Annealing of steel as a kind of heat treatment. Technology of metals

The creation of new materials and the management of their properties is the art of metal technology. One of its tools is heat treatment. These processes allow you to change the characteristics and accordingly, the scope of use of alloys. Annealing of steel is a widespread option for eliminating production defects in products, increasing their strength and reliability.

The tasks of the process and its variants

The annealing operations are performed in order to:

• Optimization of the intracrystalline structure, ordering of the alloying elements;
• Minimizing internal distortions and stresses due to rapid technological temperature changes;
• Increased compliance of objects to subsequent machining.

The classical operation is called "complete annealing," however, there are a number of its varieties, depending on the given properties and features of the tasks: incomplete, low, diffusion (homogenization), isothermal, recrystallization, normalization. All of them are similar in principle, but the modes of heat treatment of steels are significantly different.

Thermal processing based on the diagram

All transformations in the iron and steel industry, which are based on the game of temperatures, clearly correspond to the diagram of iron-carbon alloys. It is a visual aid for determining the microstructure of carbon steels or cast irons, as well as points of transformation of structures and their features under the influence of heating or cooling.

The technology of metals regulates all kinds of annealing of carbon steels by this schedule. For incomplete, low, and also for recrystallization, the "starting" temperature values are the PSK line, namely its critical point Ac ₁ . Full annealing and normalization of the steel are thermally oriented to the GSE diagram line, its critical points Ac ₃ and Ac _m . Also, the diagram clearly establishes the relationship of a certain heat treatment method with the type of material with respect to the carbon content and the corresponding possibility of conducting it for a particular alloy.

Full annealing

Objects: castings and forgings from the pre-eutectoid alloy, while the composition of the steel should fill the carbon in an amount up to 0.8%.

Goal:

• The maximum change in the microstructure obtained by casting and hot pressure, bringing the heterogeneous coarse-grained ferritic-pearlitic composition into a uniform fine-grained structure;
• Decrease in hardness and increased compliance for subsequent machining.

Technology. The annealing temperature of steel is 30-50 ° C above the critical point Ac ₃ . When the metal reaches the given thermal characteristics, they are maintained at this level for some time, allowing all the necessary transformations to be completed. Large pearlitic and ferritic grains completely pass into austenite. The next step is slow cooling with the furnace, during which ferrite and perlite, which has a fine grain and a homogeneous structure, are again separated from the austenite.

The complete annealing of steel allows to eliminate the most complex internal defects, however it is very long and energy consuming.

Incomplete annealing

Objects: doeutectoid steels, which do not have serious internal heterogeneities.

Purpose: grinding and softening of pearlite grain, without changing the ferrite base.

Technology. Heating of the metal to temperatures falling in the gap between the critical points Ac ₁ and Ac ₃ . Extraction of blanks in the furnace with stable characteristics helps to complete the necessary processes. Cooling is done slowly, together with the oven. At the output, the same pearlite-ferritic fine-grained structure is obtained. With this thermal effect, the perlite is transformed into fine-grained, the ferrite remains unchanged crystallically, and can only vary structurally, also grinding.

Incomplete annealing of steel allows you to balance the internal state and properties of simple objects, it is less energy-intensive.

Low annealing (recrystallization)

Objects: all types of rolled carbon steel, alloy steel with a carbon content of 0.65% (for example, ball bearing), parts and blanks of non-ferrous metals that do not contain serious internal defects, but require a non-power-intensive correction.

Goal:

• Removal of internal stresses and cold work due to the influence of both cold and hot deformation;
• Liquidation of negative consequences of uneven cooling of welded structures, increase of ductility and strength of joints;
• Homogeneity of the microstructure of non-ferrous metallurgy products;
• Spheroidization of lamellar perlite - giving it a granular form.

Technology.

