Döviz

WHAT IS HEAT TREATMENT?

In general, heat treatment, metal, or alloys in order to give the desired properties of the solid state is defined as the controlled heating and cooling processes
Thermal process in the Turkish Standards (TS 1112) in the definition, in solid form, one or more metals or alloys in order to gain a large number of specific features, according to instead go after each other as zamanlanarak heating and cooling processes are applied.
All the basic heat treatments applied to steels, the transformation is related to the internal structure. Conversion products, type, composition and metallographic structure can significantly affect the physical and mechanical properties of steel. In other words, the physical and mechanical properties of steel contained in a type of conversion products, and the amount depends on the metallographic structure.

HEAT TREATMENT ON THE GENERAL PRACTICE

heating
Temperatures warmed up waiting
Cooling

according to the rate of carbon contained in steel heat treatment may collect two different groups;

1-Eutectoid six steels (% C <0.8),

Over 2-Eutectoid steels (% C> 0.8)

Figure 1 - Iron - carbon equilibrium diagram

Austenitizing of the steel thermal process (ostenizasyon) once daily. Austenitizing, the steel is heated slowly until a suitable temperature, until the structure is completely transformed into austenite (ie, the internal structure of each region to show a similar structure) Tempering means. For austenitizing steel material, sub-critical temperature line (AC1) is heated to a temperature above.

Completely changed the internal structure of these temperatures, the removal of the steel, steel, warping, cracking, oxidation, decarburization (carbon atoms break the internal structure) and unwanted situations may arise, such as grain growth. Therefore, at temperatures as low as possible ostenitleştirilirler steels.

BASIC HEAT TREATMENTS

Figure 2 - Non-alloy steels, the smoothing, normalization, globalization and the annealing temperature range for hardening.

1-Stress Relieving (Relieving STRESS)

Stress relief anneal forming, casting or welding operations to reduce the internal stress arising from the steel parts, usually between 550-650 ° C after heating and cooling is the process of slowly.

2-Normalized Stress (Normalizing)

Normalized to the crystal structure of the annealed steel material is more homogeneous, finer carbide in accordance with the process and make it to the next heat of steel in order to distribute the critical temperature (in the re-crystallization temperature) is 40-60 º C above the cooling of the air tavlanıp (Figure 2).

3-WATER-MAKING PROCESS (quenching)

Up to a certain temperature (typically 850-1100 ° C) water according to the type of steel is heated, cooled in oil or salt bath quenching to provide a structure called martensite. Cooling rate, size of object, capable of hardening and quenching of steel varies depending on the environment. Water to the desired maximum speed, the most severe cooling is the speed of which serve to provide the most appropriate hardness. If the piece consists of cracks in the cooling rate is too high, too low can not be achieved if the appropriate stiffness (Figure 2).

4-tempering (Tempering)

Tempering, the steel is hardened by heat treatment at the end of quenching and cooling to resolve the tensions leading its martensitic steel özlülüğünü and to increase the resistance is usually heated to between 150-450 ° C and rapidly cooled to an appropriate reduction is the process of crispiness. To minimize cracking immediately after quenching, tempering process needs to be done.

5-Cementation (SURFACE HARDENING)

Cementation process, the process of impregnation of the carbon surface of a piece of low-carbon steel. Carbon impregnation process, the steel part of the carbon monoxide (CO) in an environment containing austenite phase temperature (850-950 ° C) occurs as a result of heating the gas-metal reaction.

Steel parts, case hardening the surface temperature to the desired depth for the advancement of the nuclear diffusion of carbon is kept long enough. This time period is called case hardening. During this period, which diffuse to the inside surface of the carbon steel component is called the depth of case hardening depth of progress.

ANNEALING
The desired structural, physical and mechanical properties to achieve and to facilitate chip removal, or cold forming of metal materials are heated to appropriate temperatures, and subsequently kept at this temperature until the necessary changes in the slow cooling process is achieved is called annealing (Fig. 3).

