Physical properties of metals. Methods for melting non-ferrous metals: melting point, density and specific volume

Density and melting point of some metals.

For metals, the following properties are most characteristic:
*metallic sheen
*hardness,
*plastic,
*ductility,
*good conductivity of heat and electricity.

All metals have a metallic crystal lattice:
positively charged ions are located at its nodes, and electrons move freely between them.
The presence of free electrons explains the high electrical and thermal conductivity, as well as the ability to be machined.

Thermal conductivity and electrical conductivity decrease in the series of metals:
Ag Cu Au Al Mg Zn Fe Pb Hg

All metals are divided into two large groups:

Black metals
They have a dark gray color, high density, high melting point and relatively high hardness.
Iron is a typical representative of ferrous metals.

Non-ferrous metals
They have a characteristic color: red, yellow, white; have high plasticity, low hardness, relatively low melting point.
A typical representative of non-ferrous metals is copper.

According to their density, metals are divided into:
*Lungs(density not more than 5 g/cm)
Light metals include: lithium, sodium, potassium, magnesium, calcium, cesium, aluminum, barium.
The lightest metal is lithium 1l, density 0.534 g/cm3.
*heavy(density greater than 5 g/cm3).
Heavy metals include: zinc, copper, iron, tin, lead, silver, gold, mercury, etc.
The heaviest metal is osmium, density 22.5 g/cm3.

Metals vary in their hardness:
*Soft: cut even with a knife (sodium, potassium, indium);
*Solid: metals are compared in hardness with diamond, whose hardness is 10. Chrome is the hardest metal, it cuts glass.

Depending on the melting point, metals are conditionally divided into :
*fusible(melting point up to 1539°C).
Low-melting metals include: mercury - melting point -38.9°C; gallium - melting point 29.78°C; cesium - melting point 28.5°C; and other metals.
*Refractory(melting point above 1539 C).
Refractory metals include: chromium - melting point 1890°C; molybdenum - melting point 2620°C; vanadium - melting point 1900°C; tantalum - melting point 3015°C; and many other metals.
The most refractory metal is tungsten - melting point 3420°C.

Steel is an alloy of iron to which carbon is added. Its main use in construction is strength, because this substance retains its volume and shape for a long time. The thing is that the particles of the body are in a position of equilibrium. In this case, the force of attraction and the force of repulsion between the particles are equal. The particles are in a clearly defined order.

There are four types of this material: ordinary, alloyed, low-alloyed, high-alloyed steel. They differ in the amount of additives in their composition. The usual contains a small amount, and then increases. Use the following additives:

• Manganese.
• Nickel.
• Chromium.
• Vanadium.
• Molybdenum.

Steel melting points

Under certain conditions, solids melt, that is, they become liquid. Each substance does this at a certain temperature.

• Melting is the process of changing a substance from a solid to a liquid state.
• The melting point is the temperature at which a solid crystalline substance melts into a liquid state. Denoted t.

Physicists use a specific table of melting and crystallization, which is given below:

Based on the table, we can safely say that the melting point of steel is 1400 ° C.

Stainless steel is one of the many iron alloys found in steel. It contains 15 to 30% Chromium, which makes it rust-resistant, creating a protective layer of oxide on the surface, and carbon. The most popular brands of this steel are foreign. These are the 300th and 400th series. They are distinguished by their strength, resistance to adverse conditions and plasticity. The 200th series is of lower quality, but cheaper. This is an advantageous factor for the manufacturer. For the first time, its composition was noticed in 1913 by Harry Brearley, who conducted many different experiments on steel.

At the moment, stainless steel is divided into three groups:

• heat resistant- at high temperatures it has high mechanical strength and stability. The parts that are made from it are used in the fields of pharmaceuticals, the rocket industry, and the textile industry.
• Rust resistant- has a high resistance to rusting processes. It is used in household and medical devices, as well as in mechanical engineering for the manufacture of parts.
• heat resistant- is resistant to corrosion at high temperatures, suitable for use in chemical plants.

The melting point of stainless steel varies depending on its grade and the amount of alloys from approximately 1300 °C to 1400 °C.

Cast iron is an alloy of carbon and iron, it contains impurities of manganese, silicon, sulfur and phosphorus. Withstands low voltages and loads. One of its many advantages is its low cost for consumers. Cast iron is of four types:

The melting points of steel and cast iron are different, as stated in the table above. Steel has a higher strength and resistance to high temperatures than cast iron, the temperatures differ by as much as 200 degrees. In cast iron, this number ranges from approximately 1100 to 1200 degrees, depending on the impurities it contains.

