Which metal heats up quickly?

Which metal has the highest heat transfer?

When choosing batteries, you need to evaluate the characteristics.

One of the most important parameters characterizing the performance of a battery is the heat transfer rate.

The operation of the entire system largely depends on the parameter

Heat transfer from heating batteries: what is it, its calculation according to the product data sheet

The amount of heat that is transferred per unit time to a certain volume per unit time is the heat transfer of the heating battery. Heat output is sometimes called thermal power because it is measured in watts .

Sometimes heat transfer is called heat flow power cal/hour in the product passport . There is a relationship between Watts and calories per hour: 1 W = 859.85 cal/hour.

The manufacturer indicates the nominal heat transfer parameter in the radiator passport. Based on this parameter, you can calculate the required number of elements for each individual room or room. If the passport indicates the power of one section is 150 W, then a section of 7 elements will produce more than 1 kW of heat.

Calculation of real heat transfer in kW

To do this, you need to decide on the number of external walls and windows. With one external wall and one window for every 10 m² of room area, 1 kW of heat will be required.

If there are two external walls , then for every 10 m² 1.3 kW will be required .

More precisely, you can calculate the required power using the formula Sxhx41:

• S is the area of ​​the room;
• h is the height of the room;
• 41 is an indicator of the minimum power per 1 cubic meter of room volume.

The resulting thermal power will be the required full power of the heating battery. Now all that remains is to divide by the power of one radiator and determine their number.

Formulas for accurate calculations

KT=1000 W/m²*P*K1*K2*K4*K7.

indicator is the amount of heat for an individual room.

P - Total area of ​​the room.

K1 is the coefficient for taking into account window openings. If there is a double window, then K1 = 1.27.

• Double glazing - 1,0,
• Triple glazing - 0,85.

K2 - coefficient of thermal insulation of walls:

• Thermal insulation is very low - 1,27;
• Laying walls with 2 bricks and insulation - 1.0;
• High quality thermal insulation - 0,85.

K3 - ratio of window area to floor area in the room:

• 50% — 1,2;
• 40% — 1,1;
• 30% — 1,0;
• 20% — 0,9;
• 10% — 0,8.

K4 - average air temperature in the room during the coldest period:

• 35 °C — 1,5;
• 25 °C — 1,3;
• 20 °C — 1,1;
• 15 °C — 0,9;
• 10 °C — 0,7.

K5 - accounting for external walls:

• 1 wall - 1,1;
• 2 walls - 1,2;
• 3 walls - 1,3;
• 4 walls - 1,4.

K6 - type of room above the room:

• Cold attic (uninsulated) - 1,0;
• Attic with heating - 0,9;
• Heated room - 0,8.

K7 - taking into account ceiling heights:

• 2,5 m — 1,0;
• 3,0 m — 1,05;
• 3,5 m — 1,1;
• 4,0 m — 1,15;
• 4,5 m — 1,2.

This calculation takes into account the maximum number of features of the room for heating.

Attention! The result must be divided by the heat transfer of one radiator and the result rounded up .

Calculation of heat transfer according to the table

Many consumers are of little interest in the process of calculating heat transfer; efficiency is more important to them. We can talk about efficiency when all parameters are taken into account. Many manufacturing companies summarize the indicators in tables, which make it easier to select batteries of the required efficiency.

Photo 1. An example of a table for calculating the heat transfer of radiators of brands such as DeLonghi, Kermi, Korado.

Example of work

From the table, select the manufacturer of interest. For example, Kermi (Germany). In the first column, select the type of radiator. type 22 radiator . Its dimensions are 400x100x300 . Product power 510 W.

If in our premises the calculated need requires a battery with a total power of 2000 W, 2000/510 = 4 such batteries will need to be installed Based on the indicated price, the total cost will be within 12 thousand rubles.

First you need to clarify whether there is room to install so many heating batteries. If there is no physical space for installation, then you need to select from other types of batteries.

Photo 2. An example of a power table for radiators from the manufacturer Kermi. Several models of heating devices are indicated.

We choose type 22. Height 600 mm, length 1000 mm . At the intersection we find the battery power - 2249 W. This means that one element is enough to heat our room with an estimated need of 2 kW.

Source: http://vashslesar.ru/uteplenie/u-kakogo-metalla-samaja-vysokaja-teplootdacha.html

Features of steel hardening

Heat treating a metal changes its characteristics. Tempering steel makes it harder and stronger. In some cases, heat treatment is carried out to refine the grain and level the structure. A simple heating and rapid cooling technology for small parts can be done at home. It is necessary to know the grade of steel and its heating temperature for hardening.

What is metal hardening?

One type of heat treatment is metal hardening. It consists of several stages performed in a certain sequence:

1. Heating metal to a certain temperature. Dwell time for leveling over the entire depth of the part.
2. Fast cooling.
3. Tempering to relieve stress and correct hardness to a specified value.

During the manufacturing process, complex parts can undergo several different types of hardening.

Based on the depth of treatment, hardening is divided into two types:

Basically, in mechanical engineering, volumetric heat treatment is used, when the part is heated to its entire depth. As a result of sudden cooling, after the completion of heat treatment, the hardness inside and outside differs by only a few units.

Surface hardening is used for parts that must be hard on top and ductile on the inside. The inductor heats the steel to a depth of 3–20 mm and immediately behind it there is a sprayer that pours water on the hot metal.

The steel is heated to austenite state. Each brand has its own temperature, determined from the table of the state of iron-carbon alloys. During sudden cooling, carbon remains inside the grain and does not enter the intercrystalline space. The transformation of the structure does not have time to occur, and the internal structure contains pearlite and ferrite. The grain becomes finer, the metal itself becomes harder.

What steels can be hardened?

When heated and rapidly cooled, internal changes in structure occur in all steels. Hardness increases only with carbon content greater than 0.4%. St. 35 according to GOST has it 0.32 - 0.4%, which means it can “get hot” - slightly change the hardness if the carbon is located at the upper limit.

Steels starting from CT45 and higher in carbon content are considered hardenable. At the same time, hardening of stainless steel with low carbon content, type 3X13, is possible. Chromium and some other alloying elements replace it in the crystal lattice and increase the hardenability of the metal.

High-alloy carbon steels contain substances that accelerate the cooling process and increase the steel's ability to harden. They require a complex step cooling system and high temperature tempering.

Temperature and heating rate

The heating temperature for hardening increases with the content of carbon and alloying substances in the steel. For St45 it is, for example, 630–650⁰, St 90HF - more than 800⁰.

