Which metals conduct heat best, and which worst?

Which metals conduct heat best?

Silver and copper conduct heat best near room temperature, then gold, then aluminium. Silver (Ag) shows 429 W/(m·K) for 99.95 % bar at 0 to 100 °C, Copper (Cu) 391 W/(m·K) for soft oxygen-free C10200 at 20 °C, and Gold (Au) 320 W/(m·K) with no temperature or purity stated. Silver's figure is 1.1 times copper's, but that sets a 0 to 100 °C value against a single 20 °C one, so read the gap as small, not exact.

Commercially pure Aluminium (alloy 1100-O, annealed, no temperature stated) comes next at 220 W/(m·K). The alloys are lower: Aluminium alloy 6061 shows 167 W/(m·K) and Aluminium alloy 7075-T6 130 W/(m·K). Of the refractory metals here, Tungsten (W) (174 W/(m·K), no temperature stated) and Molybdenum (142 W/(m·K), 20 °C) conduct best; Tantalum and Niobium (Nb) sit far lower.

Then come Free-cutting brass C36000 at 116 W/(m·K) (20 °C), die-cast zinc alloys at 109 to 115 W/(m·K) (70 to 140 °C) and Magnesium alloy AZ31B at 96 W/(m·K) with no temperature stated. For heat sinks, see our separate heat sink article.

Which metals and alloys conduct heat worst?

Ti-6Al-4V conducts worst of the metals compared here, at 6.7 W/(m·K) at 20 °C. It is followed by Inconel 625 at 9.8 W/(m·K) (annealed, 21 °C), Invar 36 (FeNi36) at 10.5 W/(m·K) (20 °C) and Inconel 718 at 11.4 W/(m·K) (annealed and aged, 70 °F; the datasheet prints 79 Btu·in/ft²·h·°F).

The stainless steels follow. Stainless steel 316 shows 14.6 for sheet, coil and bar at 20 to 100 °C, and a second source gives 13 to 17 with no temperature or form stated. Stainless steel 304L shows 16.3 at 100 °C and 21.4 at 500 °C from one table shared by several 300-series grades, so it is not a 304L-only measurement. 17-4 PH stainless steel in condition H900 shows 17.9 at 149 °C and 22.6 at 482 °C.

Commercially pure titanium and zirconium sit a little above stainless: Titanium Grade 2 shows 22 W/(m·K) (seamless tube and pipe, room temperature) and zirconium 21.5 W/(m·K) for Zircaloy-4, which has no stated temperature. Zr 702 is 22 and Zr 705 is 17.1, both as 300 to 800 K averages. Typical references put Grade 2 and room-temperature Zircaloy-4 lower, so treat both as generous (general engineering knowledge, not in our data).

The middle group is Nickel at 70.3 W/(m·K) (Nickel 200, 20 °C), Tin (Sn) at 66.6 W/(m·K) (polycrystalline), Aluminium bronze C95400 at 58.7 W/(m·K) (20 °C) and the steels, with Tantalum, Niobium (Nb) and Lead (Pb) near the bottom of that group. Alloy steel AISI 4140 shows 42.6 W/(m·K) at 100 °C. Carbon steel 1018 shows 51.9 for cold-drawn bar (an estimate from similar materials) and a 25 to 93 range for low-carbon steel in general. Treat the steel figures as indicative.

What does low thermal conductivity cost you in these alloys?

It costs you heat flow: the part is a poor heat spreader and carries a steep thermal gradient. Copper's figure is 39.9 times that of Inconel 625. The benefit is a thermal break, where a low-conductivity support limits heat flow between a hot or cold part and its structure (general engineering knowledge, not in our data).

Titanium. The Ti-6Al-4V profile calls low conductivity its weak point; the alloy is chosen for structure. Titanium Grade 2 is listed for seawater-cooled condenser tubing, where corrosion decides.

Stainless steel. Low conductivity keeps heat near a weld for longer, which adds to distortion (general engineering knowledge, not in our data). Stainless steel 304L is specified for welded parts for its low carbon limit.

Inconel. Inconel 625 is listed for thrust-chamber tubing and for heat-exchanger tubing in environmental control systems. The Inconel 718 profile says the RS-25 main combustion chamber has a structural shell of Inconel 718 lined with NARloy-Z copper alloy. The profile does not say why; the usual reading is a conductive liner inside a strong shell (general engineering knowledge, not in our data).

Invar. Invar 36 (FeNi36) shows 10.5 W/(m·K) at 20 °C, close to Inconel 625 and far below copper. It is chosen to hold a dimension across temperature changes, as our low-thermal-expansion article covers, not for heat flow.

How does thermal conductivity relate to electrical conductivity?

In metals the two usually rise and fall together, because the same free electrons carry most of both the heat and the current (general engineering knowledge, not in our data). Our data shows the match holds roughly: Silver (Ag) has the lowest resistivity in the chart and the highest thermal conductivity, and Inconel 625 the highest resistivity and one of the lowest thermal figures.

Copper is missing from the chart because its profile stores conductivity, 58 MS/m, not resistivity. In %IACS, soft copper is rated 100, annealed brass C36000 26 and aluminium bronze C95400 13, the same order as their thermal conductivity. Aluminium 1100-O is rated 59 and die-cast zinc alloys about 27, close to brass, which has a similar thermal figure.

