Plastic Nylon PA66 vs. Steel AISI 4340

Compare Plastic Nylon PA66 vs. Steel AISI 4340 strength, stiffness, weight and thermal properties.

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Plastic Nylon PA66Bulk polymer (molded or machined)

Mechanical
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Yield strength	85MPasourcetypical, dry	1,035MPasourcetypical, transverse
Ultimate tensile strength	not sourced	1,140MPasourcetypical, transverse
Elongation at break	30%sourcenominal strain at break, dry	18%sourcetypical, transverse
Young's modulus (stiffness)	3GPasourcetypical, dry	200GPasourceMIL-HDBK-5J design value (AISI 4340, quenched and tempered)
Density	1,130kg/m³sourcetypical6.94x lighter	7,840kg/m³sourcetypical
Strength to weight	75.2kN·m/kg	132kN·m/kg1.76x higher
Stiffness to weight	2.65MN·m/kg	25.5MN·m/kg9.6x higher
Poisson's ratio	0.4sourcebase material: unfilled polyamide (PA), conditioned, maker design-guide family value (Covestro Table 3-1)	0.32sourceMIL-HDBK-5J design value (AISI 4340, quenched and tempered)
Shear modulus	1.07GPa	75.7GPa70.7x stiffer in shear
Bulk modulus	5GPa	185GPa
Speed of sound	1,629m/s	5,049m/s
Thermal
Thermal conductivity	0.33W/m·Ksourcetypical	37.5W/m·Ksourcetypical114x more heat through a fin
Thermal expansion	98µm/m·Ksourcelongitudinal, 23-55 °C	11.3µm/m·Ksourcemean, -18 to 93 °C
100 mm part over a 50 °C swing	490µm growth	56.5µm growth8.67x less
Specific heat	1,700J/kg·Ksourcetypical	448J/kg·Ksourcetypical
Heats up and cools (diffusivity)	0.172mm²/s	10.7mm²/s62.1x faster
Thermal shock resistance	57.2W/m	11,678W/m204x more resistant
Melting point	260°CsourceDSC	not sourced
Max service temperature	101°CsourceIEC 60216 temperature index, 20000 h	not sourced
Values for	BASF Ultramid A3K (PA66, unreinforced injection-moulding grade), Product Information 02/2026, dry values (ISO methods)	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 Nylon PA66 stronger than Steel AISI 4340?

Steel AISI 4340 is stronger: its yield strength is 1,035 MPa against 85 MPa for Plastic Nylon PA66 (12.2x).

Which is lighter, Plastic Nylon PA66 or Steel AISI 4340?

Plastic Nylon PA66 is lighter: 1,130 kg/m³ against 7,840 kg/m³ for Steel AISI 4340.

Which is stiffer, Plastic Nylon PA66 or Steel AISI 4340?

Steel AISI 4340 is stiffer: Young's modulus 200 GPa against 3 GPa for Plastic Nylon PA66, so the same part in Steel AISI 4340 deflects less under the same load.

Which is lighter for the same job, Plastic Nylon PA66 or Steel AISI 4340?

For the same stiffness or strength: Plastic Nylon PA66 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 Nylon PA66	Steel AISI 4340
Stiff rod or tie (tension)	9.6x heavier	lighter
Stiff beam (bending)	18% heavier	lighter
Stiff panel or plate	lighter	1.71x heavier
Strong rod or tie	1.76x heavier	lighter
Strong beam	lighter	31% heavier
Strong panel or plate	lighter	1.99x 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 Nylon PA66 or Steel AISI 4340?

Steel AISI 4340 conducts heat better: 37.5 W/m·K against 0.33 W/m·K for Plastic Nylon PA66.

Which expands less with temperature?

Steel AISI 4340 expands less: 11.3 µm/m·K against 98 µm/m·K for Plastic Nylon PA66.

Among 59 materials

Stiffness against density

Strength against density