Materials science

Review questions:

• 
  What kind of structure is formed in poats after they are found?
  
• 
  What do you mean by depth of field and what does it depend on?
  
• 
  What is critical velocity and critical diameter?
  
• 
  How does the depth of penetration affect the properties of the finished part?
  
• 
  Why is the depth of penetration of steels determined by the method of end penetration?
  

7- LABORATORY WORK


SUBJECT OF TRAINING : MICROANALYSIS OF STRUCTURAL STEEL.
Purpose of the work: Independent microanalysis of structural steels and determination of their structural composition depending on the appearance of the structure.
In the course of work: to study the composition of the cleaning solutions used to reveal the microstructure, the characteristics of structural steels.


BRIEF THEORETICAL INFORMATION
Structural steels refer to steels used in the preparation of details of machines, structures and structures.
Steels used in constructions and structures are called structural steels. There is no sharp boundary between the chemical composition of construction steels and engineering steels.
Structural steels can be carbon and alloy steels. The carbon content of such steels does not exceed 0.5-0.6%. Structural steel has very fine, plastic and ductile properties. Therefore, structural steels have good technological properties. Steel must be well-worked under pressure (rolling, stamping, hammering, etc.) and cutting, well weldable. At the same time, the steel should be highly malleable and less prone to deformation, cracking (cracks) during tempering.


TYPES OF STRUCTURAL STEEL .
- carbon structural steels,
- alloy structural steels,
- very fine martensitic wear structural steels, low
alloy structural steels,
- intended for casting,
- spring-spring steel,
- ball bearing steel,
- corrosion-resistant austenite steel,
- stainless structural steels,
- heat-resistant structural steel and alloys.
The use of alloy steel as a construction material-
The reason for this is that carbon steels have high mechanical properties in small sections after heat treatment. Due to the fact that carbon steels are not deep enough, they cannot meet all the requirements for use in the production of parts with a diameter (thickness) of more than 20-25 mm.
The presence of alloying elements in steel increases the positive effect of heat treatment in improving their mechanical properties
ensures that it is fuller and more perfect.
Because alloying elements increase the ductility of steel
the strength of details also increases.
Thus, the alloying elements improve the mechanical properties of the part by increasing its ductility. The positive effect of alloying elements increases with the increase of the detail section.
Therefore, alloy steels are used only after heat treatment. It is not rational to use alloyed and therefore expensive steels without heat treatment.
The mode of thermal treatment of structural steels mainly depends on the amount of carbon in it.
Structural steels are subjected to two types of strengthening heat treatment: quenching and tempering. It should also be mentioned that medium carbon steels are usually heat treated with high release and low carbon steel with low release.
The tempering regime is determined by the condition of the critical points and the supercooling ability of austenite.
In steels with few alloying elements (for example: chrome-nickel tungsten or chrome-nickel molybdenum steels), the transition of austenite to pearlite is so slow that air cooling of large-sized parts causes the transfer of austenite to martensite. reaches the melting temperature. Another important aspect is that the martensitic structure of alloy steels can be achieved with slow cooling compared to carbon steels, because slow cooling also creates internal stress, which is a factor that increases structural strength.
We consider the microanalysis of structural steels in the following steels: low-carbon (cemented), medium-carbon (hardening), spring-spring and ball bearing steels, because the thermal treatment mode and other mechanical properties of these steels are different. close to or similar to structural steels.