The melting point of a metal is the minimum temperature at which it changes from solid to liquid. During melting, its volume practically does not change. Metals are classified by melting point depending on the degree of heating.

fusible metals

Fusible metals have a melting point below 600°C. These are zinc, tin, bismuth. Such metals can be melted in by heating them on the stove, or using a soldering iron. Fusible metals are used in electronics and engineering to connect metal elements and wires for the movement of electric current. The temperature is 232 degrees, and zinc - 419.

Medium melting metals

Medium-melting metals begin to change from a solid to a liquid state at temperatures from 600°C to 1600°C. They are used to make slabs, rebars, blocks and other metal structures suitable for construction. This group of metals includes iron, copper, aluminum, they are also part of many alloys. Copper is added to precious metal alloys such as gold, silver, and platinum. 750 gold contains 25% alloy metals, including copper, which gives it a reddish tint. The melting point of this material is 1084 °C. And aluminum begins to melt at a relatively low temperature of 660 degrees Celsius. It is a light, ductile and inexpensive metal that does not oxidize or rust, so it is widely used in the manufacture of utensils. The temperature is 1539 degrees. It is one of the most popular and affordable metals, its use is widespread in the construction and automotive industries. But in view of the fact that iron is subject to corrosion, it must be further processed and covered with a protective layer of paint, drying oil, or moisture should not be allowed to enter.

Refractory metals

The temperature of refractory metals is above 1600°C. These are tungsten, titanium, platinum, chromium and others. They are used as light sources, machine parts, lubricants, and in the nuclear industry. They are used to make wires, high-voltage wires and are used to melt other metals with a lower melting point. Platinum begins to change from solid to liquid at 1769 degrees, and tungsten at 3420°C.

Mercury is the only metal that is in a liquid state under normal conditions, namely, normal atmospheric pressure and average ambient temperature. The melting point of mercury is minus 39°C. This metal and its fumes are poisonous, so it is only used in closed containers or in laboratories. A common use of mercury is as a thermometer to measure body temperature.

Each metal and alloy has its own unique set of physical and chemical properties, not least of which is the melting point. The process itself means the transition of the body from one state of aggregation to another, in this case, from a solid crystalline state to a liquid one. To melt a metal, it is necessary to supply heat to it until the melting point is reached. With it, it can still remain in a solid state, but with further exposure and an increase in heat, the metal begins to melt. If the temperature is lowered, that is, part of the heat is removed, the element will harden.

Highest melting point among metals belongs to tungsten: it is 3422C o, the lowest is for mercury: the element melts already at - 39C o. As a rule, it is not possible to determine the exact value for alloys: it can vary significantly depending on the percentage of components. They are usually written as a number span.

How is it happening

The melting of all metals occurs in approximately the same way - with the help of external or internal heating. The first is carried out in a thermal furnace, for the second, resistive heating is used with the passage of an electric current or induction heating in a high-frequency electromagnetic field. Both options affect the metal in about the same way.

As the temperature increases, so does amplitude of thermal vibrations of molecules, structural lattice defects appear, which are expressed in the growth of dislocations, hopping of atoms, and other disturbances. This is accompanied by the breaking of interatomic bonds and requires a certain amount of energy. At the same time, a quasi-liquid layer is formed on the surface of the body. The period of destruction of the lattice and the accumulation of defects is called melting.

Depending on the melting point, metals are divided into:

Depending on the melting point choose and melting apparatus. The higher the score, the stronger it should be. You can find out the temperature of the element you need from the table.

Another important value is the boiling point. This is the value at which the process of boiling liquids begins, it corresponds to the temperature of saturated steam that forms above the flat surface of the boiling liquid. Usually it is almost twice as high as the melting point.

Both values ​​are given at normal pressure. Among themselves they directly proportional.

1. The pressure increases - the amount of melting will increase.
2. The pressure decreases - the amount of melting decreases.

Table of fusible metals and alloys (up to 600C o)

Table of medium-melting metals and alloys (from 600С o to 1600С o)

The melting points of almost all currently widely used metals are given in Table. 1. There are also mentioned some rare metals, the production and use of which is constantly growing. As you can see, the melting point of metals covers a very large range from -39 (mercury) to 3400 °C (tungsten).
Metals with a melting point below 500-600 ° C are called fusible. Low-melting metals include zinc and all other metals located in Table. 1 above him. It is also customary to single out the so-called refractory metals, referring to them those that have a higher melting point than iron (1539 ° C), i.e., according to Table. 1 is titanium and further to tungsten.

