Creating new materials and managing 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. Steel annealing is a widespread option for eliminating production defects of products, increasing their strength and reliability.
The tasks of the process and its variants
Annealing operations are performed with the aim of:
- optimization of the intracrystalline structure, ordering of alloying elements;
- minimize internal distortion and stress due to rapid technological temperature differences;
- increase the flexibility of objects for subsequent processing by cutting.
The classical operation is called “complete annealing”, however, there are a number of its varieties, depending on the specified properties and characteristics of the tasks: incomplete, low, diffusion (homogenization), isothermal, recrystallization, normalization. All of them are similar in principle, but the heat treatment modes of steels are significantly different.
Chart Based Heat Treatment
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 the points of transformation of structures and their features under the influence of heating or cooling.
Metal technology governs this schedule for all types of annealing of carbon steels. For incomplete, low, and also for recrystallization, the “starting” temperature values are the PSK line, namely, its critical point Ac 1 . Complete annealing and normalization of the steel are thermally oriented to the line of the GSE diagram, its critical points Ac 3 and Ac m . Also, the diagram clearly establishes the relationship between a specific heat treatment method and the type of material in terms of carbon content and the corresponding possibility of carrying it out for a particular alloy.
Full annealing
Objects: die-eutectoid alloy castings and forgings, while the composition of the steel should fill carbon in an amount of up to 0.8%.
Goal:
- maximum change in the microstructure obtained by casting and hot pressure, reduction of a non-uniform coarse-grained ferrite-pearlite composition to a uniform fine-grained one;
- reduced hardness and increased ductility for subsequent machining.
Technology. The temperature of steel annealing is 30-50 ° C above the critical point of Ac 3 . Once the metal reaches the specified thermal characteristics, they are maintained at this level for some time, allowing you to complete all the necessary transformations. Large pearlitic and ferritic grains completely pass into austenite. The next step is slow cooling together with the furnace, during which ferrite and perlite, which has a fine grain and a homogeneous structure, are again released from austenite.
Complete annealing of steel eliminates the most complex internal defects, but it is very long and energy-intensive.
Incomplete annealing
Objects: pre-eutectoid steels that do not have serious internal heterogeneities.
Purpose: grinding and softening pearlite grain, without changing the ferrite base.
Technology. Heating the metal to temperatures falling between the critical points Ac 1 and Ac 3 . Exposure of the workpieces in the furnace with stable characteristics contributes to the completion of the necessary processes. Cooling is slow, along with the oven. The output is the same pearlite-ferrite fine-grained structure. With this thermal effect, perlite turns into fine-grained, while ferrite remains unchanged crystalline, and can only change structurally, also being crushed.
Incomplete annealing of steel makes it possible 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 within 0.65% (for example, ball-bearing), non-ferrous metal parts and blanks that do not contain serious internal defects, but require non-energy-intensive correction.
Goal:
- removal of internal stresses and hardening due to the influence of both cold and hot deformation;
- elimination of the negative consequences of uneven cooling of welded structures, increase of ductility and strength of welds;
- uniformity of the microstructure of non-ferrous metallurgy products;
- spheroidization of lamellar perlite - giving it a granular shape.
Technology.
The parts are heated 50-100 ° C below the critical point Ac 1 . Under the influence of such influences, minor internal changes are eliminated. The whole process takes about 1-1.5 hours. Approximate temperature ranges for some materials:
- Carbon steel and copper alloys - 600-700 ° C.
- Nickel alloys - 800-1200˚.
- Aluminum alloys - 300-450˚.
Cooling is done in air. For martensitic and bainitic steels, metal technology 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 alloy steel castings.
Purpose: uniform distribution of atoms of alloying elements over the crystal lattices and the entire volume of the ingot as a result of high-temperature diffusion; softening the structure of the workpiece, reducing its hardness before performing subsequent technological operations.
Technology. The material is heated 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. Upon completion of all stages of high-temperature transformations, slow cooling follows.
A laborious, but highly efficient process for leveling the microstructure of large structures.
Isothermal Annealing
Objects: carbon steel sheet products , products from alloyed and high alloy alloys.
Purpose: improving the microstructure, removing internal defects with less time.
Technology. The metal is initially heated to full annealing temperatures and can withstand the time required to convert all existing structures to austenite. Then slowly cool by immersion in hot salt. Upon reaching the heat, 50-100 ° C below the Ac 1 point is placed in the furnace in order to maintain it at this level for the time necessary for the complete conversion of austenite to perlite and cementite. Final cooling takes place in air.
The method allows to achieve the required properties of alloy steel billets, while saving time in comparison with full annealing.
Normalization
Objects: castings, forgings and parts from low carbon, medium carbon and low alloy steel.
Purpose: streamlining the internal state, imparting the necessary 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, held and cooled in air. Thus, the speed of completion of processes increases. However, using this procedure, a rational calm structure can be achieved only when the composition of the steel is determined by carbon in an amount of not more than 0.4%. With increasing carbon content, an increase in hardness occurs. The same steel after normalization has greater hardness along with evenly spaced fine grains. The technique can significantly increase the resistance of alloys to fracture and ductility by machining.
Possible annealing defects
During the execution of heat treatment operations, it is necessary to adhere to the specified modes of temperature heating and cooling. In case of violation of the requirements, various defects can occur.
- Oxidation of the surface layer and the formation of scale. During the operation, the hot metal reacts with atmospheric oxygen, which leads to the formation of scale on the surface of the workpiece. To be cleaned mechanically or using special chemicals.
- Carbon burnout. Also occurs as a result of the influence of oxygen on the hot metal. A decrease in the amount of carbon in the surface layer leads to a decrease in its mechanical and technological properties. In order to prevent these processes, it is necessary to anneal the steel in parallel with the introduction of protective gases into the furnace, the main task of which is to prevent the alloy from interacting with oxygen.
- Overheat. It is a consequence of prolonged exposure in an oven at high temperature. It results in excessive grain growth, the acquisition of a heterogeneous coarse-grained structure, and an increase in fragility. It undergoes correction by carrying out another stage of complete annealing.
- He burned out. It occurs as a result of exceeding the permissible values of heating and aging, leads to the destruction of bonds between some grains, completely spoils the entire metal structure and is not subject to correction.
To prevent failures, it is important to clearly perform the heat treatment tasks, have professional skills and strictly control the process.
Steel annealing is a highly effective technology for bringing the microstructure of parts of any complexity and composition to the optimal internal structure and condition, which is required for the subsequent stages of thermal influences, cutting processing and commissioning of the structure.