The heating of the parts is 50-100 ° C below the critical point Ac ₁ . Under the influence of such influences, minor internal changes are eliminated. The entire technological process takes about 1-1.5 hours. Approximate values of temperature intervals for some materials:

1. Carbon steel and copper alloys are 600-700 ° C.
2. Nickel alloys - 800-1200 ° C.
3. Aluminum alloys are 300-450 ° C.

Cooling is performed in the air. For martensitic and bainitic steels, the technology of metals provides a different name for this process - high tempering. It is a simple and affordable way to improve the properties of parts and structures.

Homogenization (diffusion annealing)

Objects: large casting products, especially castings of alloy steel.

Purpose: uniform distribution of atoms of alloying elements along the crystal lattices and the entire volume of the ingot due to high-temperature diffusion; Softening the structure of the billet, reducing its hardness before performing the subsequent technological operations.

Technology. The material is heated up to high temperatures of 1000-1200 ° C. Stable thermal characteristics must be maintained for a long time - about 10-15 hours, depending on the size and complexity of the cast structure. After the completion of all stages of high-temperature transformations, slow cooling follows.

Laborious, however, highly efficient process of equalizing the microstructure of large structures.

Isothermal annealing

Objects: sheet steel of carbon steel, products from alloyed and high-alloyed alloys.

Objective: to improve the microstructure, to remove internal defects with less time.

Technology. The metal is initially heated to full annealing temperatures and withstands the time necessary for the transformation of all available structures into austenite. Then slowly cool by immersion in the red hot salt. When the heat reaches 50-100 ° C below the Ac ₁ point, it is placed in the furnace in order to maintain it at this level for the time needed to completely convert the austenite to perlite and cementite. Final cooling takes place in the air.

The method allows to achieve the necessary properties of billets from alloy steel, while saves time, in comparison with complete annealing.

Normalization

Objects: castings, forgings and details from low-carbon, medium-carbon and low-alloy steel.

Purpose: ordering the internal state, giving the desired hardness and strength, improving the internal state before the subsequent stages of heat treatment and cutting.

Technology. Steel is heated to temperatures that lie slightly above the GSE line and its critical points, soak and cool in air. Thus, the speed of completion of processes increases. However, with this procedure it is possible to achieve a rational quiet structure only if the composition of the steel is determined by carbon in an amount not exceeding 0.4%. With an increase in the amount of carbon, an increase in hardness occurs. The same steel after normalization has a great hardness together with evenly spaced fine grains. The technique makes it possible to significantly increase the resistance of alloys to fracture and the compliance with machining by cutting.

Possible annealing defects

During the execution of heat treatment operations, it is necessary to adhere to the prescribed modes of temperature heating and cooling. In case of violation of requirements, various defects may arise.

1. Oxidation of the surface layer and the formation of scale. During the operation, the heated metal reacts with the oxygen of the air, which leads to the formation of scale on the surface of the workpiece. It must be cleaned mechanically or with the help of special chemical reagents.
2. Burnout of carbon. Also occurs as a result of the effect of oxygen on the hot metal. Reducing the amount of carbon in the surface layer leads to a reduction in its mechanical and technological properties. In order to prevent these processes, steel annealing must be performed in parallel with the introduction of protective gases into the furnace, the main task of which is to prevent the interaction of the alloy with oxygen.
3. Overheat. It is a consequence of prolonged exposure in the furnace at high temperature. It has the consequence of excessive growth of grains, the acquisition of a heterogeneous coarse-grained structure, increasing brittleness. It is corrected by performing one more stage of complete annealing.
4. Burned. Occurs as a result of exceeding the permissible values of heating and soaking, leads to the destruction of bonds between some grains, completely spoils the entire structure of the metal and is not subject to correction.

To prevent malfunctions, it is important to clearly perform heat treatment tasks, have professional skills and strictly control the process.

Annealing of steel is a highly efficient technology of bringing the microstructure of parts of any complexity and composition to the optimum internal structure and state that is required for subsequent stages of thermal influences, cutting and the commissioning of the structure.