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

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Fig. 6.1. Iron diagram with alloying elements: a - with nickel, - b- with chrome. 
. 
Fig. 6.2. Scheme of heat treatment of quick cutting steels. 
Lecture № 7. 
Heat treatment. 
1. Phase transformations at heat treatment. 
2. Influence of heat treatment on mechanical properties 
3. Types of heat treatment. 
4. Annealing and normalization. 
5. Training. Superficial training. 
Phase transformations at heat treatment. 
Main types of heat treatment are heat hardening, drawback and annealing. The 
majority of the structural changes which are taking place at heat treatment is 
directly connected with the processes described by lines of the chart iron-carbon. 
Therefore also modes of heat treatment are in most cases connected with 
position of these lines. 
In the theory and practice of heat treatment symbols of critical points of the 
chart are widely applied. So, the critical points corresponding to the PSK line, 

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i.e. pearlitic transformation, are designated as A1, the GOS lines, i.e. the 
beginnings of allotropic transition, is designated as A3, the ES line 
corresponding to change of a limit of saturation, as Acm. Critical temperature of 
transformation when heating is slightly higher, than when cooling. Therefore, 
specifying a critical point, it should be noted, whether it is received when 
cooling or when heating. So, pearlitic transformation when heating will be 
designated as Ac1, and when cooling Ar1, the same belongs and to A3 point. 
Any process of heat treatment of metal consists of heating, endurance and 
cooling. For example we will consider a curve of heating of U8 steel. On 
structure this steel corresponds to a pearalitic point and at low temperatures has 
pearlitic structure. When heating to temperature 727?С there is the allotropic 
transformation leading to formation of austenite. Emergence of austenitic grains 
begins on border of ferritic and cementite phases and comes to an end at a 
constant temperature. Fine grains of austenite are as a result formed, i.e. upon 
transition through a critical point steel gets more fine-grained microstructure, 
than initial. For steel proevtectoid when heating As1, except austenite is higher, 
there will be also a ferrite which completely will turn into austenite only at 
temperatures above the top critical point of As3. At hypereutectoid steel when 
heating to As1 also instead of pearlitic sites grains ustenite are formed, but the 
grid secondary cementite round grains disappears only when heating above 
Nuclear heating plant. Temperature increase above critical points leads to 
growth of austenitic grains of subjects bigger, than temperature that is 
undesirable is higher. Significant growth in austenitic grain owing to heating is 
called as an overheat steel much higher than a critical point. In case temperature 
of heating comes nearer to the line a solidus on 100-200zs, happens at the 
expense of interaction to environment that leads to sharp decrease in durability 
of steel. 

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