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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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