High-carbon and high-alloy steels, when heated quickly, can “crack” - form small cracks on the surface and inside. They are heated in several stages. At temperatures of 300⁰ and 600⁰, exposure is done. In addition to equalizing the temperature throughout the depth, there is a structural change in the crystal lattice and a transition to other types of internal structure.

Properties of steel after hardening

After hardening of parts, structural changes occur that affect the technical characteristics of the metal:

• increases hardness and strength;
• grain decreases;
• flexibility and ductility decreases;
• fragility increases;
• abrasion resistance increases;
• fracture resistance decreases.

It is easy to obtain a high class of cleanliness on the surface of a hardened part. Raw steel is not polished, it drags on and on.

Types of steel hardening

The main parameters for hardening steel: heating temperature and cooling rate. They completely depend on the grade of steel - carbon content and alloying substances.

Hardening in one environment

When hardening steel, the environment determines the cooling rate. The greatest hardness is obtained when the part is dipped in water. This way you can heat medium-carbon low-alloy steels and some stainless steels.

If the metal contains more than 0.5% carbon and alloying elements, then when cooled in water, the part will crack - become covered with cracks or completely collapse.

High-alloy steels increase their hardness even when cooled in air.

When quenching in water, alloy steel is heated to 40–60⁰. The cold liquid will bounce off the hot surface, forming a steam jacket. The cooling rate will be significantly reduced.

Step hardening

Hardening of steels with complex composition can be carried out in several stages. To speed up the cooling of large parts made of high-alloy steels, they are first dipped in water. The residence time of the part is determined by several minutes. After this, quenching continues in oil.

Water quickly cools the metal on the surface. After this, the part is dipped in oil and cools to the critical temperature of structural transformations of 300–320⁰. Further cooling is carried out in air.

If you heat massive parts only in oil, the temperature from the inside will slow down the cooling and significantly reduce the hardness.

Isothermal hardening

It is difficult to harden metal with a high carbon content, especially tools made of tool steel - axes, springs, chisels. When rapidly cooled, strong stresses are formed in it. High temperature tempering removes some of the hardness. Hardening is carried out in stages:

1. Normalization to improve structure.
2. Heating to hardening temperature.
3. Dipping into a bath of saltpeter, heated to 300–350⁰, and soaking in it.

After hardening in a saltpeter bath, tempering is not necessary. Stresses are released during slow cooling.

Light hardening

There is no technical term for “light hardening”. When alloy steels are hardened, including heating, in a vacuum or inert gases, the metal does not darken. Hardening in a protective gas environment is expensive and requires special equipment separately for each type of part. It is used only for mass production of the same type of product.

In a vertical furnace, the part is heated, passing through an inductor, and immediately lowered below - into a salt or nitrate bath. The equipment must be sealed. After each cycle, the air is pumped out of it.

Hardening with self-tempering

During rapid cooling during the hardening process of steel, heat remains inside the part, which gradually leaves and releases the material - relieving stress. Self-tempering can only be done by specialists who know how much the time a part remains in the coolant can be reduced.

Self-tempering can be done at home if you need to slightly increase the hardness of fasteners or small parts. It is necessary to lay them on heat-insulating material and cover with asbestos on top.

Cooling methods during hardening

Methods of cooling metal during water and oil quenching are widely used in industry. The most ancient composition for hardening swords and other thin-walled objects is saline solution. Hardening was carried out by blacksmiths using forging heat and the heat generated by deformation.

Red sabers, swords, knives were dipped into the urine of red-haired guys. In Europe they were simply stuck into the bodies of living slaves. The colloidal composition containing salts and acids made it possible to cool the steel at an optimal speed and not create unnecessary stresses and leads.

Currently, various sodium salt solutions, saltpeter and even plastic shavings are used.

How to harden steel at home

The decision on how to heat metal is made based on several parameters:

• steel grades;
• required hardness;
• operating mode of the part;
• dimensions

Not all heat treatment methods are available to amateurs. You should choose the simplest ones. Most often, at home, you have to harden stainless steel when making knives and other home cutting tools.

The hardening temperature of chromium-containing steels is 900–1100⁰C. Heating should be checked visually. The metal should have a light orange - dark yellow color, uniform over the entire surface.

You can dip a thin stainless steel into hot water, lifting it into the air and lowering it again. The higher the carbon content, the more time the steel spends in air. One cycle lasts approximately 5 seconds.

Plain weldable steels are heated to a cherry color and cooled in water. Medium alloy materials should have a red color before immersion in water. After 10–30 seconds, they are transferred to oil, then placed in the oven.

When hardening, the maximum hardness that steel gives with this technology is obtained. Then it is reduced to the required value by high-temperature tempering.

Hardening at home

Equipment

Metal is heated in various ways. You just need to remember that the combustion temperature of wood cannot provide heating to the metal.

If you need to improve the quality of 1 part, just light a fire. It must be lined with bricks around the perimeter and, after laying the workpiece, partially closed on top, leaving gaps for air access. It's better to burn coal.

A separate area and a small part are heated with a gas and kerosene burner, constantly running the flame and heating it from all sides.

Making a muffle furnace requires a lot of time and resources. It is advisable to build it for constant use.

The coolant can be in a bucket or any other container that will ensure complete immersion of the part with an oil thickness of the 5 largest sections of the part:

• one part under the hardened product;
• two on top.

The part must be moved slowly in the coolant. Otherwise, a steam jacket will form.

Self-production of a chamber for hardening metal

The simplest semblance of a muffle furnace is made from refractory brick, fireclay clay and asbestos:

1. Wind copper wire onto the mandrel. For home voltage, a cross section of 0.8 mm is suitable. Leave long ends.
2. Place the spiral inside the bricks and fix it with clay, coating the entire inner surface.
3. Make a pallet inside - a platform for placing workpieces. To do this, you need to mix clay with asbestos.
4. Thermal insulating material can also be placed outside, reducing the heat transfer of the walls.
5. Connect the ends of the wire to the wires with a plug.
6. At the back, seal the hole between the bricks hermetically.
7. Build a lid in front that will open.

All materials should dry at room temperature.
This will take several days. Then you can lay the part on the insulating material and heat it. Hardening an ax at home.

Defects during hardening of steel

When hardening steel, 2 groups of defects arise:

The first are associated with uneven, spotty hardening and the discrepancy between the resulting hardness and the requirements in the drawing. Such defects are mainly caused by improper cooling or poorly performed heat treatment.

Uncorrectable items include chips, cracks, and complete destruction of parts. The reason most often lies in low-quality metal.

Hardening significantly changes the structure and performance properties of the metal. You can do it yourself using simple parts. It is necessary to know exactly the grade of steel, its hardening temperature and the cooling medium.

Source: https://metalloy.ru/obrabotka/termo/zakalka-stali

How to choose a kettle for the stove

16.02.2018 10:08:25

To ensure that your kettle is easy to use and does not “die a heroic death” after several months of use, before purchasing, pay attention to the material from which it is made, the length of the spout, volume and other parameters. We will tell you how to do this in our article.

What types of kettles are there for the stove?

Stovetop kettles are most often made from stainless steel and enameled steel. Products made from heat-resistant glass, aluminum and ceramics are less common on sale, and cast iron cookware is almost impossible to find, except at a flea market or in your grandmother's chest. They all have their pros and cons.

Stainless steel teapots

One of the most popular and successful types of teapots. Stainless steel cookware has a beautiful appearance (shiny mirror surface), as well as high strength and resistance to deformation. Stainless steel does not contain any toxic substances that can “migrate” into water, so this material is absolutely safe for health.

Many stainless steel models have a “layered” bottom made of other materials, such as copper and aluminum. This improves heat distribution and makes the kettle heat more evenly.

Aluminum teapots

Aluminum is a cheap, lightweight material that is resistant to corrosion. During the Soviet Union, it was often used to make pots, cups, cutlery and teapots, but now it is used much less frequently.

On the Internet you can find information that aluminum cookware, when heated, releases heavy metals into water and food, and they gradually accumulate in our body. This is dangerous to health, because... Excessive aluminum content can lead to the development of diseases such as osteoporosis, cancer, kidney disease, etc.

Others argue that using such utensils to boil water is safe because... When interacting with oxygen, an oxide film is formed on the surface of aluminum, which protects drinks from the “migration” of toxic substances. This film is damaged only when food is stored for a long time and exposed to acids, for example, from tomato juice or sauerkraut. Therefore, using an aluminum kettle cannot cause cancer and other diseases.

Unfortunately, it is impossible to say exactly how the use of aluminum cookware affects the human body. Too little research has been done on this issue. Whether it is worth buying such a product is up to you to decide.

Enameled steel teapots

Enameled teapots are very popular among residents of our country and the CIS countries. The “fashion” for them began during the Soviet era and continues to this day. And it’s not surprising, because they can have a variety of sizes and colors. Thanks to this, the kettle can be matched to any kitchen design. In addition, such products do not react with water when heated and do not emit hazardous substances.

Glass teapots

A glass stovetop teapot is a good way to surprise your guests. This is a great option for occasional use and a holiday table. But if you boil water in it constantly, the kettle will break faster. The product can also crack if there is a sharp fluctuation in temperature, for example, if you place a hot kettle under a cold stream of water.

The body of the product is made of fireproof glass, and the lid and handle are made of plastic. Thanks to the transparent walls, you can control the boiling process of water and its level, and the plastic reduces the likelihood of burns.

Cast iron teapots

Cast iron kettles are one of the most durable and reliable options. They are difficult to scratch or deform, and their service life is tens of years. These kettles heat up evenly and cool down slowly. During long gatherings, you do not have to constantly boil the water - it will remain hot for a long time.

Pen

When buying a kettle, be sure to pay attention to the material from which the handle is made. If it has low thermal conductivity, this will protect you from burns.

Handles can be made from:

1. Bakelite;

2. Metal;

3. Ceramics.

Bakelite is a heat-resistant material designed for temperatures not exceeding 180C. If it exceeds 180C it starts to char, so it should be kept away from the fire. Bakelite handles are comfortable to use, do not slip and are pleasant to the touch.

Enameled teapots are usually equipped with metal or ceramic handles . Metal handles get very hot, but are highly durable and resistant to shocks and falls, while ceramic handles are more fragile. They heat up slowly, but if the kettle is not removed from the stove in time, you can get burned on them.

Sometimes teapots are equipped with silicone handles . Silicone has low thermal conductivity and can withstand temperatures up to 230C.

Spout

Using a kettle with a small and wide spout, you can quickly fill a cup. However, you cannot pour a lot of liquid into it - the water level should not be higher than the lowest point at which the spout and the body of the kettle are connected. If you use a product with a narrow and long spout, the water flow will be slower. In addition, if you tilt the kettle too much, boiling water may splash out from under the lid.

Volume

The volume of the kettle can be either 0.7 liters or 3.2 liters. The choice of displacement, first of all, should depend on the number of people who will drink tea. For example, for a holiday table or a large company, choose 2–3 liter models, and for a family of four, 1.5–2 liter dishes are suitable. A kettle with a volume of 0.7 liters is enough for one person, and one liter for a couple in love.

How to properly wash enameled and stainless steel teapots

Improper care of the kettle not only shortens the service life of the product, but can also harm human health. For example, the use of abrasive cleaners and hard sponges thins the enamel coating of the cookware, causing cracks and chips to appear on it, and the kettle has to be thrown away. If a person continues to use such a kettle, the “exposed” metal reacts with water and releases substances into it that are hazardous to health.

Caring for an enamel kettle

Before using the product for the first time, “temper” it. Boil water with salt in it (1 tablespoon of salt per 500 ml of water). This will make the coating more durable.

Enameled kettles are prone to scale. To prevent its formation, clean the inside of the kettle daily with a soft sponge. Do not use strong detergents or hard sponges. Do not leave water in the kettle for a long time.

To get rid of scale, you can use citric acid, baking soda, vinegar and even potato peelings. Most often, housewives use citric acid. To remove scale, you need to:

1. Fill the kettle with a solution of water and citric acid at the rate of 25 g per 2.5 l;

2. Boil the solution and immediately remove from heat;

3. Wait until the liquid cools down;

4. Rinse under running water and remove any remaining scale with a sponge.

5. Do not wash a hot kettle in cold water. Due to sudden changes in temperature, the enamel may crack.

Caring for a stainless steel kettle

Stainless steel dishes quickly get dirty, and stains immediately appear on them. Therefore, it should be washed regularly using mild detergents or laundry soap. It is not recommended to use hard sponges and abrasive cleaners - they can scratch the surface. To remove stubborn and old stains, you can use baking soda and soap. For this:

1. Rinse the kettle under running water;

2. Lather a sponge with soap and sprinkle baking soda on top;

3. Sprinkle the kettle with baking soda;

4. Wipe the product with a sponge with the prepared mixture;

5. After 40–60 minutes, wash the dishes in warm water;

6. Repeat if necessary.

The best manufacturers of kettles for the stove

The quality of the kettle, the safety of the coating and its service life depend on the manufacturer. Buying cookware from an unknown brand can not only lead to unnecessary expenses, but also harm your health.

Some manufacturers produce cookware from low-quality material that quickly deforms and the coating chips, allowing the metal to react with water and release compounds hazardous to health. In addition, some budget models of kettles have too thin walls and bottoms, which is why they heat up unevenly and quickly become unusable, and the “heat-resistant” handle heats up and melts.

To reduce the risk of a bad purchase, pay attention to the following manufacturers:

1. Rondell;

2. Becker;

3. Vinzer;

4. Bork;

5. Esprado;

6. Wellberg.

These manufacturers are very popular among residents of the Russian Federation, CIS countries and Europe. Most reviews of kettles from these companies are positive - consumers report high quality cookware, durability and ease of use.

Attention! This material is the subjective opinion of the authors of the project and is not a guide to purchase.

Source: https://expertology.ru/kak-vybrat-chaynik-dlya-plity/

Which metal heats up faster - Metalist's Handbook

The high thermal conductivity of copper and its other useful characteristics were one of the reasons for the early development of this metal by humans. To this day, copper and copper alloys are used in almost all areas of our lives.

Copper plates

A little about thermal conductivity

In physics, thermal conductivity is understood as the movement of energy in an object from more heated small particles to less heated ones. Thanks to this process, the temperature of the object in question as a whole is equalized.

The magnitude of the ability to conduct heat is characterized by the thermal conductivity coefficient.

This parameter is equal to the amount of heat that a material 1 meter thick passes through a surface area of ​​1 m2 for one second at a unit temperature difference.

MaterialThermal conductivity coefficient, W/(m*K)

Silver	428
Copper	394
Aluminum	220
Iron	74
Steel	45
Lead	35
Brick	0,77

Copper has a thermal conductivity coefficient of 394 W/(m*K) at temperatures from 20 to 100 °C. Only silver can compete with it. And for steel and iron this figure is 9 and 6 times lower, respectively (see table).

It is worth noting that the thermal conductivity of products made from copper largely depends on impurities (however, this also applies to other metals).

For example, the rate of heat conduction decreases if substances such as:

• iron;
• arsenic;
• oxygen;
• selenium;
• aluminum;
• antimony;
• phosphorus;
• sulfur.

Copper wire

If you add zinc to copper, you get brass, which has a much lower thermal conductivity coefficient.

At the same time, adding other substances to copper can significantly reduce the cost of finished products and give them characteristics such as strength and wear resistance.

For example, brass is characterized by higher technological, mechanical and anti-friction properties.

Since high thermal conductivity is characterized by rapid distribution of heating energy throughout the entire object, copper is widely used in heat exchange systems.

At the moment, radiators and tubes for refrigerators, vacuum units and cars are made from it for rapid heat removal.

Copper elements are also used in heating installations, but for heating.

Copper heating radiator

In order to maintain the thermal conductivity of the metal at a high level (and therefore make the operation of copper devices as efficient as possible), forced airflow by fans is used in all heat exchange systems. This decision is due to the fact that as the temperature of the environment increases, the thermal conductivity of any material decreases significantly, because heat transfer slows down.

  What is the difference between drill bits for metal and wood?

Aluminum and copper - which is better?

Aluminum has one disadvantage compared to copper: its thermal conductivity is 1.5 times less, namely 201–235 W/(m*K). However, compared to other metals, these are quite high values. Aluminum, like copper, has high anti-corrosion properties. In addition, it has advantages such as:

• low density (specific gravity 3 times less than that of copper);
• low cost (3.5 times less than copper).

Aluminum heating radiator

Thanks to simple calculations, it turns out that an aluminum part can be almost 10 times cheaper than a copper part, because it weighs much less and is made of cheaper material.

This fact, along with high thermal conductivity, allows the use of aluminum as a material for cookware and food foil for ovens.

The main disadvantage of aluminum is that it is softer, so it can only be used in alloys (for example, duralumin).

For effective heat transfer, the rate of heat transfer to the environment plays an important role, and this is actively facilitated by the cooling of radiators.

As a result, the lower thermal conductivity of aluminum (relative to copper) is leveled out, and the weight and cost of the equipment are reduced.

These important advantages allow aluminum to gradually replace copper from use in air conditioning systems.

Use of copper in electronics

In some industries, for example, in the radio industry and electronics, copper is essential.

The fact is that this metal is very ductile in nature: it can be drawn into extremely thin wires (0.005 mm), and also can be used to create other specific conductive elements for electronic devices.

And high thermal conductivity allows copper to extremely effectively remove the heat that inevitably arises during the operation of electrical appliances, which is very important for modern high-precision, but at the same time compact equipment.

The use of copper is relevant in cases where it is necessary to deposit a certain shape on a steel part.

In this case, a copper template is used, which is not connected to the element being welded. Using aluminum for these purposes is impossible, as it will melt or burn through.

It is also worth mentioning that copper can act as a cathode when welding with a carbon arc.

1 - gear, 2 - template fastenings, 3 - welded gear tooth, 4 - copper templates

  Sheet metal drawing

Disadvantages of the high thermal conductivity of copper and its alloys

Copper has a much higher cost than brass or aluminum. At the same time, this metal has its disadvantages, which are directly related to its advantages.

High thermal conductivity leads to the need to create special conditions during cutting, welding and soldering of copper elements. Since copper elements need to be heated much more concentrated compared to steel.

Also, preliminary and concomitant heating of the part is often required.

Don’t forget that copper pipes require careful insulation if they make up the main line or distribution of the heating system. Which leads to an increase in the cost of network installation compared to options when other materials are used.

Example of thermal insulation of copper pipes

Difficulties also arise with gas welding of copper: this process will require more powerful torches. When welding metal 8–10 mm thick, two or three torches will be required. While one torch is used for welding, the other is heating the part. In general, welding work with copper requires increased costs for consumables.

Plasma cutting of copper

Is it possible to increase the thermal conductivity of copper?

Copper is widely used in the creation of microcircuits for electronic devices and is designed to remove heat from parts heated by electric current.

When trying to increase the speed of modern computers, developers were faced with the problem of cooling processors and other parts. One of the solutions was to split the processor into several cores.

However, this method of combating overheating has exhausted itself, and now it is necessary to look for new conductors with higher thermal and electrical conductivity.

One solution to this problem is the recently discovered element graphene. Thanks to graphene deposition, the thermal conductivity of the copper element increases by 25%. However, the invention is still at the development level.

