Intent: Comparison of through-hardenability across heavy cross-sections and Charpy V-notch fracture toughness.
Engineering Verdict & When to Choose Each
4140 or 4340: section and final condition decide the comparison
By Mat Grades Editorial Desk · Updated 2026-11-14
Use 4140 when a supplier-supported final condition meets the shaft or component requirement. Review 4340 when its alloy system and available processing evidence address a demanding section or toughness requirement. The choice should be justified by the actual duty rather than the assumption that the larger grade number is always better.
For a thick shaft, ask how the required center properties will be demonstrated. For a notched or cyclically loaded part, review geometry, surface condition and the necessary fatigue or toughness evidence. A surface hardness reading does not answer all of those questions.
Agree the machining and heat-treatment sequence before ordering. Include allowance for movement, cleanup and any finish grinding. Welding or repair requires a separate procedure assessment for the actual composition, condition, restraint and section. Do not use a generic preheat temperature as a substitute for that qualification.
SAE 4140 Alloy Steel: evidence to check
SAE 4140 Alloy Steel has 21 cataloged property rows. Elastic Modulus is reported as 205 GPa (typical; further condition not stated). Tensile Strength is reported as 930–1080 MPa (range; Quenched and tempered (voestalpine supply), Bar). A product specification must be agreed with the supplier where the record does not identify one. These are source-scoped examples rather than unrestricted design allowables. Read the full row context and certificate before using a value for acceptance; an absent value is not zero or a performance failure.
SAE 4340 Alloy Steel: evidence to check
SAE 4340 Alloy Steel has 14 cataloged property rows. Elastic Modulus is reported as 205 GPa (typical; further condition not stated). Ultimate Tensile Strength is reported as minimum 745 MPa (minimum; Annealed Bar, Bar). A product specification must be agreed with the supplier where the record does not identify one. These are source-scoped examples rather than unrestricted design allowables. Read the full row context and certificate before using a value for acceptance; an absent value is not zero or a performance failure.
Four questions before approval
- Does the supplier guarantee the required ruling section?
- Are final strength and toughness acceptance criteria stated?
- Are notches and surface finish included in the review?
- Is any repair process separately approved and inspected?
Check the linked material records for the complete source trail. Read the methodology before comparing minimum requirements with typical measurements. No direct substitution or qualified fabrication procedure is implied.
Material Comparison
Compare verified engineering materials side by side with condition-specific mechanical properties.
| Parameter | SAE 4140 Alloy Steel | SAE 4340 Alloy Steel |
|---|---|---|
| Material Family | Steel | Steel |
| Subfamily | Alloy Steel | Alloy Steel |
| SAE Designation | 4140 | 4340 |
| AISI Designation | 4140 | — |
| SPECIFICATION Designation | SAE 4140 Alloy Steel Engineering Datasheet | SAE 4340 Alloy Steel Engineering Datasheet |
| Element | SAE 4140 Alloy Steel | SAE 4340 Alloy Steel |
|---|---|---|
| C (Carbon) | 0.38 – 0.43% | 0.38 – 0.43% |
| Mn (Manganese) | 0.75 – 1% | 0.6 – 0.8% |
| Cr (Chromium) | 0.8 – 1.1% | 0.7 – 0.9% |
| Mo (Molybdenum) | 0.15 – 0.25% | 0.2 – 0.3% |
| Si (Silicon) | 0.15 – 0.35% | 0.15 – 0.35% |
| P (Phosphorus) | ≤ 0.035% | ≤ 0.035% |
| S (Sulfur) | ≤ 0.04% | ≤ 0.04% |
| Ni (Nickel) | ≤ 0.25% | 1.65 – 2% |
| Property | SAE 4140 Alloy Steel | SAE 4340 Alloy Steel |
|---|---|---|
| Density · TYPICALUnit: g/cm³ | 7.85 g/cm³ | 7.85 g/cm³ |
| Elastic Modulus · TYPICALUnit: GPa | 205 GPa | 205 GPa |
| Poisson's Ratio · TYPICALUnit: dimensionless | 0.29 dimensionless | 0.29 dimensionless |
| Mechanical Parameter | SAE 4140 Alloy SteelQuenched and tempered (voestalpine supply) | SAE 4340 Alloy SteelAnnealed Bar |
|---|---|---|
| Tensile Strength · Bar · 10–100 mm · Quenched and tempered (voestalpine supply) · RANGEUnit: MPa | 930–1080 MPa | — MPa |
| Tensile Strength · Bar · 101–250 mm · Quenched and tempered (voestalpine supply) · RANGEUnit: MPa | 850–1000 MPa | — MPa |
| Ultimate Tensile Strength · Bar · Annealed Bar · MINIMUMUnit: MPa | — MPa | ≥ 745 MPa |
| Yield Strength · Bar · 10–100 mm · Quenched and tempered (voestalpine supply) · MINIMUMUnit: MPa | ≥ 740 MPa | — MPa |
| Yield Strength · Bar · 101–250 mm · Quenched and tempered (voestalpine supply) · MINIMUMUnit: MPa | ≥ 665 MPa | — MPa |
| Yield Strength · Bar · Annealed Bar · MINIMUMUnit: MPa | — MPa | ≥ 470 MPa |
| Elongation · Bar · 10–100 mm · Quenched and tempered (voestalpine supply) · MINIMUMUnit: % | ≥ 12 % | — % |
| Elongation · Bar · 101–250 mm · Quenched and tempered (voestalpine supply) · MINIMUMUnit: % | ≥ 13 % | — % |
| Elongation at Break · Bar · in 50 mm (2 in) · Annealed Bar · MINIMUMUnit: % | — % | ≥ 22 % |
| Property | SAE 4140 Alloy Steel | SAE 4340 Alloy Steel |
|---|---|---|
| Coefficient of Thermal Expansion · TYPICAL · 100 °CCategory: THERMAL • Unit: µm/(m·K) | 12.2 µm/(m·K) | 12.3 µm/(m·K) |
| Thermal Conductivity · TYPICAL · 100 °CCategory: THERMAL • Unit: W/(m·K) | 42.6 W/(m·K) | 44.5 W/(m·K) |
| Hardness Scale | SAE 4140 Alloy Steel | SAE 4340 Alloy Steel |
|---|---|---|
| HB Hardness · Bar · 10–100 mm · Quenched and tempered (voestalpine supply) · RANGE | 269–331 HB | — |
| HB Hardness · Bar · 101–250 mm · Quenched and tempered (voestalpine supply) · RANGE | 248–302 HB | — |
| HB Hardness · Bar · Annealed Bar · MAXIMUM | — | ≤ 217 HB |