for aircraft, pressure vessel, or dynamically loaded critical components without re-engineering the design.
Why does 4340 through-harden better than 4140?
Nickel is the key difference. 4340 contains 1.65-2.0% nickel, which dramatically slows the transformation of austenite to ferrite and pearlite during quenching. This means the cooling rate at the center of thick sections is fast enough to form martensite throughout. 4140 lacks sufficient nickel, so the core of thick sections transforms to softer bainite or pearlite.
What tempering temperature should I use for 4340?
For tensile strength of 1080-1280 MPa (typical landing gear range), temper at 450-510°C after quenching from 830-860°C in oil. For lower strength and higher toughness (850-1000 MPa), temper at 580-650°C. Always temper immediately after quenching — never allow 4340 to cool to room temperature and sit untempered, as it is susceptible to quench cracking.
Is 4340 susceptible to hydrogen embrittlement?
Yes, especially at higher strength levels (>1200 MPa). Electroplating (zinc, cadmium) introduces hydrogen that can cause delayed brittle fracture. Mitigate by: (1) baking at 190-220°C for 8-24 hours within 4 hours of plating, (2) specifying lower strength tempers when possible, or (3) using mechanical plating instead of electroplating.
Can 4140 and 4340 be welded?
Both require preheating before welding due to their hardenability. For 4140: preheat 150-260°C, use E11018-M filler, post-weld temper at 590-650°C. For 4340: preheat 250-350°C, use E12018-M filler, post-weld temper at 590-650°C. Welding in the quenched-and-tempered condition will soften the HAZ — design welds in low-stress areas or weld before final heat treatment.