Конструкциялық материалдар және термоөңдеу Конструкционные материалы и термообработка Constructional materials and heat treatment Учебное пособие для специальности: 5В071200– «Машиностроение»

171 
 
 
Lecture № 1. 
Properties of materials. 
1. Physical properties. 
2. Mechanical properties. 
3. Chemical properties. 
4. Technological properties 
5. Structure of alloys. Polymorphic transformations in metals. Corrosion of 
metals 
Physical properties. 
Physical properties of materials belong to: 
The density is a mass of substance in the unit of volume. 
Color - ability of materials to reflect light rays with various refraction. 
Heat conductivity - ability of the material to carry heat through itself. 
Electrical conduction - ability of the material to carry electrical current 
through itself. 
Freeze resistance - ability of the material to maintain repeated alternate 
freezing and thawing. 
Flame resistance - ability of the material to maintain long influence of water 
and flame in the conditions of the fire. 
Ability to be magnetized – ability of the material to get magnetic properties 
under the influence of an external magnetic field. 
Mechanical properties. 
Mechanical properties of materials belong to : 
Durability is an ability of the material to maintain internal tension arising 
under the influence of the external loadings. 
Elasticity is an ability of the material to change the form under the influence of 
external loading and to restore it after loading removal. 
Plasticity is an ability of the material to change the form under the influence of 
loading and not to restore it after removal of this loading. 
Fragility is an ability of the material not to change form under the influence of 
loading to certain limit after which occurs material destruction. 
Hardness is an ability of the material to resist to introduction of other firmer 
material. 
Wear resistance - ability of the material to maintain an abrasive loadings 
action a long time. 
Chemical properties. 
Chemical properties of materials belong to: 
Solubility is an ability of the material to be dissolved in any liquid solvent. 

172 
Corrosion resistance is an ability of the material to maintain influence of 
external hostile environment. 
Alkali-resistance - is an ability of the material to maintain effect of alkalis. 
Acid-resistance - is an ability of the material to maintain effect of various 
acids. 
Gas-resistance - is an ability of the material to maintain effect of various 
gases. 
Technological properties. 
Technological properties of materials belong to: 
The machinability is an ability of the material to give in to processing by 
cutting tools. 
Castability - is an ability of the materials to fill a casting mold in the melted 
condition. 
The malleability is an ability of the materials to give in to processing by 
pressure without destruction. 
Weldability - is ability of the materials to joint by means of welding without 
cracks, leakage and other defects. 
Structure of alloys.
Mechanical mix. 
Pure metals as constructional materials aren't almost applied. They alloy with 
other metals and nonmetals for improving of mechanical properties, corrosion 
resistance and obtaining special physical properties. Properties of the received 
alloy will depend on its structure. 
The following types of alloys can be formed at the joint crystallization of 
several elements : mechanical mix, firm solution and chemical compound. 
Possibility of this or that type of an alloy is defined by nature of interaction of 
elements in the course of crystallization. Let's consider a structure of these 
alloys in case of joint crystallization of two elements A and B. 
The mechanical mix is formed at separate crystallization of components. The 
structure of an alloy will consist of crystals of substance A and B 
communication between which is carried out on borders of grains. 
Firm solution. 
Firm solution is formed when each crystal of an alloy is under construction by 
the atoms of both substances. Only one of components builds a crystal lattice, 
and other component takes place in this lattice in an atomic look. Depending on 
nature of placement distinguish firm solutions of replacement and introduction. 
At formation of firm solution of replacement atoms of one of components, for 
example B, partially replace atoms of a component A in knots of its crystal 
lattice. Firm solution of introduction is formed, when atoms of one of 
components take place in interstices of a crystal lattice of another. 
Firm solutions of replacement can be formed at joint crystallization of metals, 
and firm solutions of introduction at joint crystallization of metal with nonmetal, 