Source: https://ssk2121.com/kakoy-metall-bystree-nagrevaetsya/

Thermal conductivity of metals

All products used by humans are capable of transmitting and maintaining the temperature of the object or environment they touch. The ability of one body to transfer heat to another depends on the type of material through which the process takes place.

The properties of metals allow heat to be transferred from one object to another, with certain changes depending on the structure and size of the metal structure.

The thermal conductivity of metals is one of the parameters that determines their operational capabilities.

What is thermal conductivity and why is it needed?

The process of transferring the energy of atoms and molecules from hot objects to products with a cold temperature is carried out during the chaotic movement of moving particles. Such heat exchange depends on the state of aggregation of the metal through which the transmission passes.

Depending on the chemical composition of the material, thermal conductivity will have different characteristics.

This process is called thermal conductivity, it consists in the transfer of kinetic energy by atoms and molecules, which determines the heating of a metal product during the interaction of these particles, or is transferred from a warmer part to one that is less heated.

The ability to transfer or store thermal energy makes it possible to use the properties of metals to achieve the necessary technical goals in the operation of various components and assemblies of equipment used in the national economy.

An example of such an application would be a soldering iron that heats up in the middle part and transfers heat to the edge of the working rod, which is used to solder the necessary elements.

Knowing the properties of thermal conductivity, metals are used in all industries, using the required parameter for its intended purpose.

The concept of thermal resistance and thermal conductivity coefficient

If thermal conductivity characterizes the ability of metals to transfer the temperature of bodies from one surface to another, then thermal resistance shows an inverse relationship, i.e. the ability of metals to prevent such transfer, in other words, to resist. Air has high thermal resistance. It is he who, most of all, prevents the transfer of heat between bodies.

The quantitative characteristic of the change in temperature of a unit area per unit of time by one degree (K) is called the thermal conductivity coefficient. The international system of units usually measures this parameter in W/m*deg. This characteristic is very important when choosing metal products that must transfer heat from one body to another.

Table 1

Metal	Thermal conductivity coefficient of metals at temperature, °C
— 100		100	300	700
Aluminum	2,45	2,38	2,30	2,26	0,9
Beryllium	4,1	2,3	1,7	1,25	0,9
Vanadium	—	—	0,31	0,34	—
Bismuth	0,11	0,08	0,07	0,11	0,15
Tungsten	2,05	1,90	1,65	1,45	1,2
Hafnium	—	—	0,22	0,21	—
Iron	0,94	0,76	0,69	0,55	0,34
Gold	3,3	3,1	3,1	—	—
Indium	—	0,25	—	—	—
Iridium	1,51	1,48	1,43	—	—
Cadmium	0,96	0,92	0,90	0,95	0,44 (400°)
Potassium	—	0,99	—	0,42	0,34
Calcium	—	0,98	—	—	—
Cobalt	—	0,69	—	—	—
Lithium	—	0,71	0,73	—	—
Magnesium	1,6	1,5	1,5	1,45	—
Copper	4,05	3,85	3,82	3,76	3,50
Molybdenum	1,4	1,43	—	—	1,04 (1000°)
Sodium	1,35	1,35	0,85	0,76	0,60
Nickel	0,97	0,91	0,83	0,64	0,66
Niobium	0,49	0,49	0,51	0,56	—
Tin	0,74	0,64	0,60	0,33	—
Palladium	0,69	0,67	0,74	—	—
Platinum	0,68	0,69	0,72	0,76	0,84
Rhenium	—	0,71	—	—	—
Rhodium	1,54	1,52	1,47	—	—
Mercury	0,33	0,09	0.1	0,115	—
Lead	0,37	0,35	0,335	0,315	0,19
Silver	4,22	4,18	4,17	3,62	—
Antimony	0,23	0,18	0,17	0,17	0,21
Thallium	0,41	0,43	0,49	0,25 (400 0)
Tantalum	0,54	0,54	—	—	—
Titanium	—	—	0,16	0,15	—
Thorium	—	0,41	0,39	0,40	0,45
Uranus	—	0,24	0,26	0,31	0,40
Chromium	—	0,86	0,85	0,80	0,63
Zinc	1,14	1,13	1,09	1,00	0,56
Zirconium	—	0,21	0,20	0,19	—

What does thermal conductivity depend on?

Studying the ability of heat transfer by metal products, it was revealed that thermal conductivity depends on:

• type of metal;
• chemical composition;
• porosity;
• sizes.

Metals have different crystal lattice structures, and this can change the thermal conductivity of the material. For example, in steel and aluminum, the structural features of microparticles affect differently the rate of transfer of thermal energy through them.

The thermal conductivity coefficient can have different values ​​for the same metal when the exposure temperature changes. This is due to the fact that different metals have different melting degrees, which means that under other environmental parameters, the properties of the materials will also differ, and this will affect thermal conductivity.

Measurement methods

To measure the thermal conductivity of metals, two methods are used: stationary and non-stationary. The first is characterized by the achievement of a constant value of the changed temperature on the controlled surface, and the second - by a partial change in it.

Stationary measurement is carried out experimentally, requires a lot of time, as well as the use of the metal under study in the form of blanks of the correct shape, with flat surfaces. The sample is placed between the heated and cooled surface, and after touching the planes, the time during which the workpiece can increase the temperature of the cool support by one degree Kelvin is measured. When calculating thermal conductivity, the dimensions of the sample being studied must be taken into account.

Non-stationary research methods are used in rare cases due to the fact that the result is often biased. Nowadays, no one except scientists is involved in measuring the coefficient; everyone uses long-established experimental values ​​for various materials. This is due to the constancy of this parameter while maintaining the chemical composition of the product.

Thermal conductivity of steel, copper, aluminum, nickel and their alloys

Ordinary iron and non-ferrous metals have different structures of molecules and atoms. This allows them to differ from each other not only in mechanical properties, but also in thermal conductivity properties, which, in turn, affects the use of certain metals in various sectors of the economy.

table 2

Steel has a thermal conductivity coefficient at an ambient temperature of 0 degrees. (C) equal to 63, and when the degree increases to 600, it decreases to 21 W/m*degree. Aluminum, under the same conditions, on the contrary, will increase the value from 202 to 422 W/m*deg. Aluminum alloys will also increase thermal conductivity as the temperature increases. Only the value of the coefficient will be an order of magnitude lower, depending on the amount of impurities, and range from 100 to 180 units.

