Thermal conductivity of steel, aluminum, brass, copper

What is thermal conductivity

This term means the ability of various materials to exchange energy , which in this case is represented by heat. In this case, energy transfer passes from the hotter part to the colder part and occurs due to:

1. Molecules
2. Atoms.
3. Electrons and other particles of the metal structure.

The thermal conductivity of stainless steel will differ significantly from that of another metal - for example, the thermal conductivity of copper will be different than that of steel.

To indicate this indicator, a special value is used, called the thermal conductivity coefficient. It is characterized by the amount of heat that can pass through a material in a certain unit of time.

Indicators for steel

Thermal conductivity can vary significantly depending on the chemical composition of the metal. The coefficient of this value will be different for steel and copper. In addition, with an increase or decrease in carbon concentration, the indicator under consideration also changes.

There are other features of thermal conductivity:

1. For steel that does not have impurities, the value is 70 W/(m* K).
2. Carbon and high-alloy steels have much lower conductivity. Due to an increase in the concentration of impurities, it is significantly reduced.
3. The thermal effect itself can also affect the structure of the metal. As a rule, after heating, the structure changes its conductivity value, which is associated with a change in the crystal lattice.

Thermophysical properties of steel depending on temperature

The following thermophysical properties of steel of various grades are presented:

• steel density, kg/m3;
• specific (mass) heat capacity of steel, kJ/(kg deg);
• thermal conductivity of steel, W/(m deg);
• coefficient of thermal expansion of steel, 1/deg.

Effect of carbon concentration

The carbon concentration in steel affects the amount of heat transfer:

1. Low carbon steels have a high conductivity index. That is why they are used in the manufacture of pipes, which are then used to create the heating system pipeline. The coefficient value varies from 54 to 47 W/(m* K).
2. The average coefficient for common carbon steels is a value from 50 to 90 W/(m* K). That is why such material is used in the manufacture of parts for various mechanisms.
3. For metals that do not contain various impurities, the coefficient is 64 W/(m* K). This value does not change significantly under thermal influence.

Thus, the considered indicator for alloyed alloys may vary depending on the operating temperature.

Thermal conductivity of steel and cast iron

The table shows the thermal conductivity values ​​of steel and cast iron.
The thermal conductivities of steels are given for the following types: carbon, low- and medium-alloyed, heat-resistant and heat-resistant stainless steels, spring steels, steels for castings, tool steels, depending on temperature.

The thermal conductivity of steel in the table is presented for various grades in the temperature range from -263 to 1200°C.

The average thermal conductivity of carbon steels (type steel 08, steel 3, steel 20) at room temperature is 50...90 W/(m deg). The thermal conductivity of stainless, heat-resistant and heat-resistant martensitic steels lies in the range from 30 to 45 W/(m deg). The thermal conductivity of stainless steels (type 12Х18Н9Т) of the austenitic class is in the range from 12 to 22 W/(m deg).

The table also presents data on the thermal conductivity of cast iron.
Table of thermal conductivity of steel and cast iron

Importance in everyday life and production

Why is it important to consider thermal conductivity? A similar value is indicated in various tables for each metal and is taken into account in the following cases:

1. In the manufacture of various heat exchangers. Heat is one of the important carriers of energy. It is used to provide comfortable living conditions in residential and other premises. When creating heating radiators and boilers, it is important to ensure rapid and complete heat transfer from the coolant to the end consumer.
2. In the manufacture of outlet elements. You can often encounter a situation where you need to remove heat rather than supply it. An example is the case of heat removal from the cutting edge of a tool or gear teeth. To ensure that the metal does not lose its basic performance qualities, rapid removal of thermal energy is ensured.
3. When creating insulating layers. In some cases, the material should not conduct thermal energy transfer. For such operating conditions, a metal is selected that has a low heat conductivity coefficient.

Information about the thermal conductivity index

The process of heat transfer in the body of any substance occurs between the atomic and molecular bonds of the material, in which the temperature regime is uneven. Any substance heats up gradually, transferring heat energy from area to area. This heat transfer depends on the state of the substance. Heat conductivity depends on: 1. The state of aggregation of the substance, 2. The heating rate. 3. Density indicator. 4. Melting points. The heat conductivity coefficient is the amount of heat passing through a unit area of ​​a material in a certain period of time when temperatures change.

What does heat conductivity depend on? Aluminum has a crystal structure - a cube. At a temperature of 200C, specific gravity = 2.7 g/cm3. The melting temperature is from +657 to +660.2 0C. If aluminum is of high purity, then the metal begins to melt at +1800 to 2060 0C. The specific heat capacity during the heating period increases, and the coefficients of expansion and thermal conductivity also increase. The thermal conductivity of aluminum, compared to other metals, is considered high.

Aluminum reacts with oxygen to form an oxide film on the surface. The latter protects the metal from further oxidation. Aluminum alloys have unique properties: 1. When aluminum melts, the hydrogen present in it dissolves, which leads to the formation of pores in the metal. If the composition contains impurities of calcium, potassium or sodium, it also leads to porosity. 2. The structure of the material becomes homogeneous upon cooling if the alloy contains additives of iron, vanadium, nickel or zirconium. 3. Aluminum alloys remain inert to some chemical elements. The presence of substances such as sulfur and its derivatives precipitate, forming slag, and do not affect the change in structure and properties of the alloys. 4. Under the influence of nitrogen, phosphorus or carbon, the properties of the material do not change.

The strength of aluminum in its pure form is low, so casting technology is used extremely rarely for the production of finished products. As a rule, these are cast ingots, manufactured for further rolling and forging.

What systems require calculations?

The heat transfer coefficient is calculated for heated floors. This system is made less and less often from steel pipes, but if products made from this material are chosen as coolants, then calculations must be made. The coil is another system, when installing which it is necessary to take into account the heat transfer coefficient.

Registers are presented in the form of thick pipes connected by jumpers. The average heat output of 1 meter of this design is 550 W. The diameter ranges from 32 to 219 mm. The structure is welded so that there is no mutual heating of the elements. Then heat transfer increases. If you assemble the registers correctly, you can get a good room heating device - reliable and durable.

How to optimize the heat transfer of a steel pipe?

During the design process, specialists are faced with the question of how to reduce or increase the heat transfer of 1 m of steel pipe. To increase, you need to change the infrared radiation upward. This is done with paint. Red color increases heat transfer. It is better if the paint is matte.

Another approach is to install fins. It is mounted outside. This will increase the heat transfer area.

In what cases does it need to be reduced? The need arises when optimizing a pipeline section located outside a residential area. Then experts recommend insulating the area - isolating it from the external environment. This is done using polystyrene foam, special shells that are made from special foamed polyethylene. Mineral wool is also often used.

We make a calculation

The formula by which heat transfer is calculated is as follows:

• K – thermal conductivity coefficient of steel;
• Q – heat transfer coefficient, W;
• F is the area of ​​the pipe section for which the calculation is being made, m 2 dT is the value of the temperature pressure (the sum of the primary and final temperatures taking into account room temperature), ° C.

The thermal conductivity coefficient K is selected taking into account the area of ​​the product. Its size also depends on the number of threads laid in the premises. On average, the coefficient is in the range of 8-12.5.

dT is also called temperature difference. To calculate the parameter, you need to add the temperature that was at the outlet of the boiler with the temperature that was recorded at the entrance to the boiler. The resulting value is multiplied by 0.5 (or divided by 2). The room temperature is subtracted from this value.

If the steel pipe is insulated, then the resulting value is multiplied by the efficiency of the insulating material. It reflects the percentage of heat that was given off during the passage of the coolant.