Figure 3 - Schematic representation of the annealing process

Stress SOFTENING
Anneal softening, reducing the hardness of steel to reduce the size of the internal structure of the grain, sawdust, or to facilitate removing the parts of casting and forging to eliminate internal stresses. Nc3 six eutectoid steel, eutectoid steels are higher on the lines of AC1 are heated up to certain temperatures, keeping the internal structures inside the oven is very slow after converting to austenite is cooled.

0.2% C steel containing coarse-grained part of a six-eutectoid grains during annealing, the changes of the internal structure is as follows;

a) The first or original structure consists of coarse ferrite and pearlite grains (Figure 4).

Figure 4 - The internal structure of a low-carbon steel

Figure 5 - 0.2% C steel inner structure that contains the schematic representation of the changes taking place during the annealing process

Figure 6 - Eutectoid over the internal structure of steels

b) at a temperature just above the line AC1 fine-grained austenite transforms into pearlite, ferrite structure remains the same. If the coarse grains of ferrite If we do not change this temperature for cooling would not be any change in grain size (Figure 5).

c) at a temperature above Ac3 line, completely fine-grained austenite structure changes (Figure 5).

d) Part cooled to room temperature, fine grains of ferrite with small pearlite consists of an internal structure containing regions (Figure 5).

Here, six eutectoid steels to softening tavına tutulabilmeleri Ac3 line on the subject are of the required tavlanmalarının at appropriate temperatures. Six heat-treated eutectoid steels tutulabilmeleri healthy way, first they must have a homogeneous austenitic structure. For this purpose, which is heated to the austenitizing temperature for steel materials, every 25 mm wall thickness tempered 1 hour at a time, it is recommended.

Eutectoid steels above Ac3, 1 line, at temperatures above about 50 ° C are subject to the austenitizing process. Steels are held at these temperatures, austenite and cementite phases include. This is when the water temperature steels, cementite particles remain in the same building. Reduce the hardness of cementite phase in the structure, such as steels increases their resistance to wear. For this reason above eutectoid steels do not need to completely ostenitleşmesine. These steels Nc3, 1 line, at least at a temperature above 10 º C tavlanırlar. Eutectoid steels subjected to internal structures over tavına smoothing rough non-lamellar pearlite eutectoid cementite phase consists areas surrounding them (Fig. 6). Perlite in this building, the surrounding hard and brittle cementite network. The internal structure of grain boundaries have a thick and stiff, steel machining method makes it difficult to be processed. Therefore, softening of annealed, eutectoid steels, higher will be the last transaction.

Normalization (Normalization) Stress:

Normalization is usually annealed to reduce grain, to obtain a homogeneous internal structure, and often in order to improve the mechanical properties of eutectoid eutectoid over six steels and steels Ac3 Acm transformation temperatures can heat up to temperatures above about 40-50oC, tavlandıktan outside in calm conditions after the cooling process of the oven (Fig. 2 ).

Normalization of major objectives of the annealed;

a) reduce the grain,

b) to obtain a homogeneous internal structure,

c) above eutectoid steels, the grain boundary carbide network to distribute,

d) improving the processing of steels,

e) To improve the mechanical properties of

f) stiffness and strength of steels subjected to increase the softening tavına

be listed as. For these reasons, the normalization of annealed, heat treatment of steels can be applied.

Tavına softening of the structure of steels subjected eutectoid cementite network of over, is known to reduce the strength of these steels. Normalization of annealed, eutectoid cementite steels over the break and, in some cases greatly relieves network. Therefore, the normalized strength of the steels is increased.

Annealed in normalization of the part due to the cooling of the air from the cooling rate is relatively high. In general, the cooling rate increases, and finer pearlite austenite transformation temperature drops are obtained.

Very soft ferrite, cementite, a phase is very hard. The results are normalized to the steel structure of the cementite layers are lined up close together, or often because of the hardness of the steel increases. Therefore, the normalized stiffness and strength of the steels, these steels are subjected to smoothing tavına values ​​are significantly higher. Tavına softening some of the steels in Table 1 were subjected to the mechanical properties and states are normalized.

