Heat treatment of steel gives to metal products useful properties. Subjected to heat treatment, products from steel become stronger, they resist to wear better, it is more difficult to deform them at maximum loads. Teromobrabotka is applied when it is necessary to increase operational characteristics of products sharply.
Types of heat treatment became
On heat treatment of steel mean processes at which the structure of this material when heating changes and also at the subsequent cooling. Speed of cooling of steel is defined by features of a concrete method of processing.
At heat treatment of property of steel essentially change, however its chemical composition remains the same.
Distinguish several separate types of heat treatment of steel:
- annealing;
- training;
- normalization;
When annealing, steel heats up, and then is gradually cooled. Distinguish several type of such processing of which various extents of heating and speed of cooling are characteristic.
Training of steel is based on its recrystallization during heating to temperature exceeding a certain critical level. After a certain endurance apply the accelerated cooling. The tempered steel is characterized by nonequilibrium structure. Apply release of steel to rebalancing.
Release of steel is such type of heat treatment which is used with the purpose to reduce or to completely remove residual stress of material. During the holiday the viscosity of steel increases, its hardness and fragility decreases.
Normalization in something is similar to annealing. Difference between ways that at normalization material is cooled in the open air whereas in case of annealing the cooling is conducted in the special furnace.
Operation of heating of steel preparation
The correct carrying out this responsible operation defines quality of future product and affects labor productivity. When heating, steel is capable to change the structure and properties. Change as well characteristics of a surface of a product. In interaction with atmospheric air on the surface of steel scale appears. Thickness of its layer will depend on duration of heating and temperature of influence.
With the greatest intensity steel is oxidized at a temperature over 900 degrees Celsius. If to increase temperature to 1000 degrees, the oxidability will increase twice and if to use heating to 1200 degrees, steel will begin to be oxidized five times more intensively.
Chrome-nickel steel often call heat-resistant as their processes of oxidation do not affect. On alloyed stalyakh not too thick layer of scale is formed. It gives to metal protection, without allowing began to be oxidized further and preventing cracking when forging a product.
Steel of carbonaceous type during heating lose carbon. At the same time decrease in durability of metal and its hardness is observed. Hardening worsens. In particular it concerns preparations of the small size which then are exposed to training.
The preparations made of carbon steel can be heated very quickly. Usually they are placed in the furnace cold, without heating previously. Slow heating helps to avoid emergence of cracks in stalyakh high-carbon type.
In the course of heating, steel becomes coarse-grained. Its plasticity decreases. The allowed overheating of a product can be corrected by heat treatment, but it demands additional energy and an expense of time.
Burned through steel
If to finish heating to excessively big temperature, comes so-called burned through steel. At the same time there is a violation of structural communications between separate grains. When forging the similar preparations completely collapse.
Burned through it is considered unrecoverable marriage. When forging products from staly high-carbon type the smaller number of heatings is applied, than at manufacture of products from the alloyed steel.
Heating steel, it is necessary to watch process temperature, to control heating time. If time to increase – grows a scale layer. At the accelerated heating on steel cracks quite can be formed.
Steel chemical heat treatment
Understand the heat treatment operations connected among themselves when the surface of steel is saturated with various chemical elements at the increased temperature as such processing. As elements use nitrogen, carbon, chrome, silicon, aluminum, etc.
Superficial saturation of material metal elements which form solid solutions with iron differs in bigger power consumption. Such processes go usually long time if to compare them to saturation became carbon or nitrogen. Diffusion goes in an alpha iron lattice easier, than in a gamma iron lattice where atoms are much more densely packed.
Chemical heat treatment is used for giving of steel of the increased hardness and resistance to wear. Such processing also leads to improvement of resistance of steel to cavitation and corrosion. On the surface of steel preparations at the same time compression tension is formed; the durability and reliability of products increase.
One of types of chemical heat treatment of steel – so-called cementation. At the same time the surface of the alloyed or low-carbon steel saturate with carbon at a certain temperature. This operation is followed by training and a holiday. The processing purpose cementation consists in increase in resistance to wear, hardness became. Cementation gives the chance to increase contact endurance of a steel surface in case of a viscous core of preparation. Additional effect of cementation – endurance of preparation at torsion and a bend.
Products before cementation are required to be cleaned previously. Sometimes the surface of steel is covered with a special plastering. Usually the plastering is prepared from refractory clay to which add water and asbestos powder. Other structure for a plastering includes talc and a kaolin which part with liquid glass.
Steel nitriding
So call chemical heat treatment of a surface of a metal product by means of long endurance when heating to 600-650 degrees Celsius. Process goes in the atmosphere of ammonia. The main quality of the nitrated steel – its extremely high hardness. Nitrogen is capable to form connections with iron, chrome, aluminum which differ considerably in bigger hardness in comparison with carbides. In the water environment the nitrated steel resists corrosion better.
The steel products processed by nitriding do not warp in the course of their cooling. It is a type of heat treatment of steel widely use in mechanical engineering when it is required to strengthen durability and to raise resistance to wear. Examples of products to which with success the nitriding is applied:
- sleeves of cylinders;
- shafts;
- springs;
Steel cyanation
This process is also called nitrocementation. At such chemical heat treatment the saturation of a surface of steel is at the same time conducted by nitrogen and carbon. Then training and a holiday follow – it gives the chance to increase corrosion stability. Quite often nitrocementation is carried out in the gas or liquid environment. Liquid cyanation with success can go in fusions of salts.
Similar type of heat treatment widely applies at production tool staly, applied to fast cutting. It is possible to form details with very difficult configuration of such steel. Wide circulation of the described method is complicated by the fact that it assumes use of toxic cyanic salts.
Thermomechanical processing of steel products
So call the operations assuming not only thermal impact on steel preparation, but also its plastic deformation. Thermomechanical processing (TMO) gives the chance to receive metal of special durability. Formation of structure goes in the conditions of high density. Upon termination of thermomechanical processing the training has to follow immediately. Otherwise recrystallization can develop.
This type of processing provides the increased steel durability along with its excellent plasticity. TMO quite often use in rolling production when it is necessary to strengthen bars, pipes or springs.
Release of steel
This procedure removes consequences of training and the residual tension of metal. The viscosity of steel increases. Apply heating of preparation to temperature which is not exceeding a certain critical level to a holiday. At the same time receiving a condition of martensite is possible. Advantage of processing of a such type – a combination of plasticity, favorable for products, and durability.
Distinguish a low, average and also high holiday. The difference consists in heating temperature. It is possible to determine it by special color tables of a pobezhalost of steel.