Materials / Compare
Plastic PEEK vs. Steel AISI 4340
Compare Plastic PEEK vs. Steel AISI 4340 strength, stiffness, weight and thermal properties.
| Try it on a partA bracket to run stress or thermal on, free in your browser | Open in LessCAD | Open in LessCAD |
|---|---|---|
| Mechanical | ||
| Yield strength | 98MPasourcetypical, tensile yield | 1,035MPasourcetypical, transverse |
| Ultimate tensile strength | not sourced | 1,140MPasourcetypical, transverse |
| Compressive strength | 125MPasourcetypical, 23 C | not sourced |
| Flexural strength | 165MPasourcetypical, flexural stress at yield, 23 C | not sourced |
| Elongation at break | 25%sourcetypical, at break | 18%sourcetypical, transverse |
| Young's modulus (stiffness) | 4GPasourcetypical (nominal) | 200GPasourceMIL-HDBK-5J design value (AISI 4340, quenched and tempered) |
| Density | 1,300kg/m³sourcetypical (crystalline) | 7,840kg/m³sourcetypical |
| Strength to weight | 75.4kN·m/kg | 132kN·m/kg1.75x higher |
| Stiffness to weight | 3.08MN·m/kg | 25.5MN·m/kg8.29x higher |
| Poisson's ratio | 0.38sourcetypical | 0.32sourceMIL-HDBK-5J design value (AISI 4340, quenched and tempered) |
| Shear modulus | 1.45GPa | 75.7GPa52.2x stiffer in shear |
| Bulk modulus | 5.56GPa | 185GPa |
| Speed of sound | 1,754m/s | 5,049m/s |
| Thermal | ||
| Thermal conductivity | 0.29W/m·Ksourcetypical, average, 23 C | 37.5W/m·Ksourcetypical |
| Thermal expansion | 55µm/m·Ksourceaverage of flow/cross-flow, below Tg (<143 C) | 11.3µm/m·Ksourcemean, -18 to 93 °C |
| 100 mm part over a 50 °C swing | 275µm growth | 56.5µm growth4.87x less |
| Specific heat | 2,200J/kg·Ksourcetypical, 23 °C, DSC (Victrex PEEK 450G TDS, revision Jul 2012) | 448J/kg·Ksourcetypical |
| Heats up and cools (diffusivity) | 0.101mm²/s | 10.7mm²/s105x faster |
| Thermal shock resistance | 80.1W/m | 11,678W/m146x more resistant |
| Melting point | 343°Csourcetypical, DSC | not sourced |
| Glass transition | 143°Csourceonset (midpoint 150 C) | not sourced |
| Max service temperature | 240°CsourceUL 746B RTI, mechanical strength (continuous use index) | not sourced |
| Values for | Victrex PEEK 450G (unreinforced, injection moulded), Victrex TDS rev. March 2026; Poisson from Victrex fluid-management flyer (2020-07) | Carpenter (Latrobe) Lescalloy 4340 VAC-ARC datasheet; mechanical row for 1100 °F temper; physical properties grade-level |
Every value links to the document that states it. "Not sourced" means no citable source states it (or only a specification minimum); figures computed from it are left out too.
Questions
Is Plastic PEEK stronger than Steel AISI 4340?
Steel AISI 4340 is stronger: its yield strength is 1,035 MPa against 98 MPa for Plastic PEEK (10.6x).
Which is lighter, Plastic PEEK or Steel AISI 4340?
Plastic PEEK is lighter: 1,300 kg/m³ against 7,840 kg/m³ for Steel AISI 4340.
Which is stiffer, Plastic PEEK or Steel AISI 4340?
Steel AISI 4340 is stiffer: Young's modulus 200 GPa against 4 GPa for Plastic PEEK, so the same part in Steel AISI 4340 deflects less under the same load.
Which is lighter for the same job, Plastic PEEK or Steel AISI 4340?
For the same stiffness or strength: Plastic PEEK is lighter for a stiff panel or plate, strong beam, strong panel or plate; Steel AISI 4340 is lighter for a stiff rod or tie (tension), stiff beam (bending), strong rod or tie.
| Part that must be | Plastic PEEK | Steel AISI 4340 |
|---|---|---|
| Stiff rod or tie (tension) | 8.29x heavier | lighter |
| Stiff beam (bending) | 17% heavier | lighter |
| Stiff panel or plate | lighter | 1.64x heavier |
| Strong rod or tie | 1.75x heavier | lighter |
| Strong beam | lighter | 25% heavier |
| Strong panel or plate | lighter | 1.86x heavier |
For the same stiffness or strength, mass scales with density over stiffness (or strength) raised to a power set by how the part is loaded: 1 for a rod in tension, 1/2 for a beam in bending, 1/3 for a panel (the standard material-selection indices).
Which conducts heat better, Plastic PEEK or Steel AISI 4340?
Steel AISI 4340 conducts heat better: 37.5 W/m·K against 0.29 W/m·K for Plastic PEEK.
Which expands less with temperature?
Steel AISI 4340 expands less: 11.3 µm/m·K against 55 µm/m·K for Plastic PEEK.