Thermal expansion.

Growth per degree.

Almost every material gets bigger as it warms up. The coefficient of thermal expansion, α (or CTE), says by how much: the fractional change in length for each degree of temperature change.

The numbers look small, but lengths and temperatures multiply them. 5052 aluminum at 23.8 µm/m·K grows about 0.063″ over 48″ for a 100 °F rise. Carbon steel grows about half that. Area grows by roughly 2α and volume by roughly 3α.

Expansion comes from the shape of the bond between atoms: it is easier to pull neighbours apart than to push them together, so as atoms vibrate harder with temperature, their average spacing increases. Loosely bonded materials expand more — which is why plastics (50–200 µm/m·K) expand far more than metals, and why materials with strong bonds and high melting points tend to expand least.

α is a mean over a temperature range and rises gradually with temperature. Datasheets usually quote it for 20–100 °C; for large temperature swings, use the value for the actual range.

How it is measured.

A push-rod dilatometer (ASTM E228) heats a sample slowly in a furnace and measures its length change through a rod connected to a displacement sensor. Thermomechanical analysis (ASTM E831) does the same on small samples and is standard for plastics and composites. For composites and rolled products, CTE can differ by direction, so results are reported per axis.

How materials compare.

Titanium (8.6) expands least of the common metals, carbon steel sits at 11–12, ferritic stainless 430 at about 10, austenitic 304 and 316 at 16–17, copper alloys at 17–21, and aluminum at 23–24. Magnesium is higher still. Plastics start around 45 (PEEK) and run past 100.

Notice that 304 stainless expands about 50% more than carbon steel even though both are "steel". Austenitic stainless has a different crystal structure, and it is one of the reasons stainless distorts more when welded.

No tabulated value for AR500, PA-12 (HP MJF), PA-11 (HP MJF), PA-12 glass bead (HP MJF).

How engineers use it.

Mismatch between materials

The common problem is not growth itself but different growth. An aluminum panel bolted to a steel frame, a stainless cover on an aluminum chassis, a plastic part screwed to a metal plate — each pair moves apart as the temperature changes.

For 48″ of 5052 on A36 over a 100 °F swing, that is about 0.032″ (0.8 mm). If the fasteners sit in tight round holes at both ends, the difference turns into bowing, fastener shear or loosening. The fixes are standard: locate the part with one tight hole and use slots or oversize holes elsewhere, keep clamped lengths short, or use a compliant joint.

Thermal stress

If a part is prevented from growing, it is effectively compressed by the amount it wanted to grow. The stress that results is:

For carbon steel fully restrained over a 50 °C rise: 200,000 MPa × 11.7 × 10⁻⁶ × 50 ≈ 117 MPa — about half of A36's yield strength, from temperature alone. Aluminum's higher α is partly offset by its lower modulus: about 81 MPa under the same conditions. Thermal cycling repeats that stress, which is how solder joints, welds and bonded seams fail by fatigue.

Measurement and tolerances

Dimensions are defined at a reference temperature of 20 °C (68 °F) per ISO 1. A 24″ aluminum part measured 20 °F warm reads about 0.006″ long — enough to fail a tight tolerance. Parts fresh off a laser or out of a powder-coat oven need time to come back to room temperature before inspection.

Using expansion on purpose

Shrink fits heat a hub (or chill a shaft) so it slides on and grips when temperatures equalise. Bimetal strips bend with temperature because two bonded metals expand differently — the basis of many thermostats.

What it means in fabrication.

• Weld distortion. The weld and the metal next to it expand while hot, yield in compression and then shrink as they cool, pulling the part out of shape. Higher α and lower conductivity both make it worse, which is why 304 stainless distorts more than mild steel. Balanced weld sequences, tacks and fixtures control it.
• Laser cutting. Local heating and cooling at the kerf can release and rearrange residual stress, especially in dense cut patterns on thin sheet. See laser-cut warping.
• Powder coating. Parts and racks grow in the cure oven. Masking plugs, tight-fitting hardware and assemblies with mixed materials should be checked for clearance at temperature.
• Printed nylon. PA-12 expands many times more than metals; a printed part fastened to a metal plate across a long span needs room to move.

Common mistakes.

• Treating all steels as one CTE. Austenitic stainless is about 50% higher than carbon steel.
• Locating with two tight holes across a long, mixed-material joint. Use one datum hole and slots.
• Inspecting warm parts. Let parts stabilise, or correct the measurement to 20 °C.
• Using room-temperature CTE for a large swing. α rises with temperature; use the mean value for the actual range.