Fergana polytechnic insitute mechanical engineering faculty department of mechanical engineering and automation

LECTURE – 5. THERMALLY ACTIVATED PROCESSES AND 
DIFFUSIONS IN SOLIDS. 
5.1. Rate processes in solids. 
5.2. Atomic diffusions in solids. 
5.3. Industrial applications of diffusion processes. 
5.4. Effects of temperature on diffusion in solids. 
Keywords:
Thermal performance; critical points, "iron-cementite" phase diagram: 
phase changes on heating and cooling: ferrite, austenite, pearlite, cementite: grain 
size: coarse-grained and fine-grained steel: diffusion; isothermal diagram, critical 
cooling rate; sorbite, troostite, bainite, martensite; discharge fragility. 
When developing the technology of thermal treatment of steel, the following 
should be determined: 
1) heating mode (T:Vgirl; Vsov, 

t 
): 
2) the influence of the heating environment; 
3) cooling conditions; 
4) is the work : 
5) economic advantage of the selected technology. 
For heat treatment, steels are heated in flame furnaces, electric furnaces, salt 
baths, liquid metals. It is necessary to protect steels from oxidation and decarburization 
during heating. 
Russian scientist AA Bochvar was the first to classify types of thermal operation: 
1. The first type of softening. This group includes recrystallization softening and 
diffusion softening. 
II. The second type is softening. Phase changes occur in steels during heating 
and cooling in the annealing cycle. The purpose of the second type of softening is to 
create a balanced structure. This group includes full and partial softening. 
III. Finding steels. The purpose of the acquisition is to create an unbalanced, 
unstable state. This group of thermal performance includes full and partial heating. 

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IV. Discharge. Tempering refers to the process of heating the steel from the 
phase change temperature to a lower temperature and making the steel more stable. 
Such heat treatment increases the viscosity of steel. 
The purpose of these thermal operations is to create a balanced, stabilized 
structure. 
The following steel softening methods are used to achieve the intended goal: 
A) Recrystallizing softening. This method is used to eliminate the effect of cold 
plastic deformation. For this purpose, the steel subjected to welding is heated to a 
temperature of A 
ci ;
b) Diffusion softening. This method is used to eliminate dendrite liquefaction in 
steels. Heating temperature 1100...1150 
0 
C; 
c) Full softening. Such softening is used to create a balanced structure. Softening 
is carried out t=A 
c3 
+ (30-50 
0 C);
g) premature softening. Such softening is mainly used to reduce internal stresses 
and to form granular pearlite. Steel A 
ci 
-A 
s3 in cold tempering
 
heated to a range of 
temperatures; 
d) Freezing softening. This method is carried out at a temperature higher than 
A 
for post-eutectoid steels . After such treatment, the cementite of pearlite has a granular 
form; 
e) Isothermal softening. The formation of a homogeneous ferrite cementite 
mixture is carried out at a uniform, constant, temperature higher than A 
s3 
or A 
s1 . 
This 
type of annealing is advantageous from an economic point of view, because in this 
method
it is easy to control the temperature, depending on the cooling rate . Isothermal 
softening is more commonly used for alloy steels: 
j) Normalization. Thermal treatment is performed by heating this type of steel to 
temperatures higher than A 
s3 
by 30...50 
0 
s and then consisting of air cooling. 
Normalizing heat treatment is often the final heat treatment for steels. 
In all types of annealing, the steel heated to the optimum temperature is held at 
this temperature for a certain period of time to complete the phase changes, and then 
the steel is cooled slowly (often with a furnace). 

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