Copper, with a temperature change within the same limits, will reduce thermal conductivity from 393 to 354 W/m*deg. At the same time, copper-containing brass alloys will have the same properties as aluminum ones, and the thermal conductivity value will vary from 100 to 200 units, depending on the amount of zinc and other impurities in the brass alloy.

The thermal conductivity coefficient of pure nickel is considered low; it will change its value from 67 to 57 W/m*deg. Alloys containing nickel will also have a coefficient with a reduced value, which, due to the content of iron and zinc, ranges from 20 to 50 W/m*deg. And the presence of chromium will reduce the thermal conductivity in metals to 12 units, with a slight increase in this value when heated.

Application

The state of aggregation of materials has a distinctive structure of molecules and atoms. This is what has a great influence on metal products and their properties, depending on their purpose.

The different chemical composition of components and parts made of iron allows them to have different thermal conductivities. This is due to the structure of metals such as cast iron, steel, copper and aluminum. The porosity of cast iron products promotes slow heating, and the density of the copper structure, on the contrary, accelerates the heat transfer process. These properties are used for rapid heat removal or gradual heating of inert products. An example of using the properties of metal products is:

• kitchen utensils with various properties;
• pipe soldering equipment;
• irons;
• rolling and sliding bearings;
• plumbing equipment for heating water;
• heating devices.

Copper tubes are widely used in radiators of automobile cooling systems and air conditioners used in everyday life. Cast iron radiators retain heat in the apartment, even with an inconsistent supply of coolant at the required temperature. And radiators made of aluminum contribute to the rapid transfer of heat to the heated room.

When high temperatures occur as a result of friction of metal surfaces, it is also important to take into account the thermal conductivity of the product. In any gearbox or other mechanical equipment, the ability to remove heat will allow the mechanism parts to maintain strength and not be subject to destruction during operation. Knowledge of the heat transfer properties of various materials will allow you to competently use certain alloys of non-ferrous or ferrous metals.

Source: https://prompriem.ru/metally/teploprovodnost.html

Metal induction heater. Principle of operation

The technology of induction heating of workpieces is in demand not only in hot stamping shops. Compact inductors are necessary, in particular, for car service centers engaged in the manufacture and repair of steel parts from profiled rolled products. Purchasing an industrial inductor is expensive. Is there an alternative?

How does an induction heater work?

To implement the induction heating process, a well-known physical principle is used, when, for deformation in a hot state, the workpiece is placed in the magnetic field of a ring-shaped inductor. Such a coil is powered by an electrical alternating current of frequency sharply higher than usual (50 or 60 Hz).

The operating principle of an induction heater is as follows. Eddy currents created in an electromagnetic field (they have another name - Foucault currents) produce heating of the metal. Direct contact between the workpiece and the heating element is not necessary; it is only important that the inductor evenly covers the heated surface of the metal. Using a transformer, the installation is connected to a generator, which provides the required power and frequency.

Induction heating can provide a relatively rapid increase in the temperature of surface layers. In particular, heating a bar stock with a cross-section of 3540 mm and a length of 140-150 mm will require about 2025 s.

Approximate ranges of correspondence between the best current frequency and the cross-section of a round bar are given in the table.

Diameter, mm	2040	4060	6080	80100	100120
Frequency, kHz	10040	4010	104	41	10,5

For strip metal, induction heating is less advantageous than for a round bar, since the distance between the inner diameter of the coil and the metal is not constant.

Typically, a frequency of 10 kHz is used, then the efficiency of the induction heater reaches its maximum. The frequency is adjusted depending on:

• required heating performance;
• temperature of the heated metal;
• cross-sectional dimensions.

The designs of industrial inductors are equipped with devices for automatic loading and unloading of heated workpieces. This is necessary because the interval between heating and plastic deformation of the metal is minimal.

The heating time of steel blanks is short: for a 20 mm section it is only 10 s, so the loss of metal into scale is insignificant.

DIY induction heater

There are a number of known designs of inductors made from a welding inverter, the operating principle of which can be used to induce Foucault eddy currents in metal.

Making a homemade inductor is as follows. First, you will need to make a durable case in which there will be a mounting unit for the heated workpiece. The case must be hardened so that it does not deform under the influence of possible impacts.

It is even better if the material is subjected to nitriding: in this case, two advantages are realized - an additional increase in hardness due to a more complete transformation of retained austenite into martensite, and an improvement in the skin effect, when a more powerful current flows along the outside of the workpiece.

Strength is assessed by spark test.

The next stage is the manufacture of the heating coil. It is made from individually insulated wires: in this case, power losses will be minimal. A copper tube is also suitable - it has a large surface area along which eddy currents will be induced, while the inductor’s own heating due to the high electrical conductivity of copper is practically absent.

After connecting the coil to the water cooling system and checking the pumping system, the inductor is ready for operation.

Working diagram

The heater includes the following components:

1. An inverter unit designed for a voltage of 220-240 V, with a current of at least 10 A.
2. Three-wire cable line (one wire is ground) with a normally open switch.
3. Water cooling system (it is highly advisable to use water purification filters).
4. A set of coils that differ in internal diameters and lengths (for limited volumes of work, you can get by with one coil).
5. Heating block (you can use a module with power transistors produced by Chinese companies Infineon or IGBT).
6. Damper circuit with several Semikron capacitors.

The high-frequency oscillation generator is the same as that of the base inverter. It is important that its operational characteristics fully comply with those indicated in the previous sections.

After assembly, the unit is grounded and the heating induction coil is connected to the inverter power supply using connecting cables.

Approximate operational capabilities of a homemade metal induction heater:

• Highest heating temperature, °C – 800.
• The minimum power of the inverter is 2 kVA.
• PV switch-on duration, no less than 80.
• Operating frequency, kHz (adjustable) - 1.05.0.
• Internal diameter of the coil, mm – 50.

It should be noted that such an inductor will require a specially prepared workplace - a waste water tank, a pump, and reliable grounding.

Source: https://proinstrumentinfo.ru/induktsionnyj-nagrevatel-metalla-iz-svarochnogo-invertora-shema/

Melting point of metals. The most refractory and fusible metal:

Almost all metals are solids under normal conditions. But at certain temperatures they can change their state of aggregation and become liquid. Let's find out what is the highest melting point of metal? Which is the lowest?

Melting point of metals

Most of the elements in the periodic table are metals. There are currently approximately 96 of them. They all require different conditions to turn into liquid.

The heating threshold of solid crystalline substances, above which they become liquid, is called the melting point. For metals it varies within several thousand degrees. Many of them turn into liquid with relatively high heat. This makes them a common material for making pots, pans and other kitchen utensils.

