Steel AISI 4340 vs. Titanium

Compare Steel AISI 4340 vs. Titanium strength, stiffness, weight and thermal properties.

Mechanical	Steel AISI 4340	Titanium
Yield strength	1,035MPa117% stronger	885MPa2
Ultimate tensile strength	1,140MPa121% stronger	940MPa2
Elongation at break	18%113% more ductile	16%2
Young's modulus (stiffness)	200GPa3	107–122GPa4
Density	7,840kg/m³1	4,430kg/m³21.77x lighter
Strength to weight	132kN·m/kg	200kN·m/kg1.51x higher
Stiffness to weight	25.5MN·m/kg	25.8MN·m/kg1% higher
Poisson's ratio	0.323	0.3422
Shear modulus	75.7GPa1.77x stiffer in shear	42.7GPa
Bulk modulus	185GPa1.53x higher	121GPa
Speed of sound	5,049m/s	5,084m/s

Stiffness against densityHigher and further left: stiffer for its weight

Strength against densityHigher and further left: stronger for its weight

Thermal	Steel AISI 4340	Titanium
Thermal conductivity	37.5W/m·K15.68x more heat through a fin	6.6W/m·K4
Thermal expansion	mean11.3µm/m·K1	9µm/m·K426% less
100 mm part over a 50 °C swing	56.5µm growth	45µm growth26% less
Specific heat	448J/kg·K1	580J/kg·K429% higher
Heats up and cools (diffusivity)	10.7mm²/s4.15x faster	2.57mm²/s
Thermal shock resistance	11,678W/m3.13x more resistant	3,730W/m
Melting point	not sourced	melting range1,630–1,650°C2
Max service temperature	not sourced	400°C2

Thermal conductivity against densityHigher and further left: spreads more heat for its weight

Thermal expansion against thermal conductivityLower and further right: warps less when heated

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Sources4 documents: Carpenter Technology, TIMET, U.S. Department of Defense

Steel AISI 4340: Carpenter (Latrobe) Lescalloy 4340 VAC-ARC datasheet; mechanical row for 1100 °F temper; physical properties grade-level
Titanium: TIMETAL 6-4 (Ti-6Al-4V Grade 5) annealed plate typical (TIMET properties manual) + TIMET datasheet TMC-0150 physical properties

1
Lescalloy 4340 VAC-ARC Data SheetCarpenter Technologyarchived copySteel AISI 4340 · Yield strength: typical, transverseSteel AISI 4340 · Ultimate tensile strength: typical, transverseSteel AISI 4340 · Elongation at break: typical, transverseSteel AISI 4340 · Density: typicalSteel AISI 4340 · Thermal conductivity: typicalSteel AISI 4340 · Thermal expansion: mean, -18 to 93 °CSteel AISI 4340 · Specific heat: typical
2
TIMETAL 6-4 PropertiesTIMETarchived copyTitanium · Yield strength: typical, plate (>0.187 in), annealed, 0.2% offsetTitanium · Ultimate tensile strength: typical, plate, annealedTitanium · Elongation at break: typical, in 2 in., plate annealedTitanium · Density: nominalTitanium · Poisson's ratio: mean of TIMET measurementsTitanium · Melting point: melting rangeTitanium · Max service temperature: upper recommended use temperature
3
MIL-HDBK-5J, Metallic Materials and Elements for Aerospace Vehicle Structures (2003)U.S. Department of DefenseSteel AISI 4340 · Young's modulus (stiffness): MIL-HDBK-5J design value (AISI 4340, quenched and tempered)Steel AISI 4340 · Poisson's ratio: MIL-HDBK-5J design value (AISI 4340, quenched and tempered)
4
TIMETAL 6-4 Data SheetTIMETarchived copyTitanium · Young's modulus (stiffness): typical range, 20 °CTitanium · Thermal conductivity: typical, 20 °C, mill annealedTitanium · Thermal expansion: typical, mean 0-100 °CTitanium · Specific heat: typical, 20 °C

Each number points to the document that states it, with the exact basis. Values without a number are computed from the sourced ones. "Not sourced" means no citable source states it (or only a specification minimum); figures computed from it are left out too.

Questions

Mechanical

Is Steel AISI 4340 stronger than Titanium?

Steel AISI 4340 is stronger: its yield strength is 1,035 MPa against 885 MPa for Titanium (17%).

Which is lighter, Steel AISI 4340 or Titanium?

Titanium is lighter: 4,430 kg/m³ against 7,840 kg/m³ for Steel AISI 4340.

Which is stiffer, Steel AISI 4340 or Titanium?

Steel AISI 4340 is stiffer: Young's modulus 200 GPa against 115 GPa for Titanium, so the same part in Steel AISI 4340 deflects less under the same load.

Which is lighter for the same job, Steel AISI 4340 or Titanium?

For the same stiffness or strength: Titanium is lighter for a stiff rod or tie (tension), stiff beam (bending), stiff panel or plate, strong rod or tie, strong beam, strong panel or plate.

Part that must be	Steel AISI 4340	Titanium
Stiff rod or tie (tension)	1% heavier	lighter
Stiff beam (bending)	34% heavier	lighter
Stiff panel or plate	47% heavier	lighter
Strong rod or tie	1.51x heavier	lighter
Strong beam	1.59x heavier	lighter
Strong panel or plate	1.64x heavier	lighter

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 is more ductile, Steel AISI 4340 or Titanium?

Steel AISI 4340 stretches further before it breaks: 18% elongation at break against 16% for Titanium.

Thermal

Which conducts heat better, Steel AISI 4340 or Titanium?

Steel AISI 4340 conducts heat better: 37.5 W/m·K against 6.6 W/m·K for Titanium.

Which expands less with temperature?

Titanium expands less: 9 µm/m·K against 11.3 µm/m·K for Steel AISI 4340.