173 
for example iron with carbon. Firm solutions can be designated by letters of the 
Greek alphabet α, β, γ, etc. So, for example, firm solution on the basis of a 
crystal lattice of substance A can be designated with A (B) or α. The structure of 
such alloy consists of identical crystals of firm solution and under a microscope 
looks as structure of a pure metal or a chemical compound. Values of 
properties of an alloy - firm solution can be both above and below properties of 
initial components. 
Chemical compound. 
The chemical compound is formed when crystallizing components mutually 
chemically are active. Every arising crystal of an alloy will be under 
construction with atoms of substance A and B in the proportion defined by the 
formula of connection. The crystal lattice of a chemical compound will be new, 
unlike the crystal lattice of initial components. Such alloy strongly differs from 
the components forming it by properties 
The structure of an alloy defines its properties therefore it is important to know 
how this structure will change at temperature variations and alloy composition. 
Dependence between of an alloy structure, its composition and temperature is 
described by means of state diagram. 
Polymorphic transformations in metals. 
Many metals can exist in the different crystal forms depending on temperature 
or in different updating. As a result of polymorphic transformation atoms of the 
crystal body having a lattice of one type, are reconstructed in this way that the 
crystal lattice of other type is formed. The polymorphic updating which steady 
at lower temperature is accepted to designate a letter α, at higher β, then γ etc 
for the majority of metals. 
Transition of pure metal from one polymorphic updating in another in the 
conditions of balance proceeds at constant temperature (a critical point) and is 
accompanied by heat release if transformation goes at cooling, and heat 
absorption in case of heating. 
As a result of polymorphic transformation the new crystal grains having other 
size and form are formed. Therefore such transformation is called 
recrystallization. 
Polymorphic transformations occur in the pure metals, in alloys, in chemical 
compounds. 
Polymorphic transformation is accompanied by spasmodic change of all 
properties of metals and alloys: specific volume, thermal capacity, heat 
conductivity, electrical conduction, magnetic properties, mechanical and 
chemical properties etc. 
Corrosion of metals.
Metal materials contact with the environment surrounding, are exposed with 
this speed to destruction. Metals enter oxidation-reduction reactions with the 

174 
substances being in environment, and are oxidized. It is the reason of 
destruction. 
The spontaneous destruction of metal materials occurring under chemical 
influence of environment, is called as corrosion. 
Main types of corrosion are:
- Corrosion in gases (gas corrosion) -
- Corrosion in solutions of electrolytes (electrochemical corrosion). 
Corrosion in gases occurs at the increased temperatures when moisture 
condensation on a surface of metal is impossible. Details of internal combustion 
engines, fittings of furnaces etc. are exposed to gas corrosion. Gas corrosion is 
undergone by the metal subjected to thermal processing. As a result of gas 
corrosion on a surface of metal oxides, sulfides and other components are 
formed. 
All cases of corrosion belong to electrochemical corrosion in water solutions 
and corrosion of the metal being in the damp atmosphere. As a result of 
electrochemical corrosion oxidation of metal can lead to formation of insoluble 
products (for example, rusts) and to transition of metal to solution in the form of 
ions. 
Speed of corrosion depends by nature of metal and an oxidizer, on concentration 
of an oxidizer, and also from the content of various impurity in metal and in the 
corrosion environment - in the atmosphere and in solution. 
To protection against corrosion is applied the various methods. The most 
important of which are: 
- application of chemically resistant alloys., 
- protection of a surface of metal by coverings., 
- processing of the corrosion environment., 
- electrochemical methods. 
For manufacturing of the equipment which is treating to effect of corrosion 
gases, apply heat resisting alloys: heat resisting steel and iron, alloys on the 
basis of nickel or cobalt. From chemically resistant alloys most widely use 
corrosion-proof and acid proof steel. 
The coverings applied to protection of metals, are subdivided on: 
- metal which form on the surface protective films (chrome, nickel, aluminum, 
etc.)., 
- nonmetallic coverings varnishes, paints, enamels, phenol-formaldehyde and 
other pitches., 
- the coverings created by chemical or electrochemical processing of metal, 
representing protective oxide or salt films (the aluminum oxide-formation, a
steel phosphate coating). 
The method of processing of environment consists at a distance of solution in 
which the protected detail is maintained, dissolved oxygen or in addition to this 
solution of the substances which are slowing down corrosion, - inhibitors. It is 
applied, when the volume of liquid is limited. 

175 
Electrochemical methods are applied in the environments which are well 
carrying out an electric current. 
Figure 1.1. Structure of the steel ingot casting from quiet steel. 
1-external layer consisting of small crystals. 
2 zone of the column extended to the center of an ingot of crystals. 
3-large randomly focused crystals. 
4-shrinkable emptiness in friability. 

жүктеу/скачать 1,99 Mb.

Достарыңызбен бөлісу: