304 Stainless Steel Thermal Conductivity
Use about 15 W/(m·K) at 20°C as a typical reference for 304 stainless steel. Conductivity increases with temperature: Cleveland-Cliffs lists 16.2 at 100°C and 21.4 at 500°C. For a calculation, keep the temperature, units and source with the value.
Why do sources list both 15 and 16.2 W/(m·K)?
They may describe different temperatures or datasets. Outokumpu’s Core datasheet gives 15 W/(m·K) at 20°C for Core 304/4301. Cleveland-Cliffs’ 304/304L datasheet gives 16.2 at 100°C. Copying the second number into a room-temperature calculation loses its stated condition.
These values also are not strict upper and lower limits. Grade chemistry, material condition and measurement method can differ between sources. A historical NBS measurement report on one Type 304 sample gives 14.8 at 20°C and 16.7 at 100°C. This is evidence of a particular sample, not a replacement specification for every 304 product.
For an initial estimate near 20°C, use 15. For a wider temperature range, use a traceable temperature-dependent dataset matched to the material and analysis. Do not treat a typical physical-property table as a measured guarantee for the sheet or tube being purchased.
304 thermal conductivity by temperature
The table separates producer values from calculated values. NIST entries use the published UNS S30400 polynomial with temperature in kelvin; Celsius values are converted using T(K) = T(°C) + 273.15.
| Temperature | k, W/(m·K) | k, Btu/(h·ft·°F) | Data basis |
|---|---|---|---|
| -196.15°C / 77 K | 7.92 | 4.58 | NIST curve fit |
| -173.15°C / 100 K | 9.22 | 5.33 | NIST curve fit |
| -100.00°C / 173.15 K | 11.87 | 6.86 | NIST curve fit |
| 0.00°C / 273.15 K | 14.60 | 8.43 | NIST curve fit |
| 20.00°C / 293.15 K | 15.13 | 8.74 | NIST curve fit |
| 20°C / 293.15 K | 15 | 8.67 | Outokumpu, Table 7 |
| 100°C / 373.15 K | 16.2 | 9.36 | Cleveland-Cliffs, p. 4 |
| 200°C / 473.15 K | ≈17.5 | 10.11 | Linear interpolation; not a published test point |
| 300°C / 573.15 K | ≈18.8 | 10.86 | Linear interpolation; not a published test point |
| 400°C / 673.15 K | ≈20.1 | 11.61 | Linear interpolation; not a published test point |
| 500°C / 773.15 K | 21.4 | 12.36 | Cleveland-Cliffs, p. 4 |
Teal: NIST fit. Dark square: Outokumpu at 20°C. Brown dots: Cleveland-Cliffs values. The dashed brown segment is linear interpolation between 100 and 500°C; the separate sources are not joined into a single material curve.
The NIST conductivity data cover 4–300 K; its stated equation range is 1–300 K. The reported 2% fit error is relative to its underlying data, not an uncertainty guarantee for any supplied component. Do not extrapolate this cryogenic fit above 300 K (26.85°C).
For preliminary work between the two Cleveland-Cliffs anchors, linear interpolation gives k ≈ 16.2 + 0.013(T − 100), with T in °C and k in W/(m·K). Use this only over 100–500°C; a straight line between two typical values is not a validated curve for a critical simulation.
Convert the units without losing the length term
Thermal conductivity is a material property. W/(m·K) is equivalent to W/(m·°C) when the temperature term is a difference. It is different from W/(m²·K), which is used for a surface heat-transfer coefficient or overall U-value.
| Convert from W/(m·K) to | Operation | Example: k = 15 |
|---|---|---|
| Btu/(h·ft·°F) | Divide by 1.730735 | 8.67 |
| Btu·in/(h·ft²·°F) | Divide by 0.1442279 | 104.0 |
| W/(mm·K) | Divide by 1,000 | 0.015 |
Btu conversions use NIST SP 811, Appendix B.8. The inch-based number is 12 times the foot-based number. Keep units attached when moving data into an FEA material card.
How does 304 compare with other heat conductors?
304 conducts heat much less readily than copper or aluminum. Some ferritic stainless grades also have higher conductivity. That comparison helps explain heat spreading; it does not decide corrosion suitability or permit a material substitution.
| Material and source condition | k, W/(m·K) | What the comparison means |
|---|---|---|
| 304 stainless; NIST at 295 K | 15 | Baseline for this near-room-temperature comparison. |
| Aluminum; NIST at 295 K | 235 | A generic metal reference, not a value for every aluminum alloy or temper. |
| Copper; NIST at 295 K | 400 | Strong heat spreading; confirm the actual copper grade for design. |
| Core 441/4509 ferritic stainless; Outokumpu at 20°C | 25 | Higher k than 304, but different grade chemistry, forming and corrosion behavior. |
Sources: NIST reference table at 295 K and Outokumpu Table 7. The 295 K references correspond to about 22°C. Values at nearby temperatures are suitable for this broad comparison, not precision substitution calculations.
Why 304 spreads heat relatively slowly
Heat in a metal travels through moving electrons and vibrations of the atomic lattice. In an alloy such as 304, the different atoms and defects scatter these heat carriers. This helps explain why its iron–chromium–nickel matrix conducts much less effectively than pure copper.
Do not attribute the difference only to crystal structure or only to nickel content. Composition, structure, temperature and condition act together. The micrograph shows a material structure; it cannot supply a conductivity value.
304 versus 304L
Outokumpu lists the same room-temperature k for Core 304 and its 304L grades. Lower carbon in 304L reduces susceptibility to carbide precipitation during welding. It is not a large conductivity upgrade. For that separate grade decision, see what changes when welding 304 versus 304L.
Calculate wall conduction using the two metal-surface temperatures
For steady, one-dimensional heat flow through a flat wall with constant k, uniform area and no internal heat generation:
Q is heat flow in W; A is area in m²; L is thickness in m. Th,s and Tc,s are the hot and cold metal-surface temperatures. A temperature difference of 1°C equals 1 K.
Illustrative calculation: 1.5 mm sheet near room temperature
Assume the two metal faces are maintained at 25°C and 15°C, the area is 0.01 m², and k is approximated as 15 W/(m·K) over this small temperature interval. This is a hypothetical boundary condition, not a measured production result.
- Convert thickness: 1.5 mm = 0.0015 m.
- Calculate Q = 15 × 0.01 × 10 ÷ 0.0015 = 1,000 W.
- Heat flux q″ = Q/A = 100,000 W/m².
- Wall resistance R = L/(kA) = 0.010 K/W.
The large heat flux means those face temperatures require substantial heat delivery and removal. It does not mean that air at 25°C and 15°C would automatically produce this result.
For a wide temperature interval, use k(T)
When conductivity varies appreciably, the steady one-dimensional result is Q = (A/L) ∫ k(T) dT, integrated from the cold face to the hot face. Evaluating k at the mean temperature is an approximation; it reproduces the integral for a linear k(T) within one valid interval. It should not silently bridge unrelated datasets or exceed their temperature limits.
Why a low-conductivity metal can still work in a heat exchanger
A thin stainless wall can have low thermal resistance even though its conductivity is modest. The wall is only one part of the path: the fluids must transfer heat to and from its surfaces, and deposits or contacts can add resistance.
For a clean flat wall with equal reference areas, omitting fouling and contact terms. U and the two fluid-film coefficients h are in W/(m²·K).
The driving temperatures for U are the bulk fluid temperatures, unlike the metal-face temperatures in the preceding wall-only equation. Radiation, fouling, geometry and changing fluid temperatures need additional treatment where relevant.
Illustrative calculation: thinning the wall barely changes U
Assume k = 15 W/(m·K), L = 1.5 mm, and both film coefficients equal 500 W/(m²·K). These are chosen teaching inputs, not recommended design coefficients. The resistance per unit area is 0.002 + 0.0001 + 0.002 = 0.0041 m²·K/W, so U ≈ 244 W/(m²·K).
Halving the wall to 0.75 mm gives U ≈ 247 W/(m²·K), an increase of only about 1.2%. In this example, the fluid films dominate. Actual film coefficients depend on the fluid, flow and geometry; pressure, strength and corrosion allowance can also prevent thinning.
For a cryogenic support, the objective may instead be to limit heat leak. Conductivity, cross-sectional area, path length and the entire temperature interval matter. Lower k helps relative to a better conductor, but 304 itself is not thermal insulation.
What these properties mean when welding 304
Localized heating creates temperature gradients and expansion. Thin 304 sheet can distort when heating and restraint are uneven. Conductivity helps explain this behavior, but a room-temperature number cannot predict penetration, cooling rate or weld quality.
For process trials, keep material grade and condition, thickness, joint gap, clamping, travel path and starting part temperature consistent. Check dimensional change after release from the fixture, alongside the required weld inspection. Closely spaced seams and repeated passes can change the starting temperature of later welds.
Laser power and speed also interact with spot size, absorption, shielding and joint geometry. Use the welding heat-input calculator for its defined energy-per-length calculation, and the 304 thin-sheet welding guide for the wider process discussion. Neither replaces a qualified procedure where one is required.
Choose data that match the part and the calculation
- Identify the material. Record 304/UNS S30400 or 304L/UNS S30403, product form and supplied condition. If the part is heavily cold worked, ask whether the chosen dataset represents that condition.
- Define the temperature span. Use the actual range inside the metal. For a transient simulation, also supply consistent density, heat capacity and any required phase-change data.
- Record the data method. Keep source, table or equation, units, interpolation rule and range. Run a sensitivity check if uncertainty in k could change the design decision.
- Build the complete heat path. Include the relevant fluid films, contacts, insulation and radiation. A polished finish may change surface radiation or contact behavior; do not model that by simply increasing bulk k.
A material certificate is not automatically a conductivity test report. If thermal performance is an acceptance requirement, specify the needed property evidence. For the separate purchasing documents, see the 304 plate specification guide.
Sources and data basis
- Outokumpu Core range datasheet: Table 7, p. 10, for properties at 20°C and mean expansion over 20–100°C.
- Cleveland-Cliffs 304/304L product data: p. 4, for typical conductivity at 100 and 500°C.
- NIST Cryogenic Material Properties: UNS S30400: conductivity coefficients, data range and curve-fit error.
- NIST reference tables: comparison values at 295 K.
- NIST SP 811, Appendix B.8: International Table Btu conductivity conversions.
- NBS Report 9869: a historical measured Type 304 sample; its data demonstrate source and specimen dependence.
Producer values are typical reference data. The interpolated table entries and worked examples are calculations for the stated assumptions, not material acceptance limits or equipment ratings.
Planning a 304 laser-welding trial?
Send Oceanplayer Laser the grade, thickness and joint drawing, along with the required penetration, appearance and dimensional tolerances. These details make the process discussion specific to your part.