Table 1 - tavına softening of steels subjected to the mechanical properties and the normalized states

Stress globalization:

Globalization annealed, steel AC1 temperature for a long time holding the line and the area around the tavladıktan after release, with a slow cooling process of transformation of carbides spherical shape (Figure 2). This process, with controlled cooling after ostenitleştirmeden yapılabilir.Yumuşatma anneal process, as described above annealed eutectoid steels that are hard and brittle cementite grains of the presence of internal structures are not suitable for handling. This kind of globalization in order to increase the ductility of the annealed steels, and is used to facilitate processing.

Globalization anneal is performed with one of the following methods.

a) Steel material to a temperature just below the line of AC1 (eg 700oC) for a long time (15-25 hours) annealed.

b) Steel material, lower critical temperature line (AC1) between the heated and cooled down to temperatures just above and below it, as that is tempered for release.

c) Material AC1 tavlandıktan at a temperature above the critical temperature line, or in the oven and then cooled very slowly, at a temperature just below the line of AC1 or kept for a long time.

High sıcaklıtaki annealing process, within the pearlitic structure of the steel will cause the dissolution of the cementite network break down. As a result of globalization annealed, which could be distributed within the ferritic matrix with an internal structure consisting of spherical carbides are obtained. Globalization at the end of the annealed hardness of the steel is reduced, in turn, increases ductility. As a result of this transaction, eutectoid steels suitable for handling becomes higher.

Globalization of annealed, high-carbon steels are more. Low carbon steels are subjected to tavına rarely globalization. Because this kind of globalization annealed steels and soften at the end of this ultra-softening leads to some difficulties during machining. Medium-carbon steels in the plastic forming process, for them to win sufficient degree of ductility before, sometimes undergo tavına globalization. Annealed during the annealing period of globalization should be set well. If the steel, for longer than necessary tavlanırsa cementite particles combine to show elongation, and this adversely affects the ability of the steel processing.

Softening, globalization and normalization processes are applied in order to make it suitable for handling steel. However, according to the rate of carbon steel in heat treatment applied is selected.

Stress Relieving Stress, and Search:

Stress Relieving, casting, welding and internal stresses resulting from cold forming processes, to reduce the transformation temperatures of metallic materials after heating and slow cooling to a temperature under the appropriate process. This process is sometimes referred to as the transformation temperature or annealed below the critical temperature. Stress of steel at temperatures between 540oC and 630oC tavına are subject to removal.

In December the annealed, stress reduction is a process very similar to tavına, eutectoid steel sheet metal and wire construction of six cold-steel materials in order to continue to shape the AC1 to a temperature just below the transformation temperature (550-680oC) is heated up, only after the slow cooling process of recrystallization.

Sealed quench:

After tempering, steels rapidly cooled from a slow or moderate, which could be dissolved in austenite by diffusion of carbon atoms are divided into austenite structure. The cooling rate is increased, the diffusion of carbon atoms with the solid solution can not find enough time to leave. Even though a little movement of iron atoms, carbon atoms in the solution consists of a different structure due to imprisonment. Rapid cooling of this structure as a result of "martensite" is called.

The most important reason for the high hardness of martensite, lattice structure is distorted. Martensitic transformation occurs during the growth of steel materials in a quantity by volume. Such growth in volume, creating a very high level of local stresses in the structure of steel or plastic shape change causes the extremely skewed. Çapılması lattice structure, water hardness and strength of the steels increases.

After quenching, the martensite appears under the microscope in the form of a needle or spike, and sometimes will see a unique straw-like beam. In most steels, the martensitic structure is too faint vague and, therefore, is not easily distinguishable. High-carbon steel creates a fund in the back of retained austenite, martensite needle or spike-shaped structure will look more pronounced.

Martensitic transformation occurs only during cooling. For this reason, the conversion is independent of time, ie decrease in temperature alone is cool. The most important feature of the martensite, a phase is very hard. Steels, the most severe phase after sementitten martenzittir. High hardness values, but to obtain sufficient amounts of carbon steels