Materials science

Microstructure of low-carbon (cementable) steels
Carbon steels include steels with a carbon content of up to 0.25%. 30 XGT, 18 XNM, 12 XNZ, 18 X2N4V.
Carbon steels have a ferrite-pearlite structure in an annealed state. Alloyed low-carbon steels have a ferrite-pearlite structure after annealing, and a low-carbon martensite after quenching.
Microstructure of 18 XGT brand steel is: a) ferrite-pearlite after softening, b) after tempering – low-carbon martensite.
Microstructures of carbon steels
Such steels include steels containing from 0.25% to 0.6% carbon. Carbon grades 35, 40, 45, 50 and alloyed 40X, 40XR, 30XN, 40XGR, 30XGT, 30XGS, 40XN, 40XNR, 40XGNR, 40XNM, 42XMF, 30XNZ, 30XN 23F, 38XNZMF.
The total amount of alloying elements in these steels does not exceed 3-5%.
Typical thermal treatment of such steels consists of quenching and high (high) tempering (350º-650ºС).
The structure of carbon steels in the annealed state is ferrite-pearlite, and in the hardened state it consists of martensite.
The microstructure of most medium-carbon alloy steels after annealing is ferrite-pearlite, and after refinement (from annealing and high tempering) sorbitol.
MICROSTRUCTURE OF SPRING-RESTRESS STEEL
Carbon content for springs and springs was 0.5-0.7%
Carbon steels are used, and most manganese and silicon are added to them. 50 XF grade steel is used for special springs. Chromium and vanadium are added to this steel, which gives it high elasticity properties. Brands of spring steels: 65, 70, 75, 65G, 50 S2, 55 S2, 60 S2, 70 S3A, 60 SG, 50 XFA, 50 XGFA,
60 S2XA.
Springs and springs are thermally treated in oil or water at 800-850ºС in order to obtain a high elastic limit, then released at 350-500ºС (depending on the grade of steel) and a trostite structure is obtained.
Bearing steels .
In order for bearing steel to readily accept tempering (ie have a low critical tempering rate), oil can be used as the tempering medium, and the steel is alloyed (usually with chromium).
According to GOST 801-60, four grades of steel were obtained for ball bearings: SHX6, SHX9, SHX15, SHX15SG.
Thermal processing of ball bearing details (balls, rollers, rings) consists of two main operations, i.e. pressing and releasing.
The forging is in oil, heated to 630º-640º C, and 150-160º C is released for 1-2 hours, which provides the steel with a hardness not lower than NRS62. And the structure should be in the form of very fine needle-like martensite, which is freed with excess carbide, distributed uniformly. If the temperature regime is not observed during short-term heat treatment, the quality of the bearings will decrease, that is, their durability will be affected.


ASSIGNMENT

1. 
   Study of microstructures of structural steels.
   
2. 
   Describe the studied microstructure in the form of a scheme and write an explanatory letter.
   
3. 
   Make a report on the work.