Do not convert one property into the other for design, because conditions and sources differ. Use %IACS as a quick check, not a replacement.

Why do alloys conduct heat worse than pure metals?

In every pair our data lets us compare, the alloy conducts worse than its base metal. As general engineering knowledge, not in our data, alloying atoms scatter the electrons that carry the heat. The pairs come from different suppliers, so read them as direction only.

Aluminium 1100-O is 1.7 times the 7075 figure. Copper's 391 W/(m·K) falls to 116 W/(m·K) for free-cutting brass and 58.7 W/(m·K) for aluminium bronze. Nickel's 70.3 W/(m·K) falls to 9.8 W/(m·K) in Inconel 625. Titanium Grade 2 shows 22 W/(m·K) against 6.7 W/(m·K) for Ti-6Al-4V, 3.3 times apart on the supplier's generous tube figure.

Does thermal conductivity change with temperature?

Yes, and the direction depends on the metal. In our data, Stainless steel 304L, 17-4 PH stainless steel, Inconel 625 and Inconel 718 conduct better when hot: 304L rises from 16.3 at 100 °C to 21.4 at 500 °C, 17-4 PH H900 from 17.9 at 149 °C to 22.6 at 482 °C, and Inconel 625 from 9.8 W/(m·K) at 21 °C to 25.2 at 982 °C. Inconel 718 rises from 79 Btu·in/ft²·h·°F at 70 °F to 148 at 1200 °F, almost double. Niobium C-103 rises from 38.1 at 811 °C to 44.7 at 1304 °C. Stainless steel 316 has no temperature curve in our data, so its hot behaviour is inferred, not shown. A low room-temperature figure understates hot-service heat flow.

Pure nickel is the exception. Nickel 200 goes the other way: 75.5 at -100 °C, 70.3 W/(m·K) at 20 °C, 66.5 at 100 °C and 55.4 at 400 °C.

Cooling splits the near-pure metals from an alloy in the curve-fit values our profiles hold. Alloy 1100 aluminium rises from 211.8 W/(m·K) at 293 K to 290.2 at 77 K, and oxygen-free copper with an RRR of 100 from about 397 to 547. The 6061-T6 fit goes the other way, from about 154 to 83.5. Check these calculated values against a measured curve before a cryogenic design.

Terms used

Thermal conductivity

how fast heat flows through a material for a given temperature difference. Higher means heat moves more easily. Our data gives it in W/(m·K).

Electrical resistivity

how strongly a material resists electric current. Lower means a better electrical conductor. The chart uses ohm metres (Ω·m); e-8 means a very small number.

%IACS

electrical conductivity as a percentage of the International Annealed Copper Standard. Our copper profile lists soft oxygen-free copper at 100 %IACS.

Temper

the condition a metal is supplied in, such as O (annealed) or T6 (heat treated and aged).

Curve-fit value

a value calculated from an equation fitted to measured data, not a direct reading.

Thermal gradient

the temperature difference across a part, from its hot side to its cool side. A low-conductivity part carries a steeper gradient for the same heat flow.

Residual resistivity ratio (RRR)

a measure of how pure and well annealed a metal is, used for cryogenic copper. A higher ratio gives a higher conductivity at 77 K.

What to check before you order

1. Ask for conductivity at your operating temperature, not only at room temperature. In our data 304L, 17-4 PH and the Inconels rise when hot and pure nickel's falls; 316 has no curve.
2. Name the grade, temper and form on the order. Each figure here is for one condition, for example annealed 1100-O, T6 6061 or soft C10200 copper.
3. Ask whether the figure is measured, typical, a minimum or an estimate, and at what temperature. Steel 1018's cold-drawn figure is an estimate.
4. For a heat path, ask for the test method (for example laser flash, ASTM E1461) or a certified %IACS value; lot-level thermal tests are rare.
5. For a heat sink, joint and spreading resistance often matter more than the metal, and cast aluminium alloys such as A380 conduct less than wrought 6061 (general engineering knowledge, not in our data).
6. For cryogenic copper, ask for purity or RRR and the anneal.

What do we not know yet?

• This set has no cast aluminium grade such as A380, no tough-pitch copper such as C110, and none of the thermal-management metals beryllium copper, CuCrZr or copper-tungsten (general engineering knowledge, not in our data), so the best-conductor ranking stops at what is profiled. No profile stores thermal diffusivity, which sets how fast a part heats and cools. Specific heat is missing for titanium Grade 2, tantalum, lead, tungsten and brass C36000.
• Most metals hold one temperature only. Only nickel, stainless 304L and 17-4 PH, Inconel 625 and 718 and niobium C-103 give two or more temperatures, and copper, aluminium 1100 and 6061 have 77 K curve fits. No temperature curve is stored for silver, gold, tungsten, molybdenum, stainless 316 or Ti-6Al-4V.
• No test temperature is stated for gold, tungsten, lead, tantalum, magnesium AZ31B, pure niobium, polycrystalline tin or Zircaloy-4. The steel data is thin: 1018 has an estimate and a generic range, and 4140 one value at 100 °C.
• These 28 profiles hold no data on direction (anisotropy), porosity in castings, cold work in copper beyond %IACS, thermal resistance of joints or coatings.
• Copper stores only conductivity (58 MS/m) and molybdenum's resistivity is in a different unit, so neither is in the resistivity chart.
• Cost and availability are not compared here.

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