Silver (962 °C), aluminum (660.32 °C), gold (1064.18 °C), nickel (1455 °C), platinum (1772 °C), etc. have average melting points. There is also a group of refractory and low-melting metals. The first need more than 2000 degrees Celsius to turn into liquid, the second need less than 500 degrees.

Low-melting metals usually include tin (232 °C), zinc (419 °C), and lead (327 °C). However, some of them may have even lower temperatures. For example, francium and gallium melt in the hand, but cesium can only be heated in an ampoule, because it ignites with oxygen.

The lowest and highest melting temperatures of metals are presented in the table:

Refractory	Low-melting
Tungsten	3422 °C	Mercury	-38.87 °C
Rhenium	3186 °C	Gallium	26.79 °C
Tantalum	3017 °C	France	27 °C
Osmium	3033 °C	Cesium	28.5 °C
Molybdenum	2623 °C	Rubidium	39.31 °C
Niobium	2477°C	Potassium	63.5 °C
Iridium	2466 °C	Sodium	97.8 °C

Tungsten

Tungsten metal has the highest melting point. Only the nonmetal carbon ranks higher in this indicator. Tungsten is a light gray shiny substance, very dense and heavy. It boils at 5555 °C, which is almost equal to the temperature of the Sun's photosphere.

At room conditions, it reacts weakly with oxygen and does not corrode. Despite its refractoriness, it is quite ductile and can be forged even when heated to 1600 °C. These properties of tungsten are used for incandescent filaments in lamps and picture tubes and electrodes for welding. Most of the mined metal is alloyed with steel to increase its strength and hardness.

Tungsten is widely used in the military sphere and technology. It is indispensable for the manufacture of ammunition, armor, engines and the most important parts of military vehicles and aircraft. It is also used to make surgical instruments and boxes for storing radioactive substances.

Mercury

Mercury is the only metal whose melting point is minus. In addition, it is one of two chemical elements whose simple substances, under normal conditions, exist in the form of liquids. Interestingly, the metal boils when heated to 356.73 °C, and this is much higher than its melting point.

It has a silvery-white color and a pronounced shine. It evaporates already at room conditions, condensing into small balls. The metal is very toxic. It can accumulate in human internal organs, causing diseases of the brain, spleen, kidneys and liver.

Mercury is one of the seven first metals that man learned about. In the Middle Ages it was considered the main alchemical element. Despite its toxicity, it was once used in medicine as part of dental fillings, and also as a cure for syphilis. Now mercury has been almost completely eliminated from medical preparations, but it is widely used in measuring instruments (barometers, pressure gauges), for the manufacture of lamps, switches, and doorbells.

Alloys

To change the properties of a particular metal, it is alloyed with other substances. Thus, it can not only acquire greater density and strength, but also reduce or increase the melting point.

An alloy can consist of two or more chemical elements, but at least one of them must be a metal. Such “mixtures” are very often used in industry, because they make it possible to obtain exactly the qualities of materials that are needed.

The melting point of metals and alloys depends on the purity of the former, as well as on the proportions and composition of the latter. To obtain low-melting alloys, lead, mercury, thallium, tin, cadmium, and indium are most often used.

Those containing mercury are called amalgams. A compound of sodium, potassium and cesium in a ratio of 12%/47%/41% becomes a liquid already at minus 78 ° C, an amalgam of mercury and thallium - at minus 61 ° C.

The most refractory material is an alloy of tantalum and hafnium carbides in 1:1 proportions with a melting point of 4115 °C.

Source: https://www.syl.ru/article/374078/temperatura-plavleniya-metallov-samyiy-tugoplavkiy-i-legkoplavkiy-metall

Metal tarnish colors

Tarnish colors are a spectrum of colors formed on the surface of iron alloys as a result of the appearance of an oxide film. They are formed when metal surfaces are heated to certain temperatures without the participation of water. Tarnished colors are a defect in the welded joint.

Origin

In nature, tarnish colors form on the surface of many minerals, including pyrite and chalcopyrite. Due to oxidation, they become covered with a thin oxide film, which refracts sunlight. As a result of interference, the metal surface is painted in different colors.

The brightness of the tarnish depends on the thickness of the oxide film and the wavelength. The brightest tarnish colors form on copper minerals. Also, the color depends on the quality of the metal. If an element contains a large number of metal ions, it turns blue.

In the presence of chromophores, minerals turn red.

Tarnish colors can also form naturally on the surfaces of old glass or coins. Discoloration may be due to prolonged contact of these materials with the ground. If there is a fatty film on them, they turn rainbow-colored. The tarnish hides the real color of the metal. Therefore, it is impossible to determine its true color on a fresh fracture. It is recommended to determine the color by examining the oxide film.

Artificial tarnish colors are formed on the surface of metal workpieces during welding or hardening. They appear when metals are heated to critical temperatures without the participation of water molecules or other liquids. During heating, the formation of an oxide film occurs.

Its thickness is several molecules and decreases as it heats up. This is due to the phenomenon of diffusion - the process of penetration of the smallest particles of one chemical element into another. In this case, the interaction of metal and oxygen atoms occurs.

On carbon steels, oxide films appear faster than on alloy steels.

The process of coating steel and iron with a layer of oxide film is called bluing. After this procedure, the corrosion resistance of the product increases. Treated parts do not rust. The bluing procedure allows you to give the product a color, even if the metal surface, due to operating conditions, cannot be painted.

During bluing, the workpiece is rubbed with mineral oil and heated on an iron sheet. After the oil liquid burns out, tarnished colors appear on the workpiece. For the desired color, it is necessary to heat the part to the appropriate temperature. The resulting oxide layer is moisture resistant and is not exposed to air.

The rate of formation of oxide films is influenced by the following factors:

1. Surface structure: hardened parts oxidize at a faster rate.
2. Contamination of the product: surfaces coated with oil become charred during prolonged heating, which leads to the formation of soot. For this reason, an uneven and thin oxide film is formed.
3. Presence of roughness: if a workpiece with a rough surface is heated, the oxide film becomes dense. If a part is polished before the heat treatment procedure, a thin film of oxides is formed.
4. Heating equipment: If special heating furnaces are used during heat treatment, capable of maintaining a stable temperature, then the oxide film will be dense. At home, you can also use ovens, gas burners or metallurgical furnaces (forges).

Thin oxide films absorb light waves with a shorter wavelength, but reflect them with a longer wavelength. The color of metal parts changes depending on the temperature and density of the oxide film. The thicker the oxide film, the lighter the color. Blue or violet color is produced when the longest wavelengths are reflected from the spectrum.

