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Transformations in alloys in solid state.






Transformations in austenite are stipulated by two reasons: by the decreasing of carbon solubility at temperature decreasing and by iron polymorphism. The ES line shows the decreasing of carbon solubility in austenite and the temperature of cementite precipitation starting (Fe3C). During cooling of alloys lower the ES line the growth of cementite crystals takes place. At this process carbon content in austenite changes along the ES line.

The GS line characterizes the influence of carbon to polymorphic transformation and shows the starting of ferrite precipitation from austenite. The precipitated ferrite practically does not contain carbon, therefore polymorphic transformation is accompanied by diffusion transition of carbon from ferrite in austenite. In accordance with alloys cooling in GSP area the growth of ferrite crystals is accompanied by increasing the carbon contents in austenite along the GS line.

In the S point austenite contains 0, 8% of carbon. From this austenite crystals of ferrite and cementite precipitate at the constant temperature 727°C simultaneously. Full structure change of austenite with the creation of small-grain dispersed lamellar mixture of ferrite and cementite take place. This mixture by analogy with eutectic is called eutectoid. Eutectoid in iron-carbon alloys is called perlite (this name is given in connection with nacreous tint of microsection surface).

At the temperature of 727°C austenite contains 0, 8% of carbon. The transformation of this austenite to perlite takes place in all alloys along the PSK line. This line is named as the line of eutectoid (or perlite) transformation.

At slow cooling cementite and ferrite (of which perlite consists) grow along planes of austenite crystals and create thin layered structure. This structure is called lamellar perlite. Cementite plates in perlite are strongly connected with ferrite and make ferrite to distort significantly. These distortions cause high mechanical properties of steel with perlite structure. Soft ferrite interlayers absorb energy at the impact. It ensures of high impact strength of steel.

Secondary structure change of low-carbon steels (up to 0, 2% of carbon) finishes by full transition of austenite in ferrite. From this ferrite at further cooling along the PQ line the precipitation of a small quantity of cementite is observed.

Steel with the carbon content of from 0, 02% up to 0, 8% are called hypoeutectoid steels. Their structure is the mixture of ferrite and perlite crystals. The more carbon steel has, the more perlite structural component steel has. Steels with the carbon content from 0, 8 up to 2, 14% are called hypereutectoid steels. Their structure is the perlite and cementite grid on the grains boundaries.

Low-carbon steels with the carbon content up to 0, 2% (0, 8кп, 10, 15, 20) have high plasticity. These steels are used for manufacturing of articles of complicated form, which are punched from a sheet, and also for welded constructions. Steels of the marks 10, 15, 20 are applied for manufacturing of carburized articles. Medium-carbon steels are used after heat treatment. Their usual heat treatment is thermal improvement. They are applied for manufacturing of axes, shafts, pistons, couplings, and bandages.

Become of the marks 55, 60 with the carbon content of 0, 5-0, 6% are applied for manufacturing of spring articles.

High-carbon steels У7, У8, У10, У12 are treated to receive high strength and hardness of these steels (after quenching and low-temperature tempering). These steels are applied for manufacturing of measuring and cutting tools.






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