Steel normalization refers to quenching processes by heating to a certain temperature and cooling. Heat treatment has various modes for each type of metal. As a result of applying the technology, the material becomes stronger due to the elimination of defects. The latter inevitably appear as a result of previous stages of the manufacture of steel products.
Technology purpose
Normalization of steel can be carried out in garage conditions with the appropriate equipment. The advantage of the technology is to obtain a thin eutectoid. The structure of this layer directly affects the strength and stiffness of the metal.
Since the normalization of steel is carried out to improve the quality of the product, accordingly, the cost of its manufacture increases. Technology is applied only when necessary. For lightly loaded parts, it is not necessary to carry it out. Often it is applicable for the production of long metal.
Technology can be a substitute for procedures such as quenching with high tempering, classic annealing. Normalization of medium-carbon steel does not give high strength, comparable to the structure after hardening. But it does not lead to severe deformation and helps to get rid of internal cracks.
The essence of technology
Normalization of steel refers to the thermal processing method. There are several metal heating technologies that differ in conditions:
- The heating temperature for metals and alloys is different.
- Heated holding time.
- The type of cooling often lasts longer due to heat exchange with the environment.
It is slow cooling that makes it possible to obtain a homogeneous steel composition. The purpose of annealing is a homogeneous metal structure, the desire to remove shells and voids, small cracks.
The following types of annealing are used, which are common to reduce local thickening after hot and cold rolling:
- Diffusion - changes the chemical composition.
- Full - affects the entire structure, helps to achieve uniformity.
- Recrystallization - removes the hardening of steels.
- Incomplete - makes steel more malleable for metalworking.
- Isothermal is the most optimal way to reduce the strength of steel.
- Spheroidizing - converts flat perlite grains to spherical.
The temperature of normalization of steel was selected empirically for each type of alloy. After casting or cold rolling, not a single workpiece is obtained an ideal structure. Additional heat treatment, annealing, helps to correct the situation.
Chemical correction
Normalization and hardening became necessary to correct internal heterogeneities after casting. Shaped castings and ingots are subjected to heat treatment. Most often this is required for alloy steel products.
To fix steel defects, it is necessary to heat to a very high temperature. In this state, the atoms of the alloying elements begin to move. They are uniformly redistributed in terms of internal volume.
At 1100 degrees, the optimal heat treatment of steel takes place. Diffusion normalization lasts about 10-20 hours in a heated state and very slow cooling follows.
Full annealing
Normalization and hardening of steel with a hypereutectoid is necessary to correct the structure that is disturbed by heating during the manufacturing of castings and forgings processed by pressure. The treatment temperature should exceed the critical point when perlite begins to convert to austenite.
The temperature rise should occur strictly 30-50 degrees above the critical point Ac3. This value for alloy steels is taken from the tables, and for carbon steel is determined from the state diagram. Normalization process:
- The initial stage is heating 30-50 degrees above the critical temperature Ac3. The formation of austenitic grains.
- Exposure at high temperature is accompanied by the growth of austenitic grains.
- Long-term uniform cooling - small crystals of austenite break up into several pearlite grains. A uniform filling of the structure with a ferrite pearlite layer occurs.
Incomplete annealing is required to reduce the hardness of metals. More often it is necessary under the conditions of metal cutting. As a result of normalization, the excess tension of the steel is eliminated. Unlike complete annealing, the whole process occurs at lower temperatures. Accordingly, less time is spent.
Processing of complex alloy steels
In the process of isothermal normalization, hard metals become more malleable to cutting. Heating occurs at the following temperatures:
- Structural steels - not higher than 30-50 degrees of the critical point Ac3.
- Tool steels - 5-100 degrees higher than Ac1 point.
In contrast to the methods considered, in isothermal annealing, cooling of steel immersed in molten salt is performed. Natural cooling is carried out after the temperature drops to 700 degrees. At this point, austenite completely passes into pearlite grains.
Correction of the broken structure of metals and alloys
Two-stage steel cooling allows you to convert perlite plates to grains. Heating occurs to a temperature above the Ac1 point. Then it is reduced to 700 and maintained to 500 degrees. Further, in the air, the metal cools for a long time. This normalization is called spheroidizing. As a result, the product can be easily cut. This is how metals containing 0.65% carbon are treated.
Hardening is the formation of stronger metal areas after cold stamping or drawing. Recrystallization annealing removes this defect - the brittleness of steels is eliminated by heating to 700 degrees (below Ac1). At this moment, the crystallization lattice of metals is restored. The structure becomes fine-grained and homogeneous. Light annealing can also be carried out, restoring the properties of the steels after rolling the sheet in order to maintain a shiny surface.