If the oxide film reflects short wavelength waves, the metal surface turns yellow. Light colors correspond to a high heating temperature, light colors correspond to a lower heating temperature.

For this reason, many craftsmen often use tarnish colors to determine the degree of hardening of products, steel filings and cutting tools used during turning operations.

Despite these factors, it is impossible to accurately determine the temperature of the metal using tarnish colors, because the value of this indicator is influenced by the following factors:

• heating time: the period of time during which a metal part heats up to ambient temperature in the absence of heat transfer.
• the presence of various impurities in the metal composition;
• features of lighting in the room where welding or hardening of workpieces was carried out;
• heating rate: change in the temperature of a product per unit time when it is heated.

In modern industry, temperature control is carried out using special devices - pyrometers. They are equipped with special sensors that determine the degree of heating of the workpiece using a laser.

Tarnish colors are used in the manufacture of working tools, laser marking and external processing of iron, copper, aluminum and brass products.

If it is necessary to produce tools with a high density (razor blades, objects for surgical operations, cutting edges of incisors and grabbers), then the tarnish should be of a bright color: red, orange or yellow.

Tools used in the woodworking sector are heated to purple and green tones. To achieve elasticity when making saws, knives, forks and springs, it is necessary to heat the workpieces until blue or black colors appear.

During the heating process, the metal workpiece becomes flexible, which allows the master to give it the desired shape. After this process, the product is hardened at certain temperatures.

According to the recommendations of experts, the optimal temperature for hardening metals is 700–800 °C. In this case, the product is painted in different shades of red or pink. If these values ​​are exceeded by 300 °C, the workpiece turns orange or yellow.

At high temperatures, overheating occurs, which negatively affects the strength of the product.

Hardening improves the following parameters of the metal surface:

1. Hardness: This indicator is nominal. It is written in the Rockwell scale and measured in HRC. Hardness determines the degree of resistance of a metal to mechanical damage. When soft products come into long-term contact with other surfaces, marks remain, which impairs their cutting properties. The hardness of European knives is 60 HRC, Asian knives are 70 HRC.
2. Elasticity: this parameter determines the degree of deformation of the metal during bending and impact. If the steel is hardened, when bent by 10-30° it will return to its original position. When overheated, the elasticity of the surface decreases, which leads to tool breakage.
3. Wear resistance: this criterion shows the overall resistance of the metal (resistance to abrasive wear, resistance to heavy loads). With proper hardening, the product will be able to function stably for a longer period.

After hardening, the workpiece acquires high hardness. To restore its strength, it is necessary to carry out a tempering procedure, which is repeated heat treatment of the part. The metal product is heated to lower temperatures and cooled. Between hardening and cooling, the metal surface is also completely cooled by immersing it in a salt solution or oil. When choosing a vacation, you must consider the following features:

1. For products subject to deformation or shock loads, high-temperature tempering must be used: up to 700 °C.
2. For light blades, medium temperature tempering is used: up to 500 °C.
3. To ensure optimal hardness, low-temperature tempering is used: up to 250 °C. But in this case, the product will not be able to withstand high impact loads and will be easily deformed.

Temperature of colors of tarnish and heat

During the holidays, incandescent colors appear. From them you can determine to what temperature the workpiece has heated up. Unlike tarnish, heat colors change as the metal surface cools. The transition between colors is carried out in a strict sequence, but at a fast speed, so the master must carefully control the heat treatment process.

Steel tarnish color scale

The color of carbon parts at appropriate temperatures is indicated in the following steel tarnish color scale:

Temperature of tarnish colors for carbon steels

Color	Temperature limits, °C
Citric	220 – 229
Yellow (straw color)	230 – 245
Gold	246 – 255
Earthy or brown	256 – 264
Scarlet or red-orange	265 — 274
Purple	275 – 279
Amethyst	280 – 289
Heavenly	290 – 294
Twitter	295 – 299
Indigo Crayola	300 – 309
Light blue	310 – 329
Aquamarine	320 — 339

On workpieces made of stainless steel 12Х18Н10Т, containing 18% chromium, 10% nickel and 1% titanium (values ​​defined in GOST 5632-2014), tarnish colors are formed at other temperatures. This is due to the fact that this material is corrosion-resistant and heat-resistant. Therefore, during quenching and cooling, the smallest particles of metals and oxygen interact more slowly, which prevents the formation of an oxide film during quenching and heating.

The following table of tarnish colors shows the characteristics of color changes in stainless steel products:

Temperature of tarnish colors for stainless steels

Color	Temperature limits, °C
Light straw	300 – 399
Golden

Source: https://stankiexpert.ru/spravochnik/materialovedenie/cveta-pobezhalosti.html

Why is metal cold?

We have all noticed more than once that even in a warm room, metal objects still feel cold to the touch. Why is this happening? Why doesn't metal heat up on its own?

November 21, 2018

Let's start with the fact that metal objects are not always cold. Remember what a metal spoon becomes in hot water. For example, if you place a wooden spoon in boiling water, it will heat up. But a metal spoon that has been in boiling water will heat up much more. If handled carelessly, you can even get scalded by forgetting metal cutlery in a hot pot or frying pan.

Share the warmth

The secret lies in thermal conductivity - the ability of a body to transfer heat to another body, from more heated parts to less heated ones.

Different objects have different thermal conductivities. For metal it is extremely high. In practice, this can be confirmed by simply touching a metal object.

Take any metal object in your hand, for example the same spoon (that has not been in boiling water!) or metal keys. Our normal body temperature is 36.6°C. When we touch an object that is less hot than our body, we ourselves begin to transfer heat to it. The surface temperature of the skin becomes lower, and we feel the coldness of the object.

Such different thermal conductivities

Our body heat begins to heat the top layer of a cool object. If an object has high thermal conductivity (like our metal spoons or keys), then the energy begins to rapidly spread throughout the entire object. The temperature increases slightly, heat transfer continues. However, the object remains cold.

If the object has low thermal conductivity (for example, like our wooden spoon), then the upper layers heat up much faster. Often, heating occurs instantly, and we do not even have time to notice that the object was cool. Once the heat is transferred, heat transfer practically stops. The object became warm.

What happens to hot bodies?

In hot objects, processes occur in a different order. The thermal conductivity of metallic bodies is high due to free electrons responsible for metallic electrical conductivity. Electrons in metal bodies move rapidly throughout the entire volume, transferring heat to all parts of the object.

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Source: https://rosuchebnik.ru/material/pochemu-metall-kholodnyy/