From the data in Table. 1 shows that the densities of metals at room temperature also have a very wide range. The lightest metal is lithium, which is about 2 times lighter than water. In technology, it is customary to single out a group of light metals that serve as the basis for structural metal materials in aviation and rocket science. Light metals include those whose density does not exceed 5 g/cm3. This group includes titanium, aluminum, magnesium, beryllium, lithium.
Along with the density, denoted by the letter d, the inverse value is used to describe the properties of metals - the specific volume v = 1d (cm3 g).
With increasing temperature, the density of all metals in the solid state decreases, and the specific volume increases accordingly. An increase in the specific volume of a solid metal that does not undergo polymorphic transformations upon heating by Δt can be quite accurately described by the linear dependence vtvt=vtv20°C (1+βtv Δt), where βtv is the temperature coefficient of volume expansion. As is known from physics, βtv=3α, where α is the temperature coefficient of linear expansion in a given temperature range. For most metals, heating from room temperature to the melting temperature causes an increase in volume by 4-5%, so that dtvtmelt = 0.95/0.96dtv20°C.
The transition of a metal to a liquid state is accompanied in most cases by an increase in volume and a corresponding decrease in density. In table. 1 this is expressed through the change in specific volumes Δv = 100 (vl - vtv)/vl, where vl and vtv are the specific volumes of liquid and solid metal at the melting temperature. It can be shown that Δv \u003d 100 (vl - vtv) / vl \u003d Δd \u003d 100 (dtv - dl) / dtv. The decrease in density during melting is expressed as a few percent. There are several metals and non-metals that exhibit an inverse change in density and specific volume upon melting. Gallium, bismuth, antimony, germanium, silicon decrease in volume during melting, and therefore their Δv has a negative value. For comparison, it can be noted that for Veda Δv = -11%.
A slight change in the volume of metals during melting indicates that the distances between atoms in a liquid metal differ little from the interatomic distances in the crystal lattice. The number of nearest neighbors for each atom (the so-called coordination number) in a liquid is usually slightly less than in a crystal lattice. For metals with close-packed structures, the coordination number during melting decreases from 12 to 10-11, for metals with o. c. structure, this number changes from 8 to 6. In a liquid metal near the melting point, the short-range order is preserved, in which the arrangement of neighboring atoms at a distance of up to about three atomic diameters remains similar to what it was in the crystal lattice, which, as is known, has also far away. During melting, metals do not observe a fundamental change in a number of properties: thermal conductivity, heat capacity; the electrical conductivity remains of the same order as in a solid metal near the melting point.
An increase in the temperature of a liquid metal causes not only a gradual change in all its properties, but also leads to gradual structural rearrangements, which are expressed in a decrease in the coordination number and the gradual disappearance of short-range order in the arrangement of atoms. The increase in the specific volume of liquid metal caused by an increase in temperature can be approximately described by the linear dependence vzht = vzhtpl (1 + βl Δt). The temperature coefficient of volumetric expansion of liquid metal is significantly greater than that of solid metal. Usually βl = 1.5/3βtv.
Alloys, both in the solid and in the liquid state, are generally not perfect solutions, and the fusion of two or more metals is always associated with a change in volume. As a rule, there is a decrease in the volume of the alloy compared to the total volume of pure components, taking into account their content in the alloy. However, for technical calculations, the decrease in volume during fusion can be neglected. In this case, the specific volume of the alloy can be determined by the rule of additivity, i.e., from the values ​​of the specific volumes of pure components, taking into account their content in the alloy. Thus, the specific volume of the alloy, which consists of components A, B, C, ..., X, contained in percentage by weight in the amount of a, b, c, ..., x is

where vA, vB, vC, vX are the specific volumes of pure components at the temperature for which the specific volume of the alloy is calculated.
The change in the volume of liquid metal before and during crystallization predetermines the most important casting property - volumetric shrinkage, which manifests itself, as will be shown later, in the form of shrinkage cavities and porosity (looseness) in the body of the casting.
The maximum possible value of the relative volumetric shrinkage of the casting is equal to Δvmax = 100 (vЖt - vТвtmelt)/vЖt, where vЖt is the specific volume of liquid metal at pouring temperature t; ttvtpl - specific volume of solid metal at the melting point.
The experimentally detected volumetric shrinkage in castings is usually less than Δvmax. This is explained by the fact that when the mold is filled, the melt cools and crystallization may even begin, so the initial state of the melt in the mold is not characterized by the specific volume vtl. Cooling the hardened casting to room temperature does not affect the relative volumetric shrinkage.
In castings from metals and alloys with negative values ​​of Δv (see Table 1), it is not shrinkage that is found, but the so-called growth - extrusion of the melt onto the surface of the castings.