HEAT TREATMENTS IN STEELS

• Heat Treatment Directions
• Heat Treatment General Practice
• Issues to be aware of the Application of Heat Treatment
• Basic Heat Processes
  • Stress Relieving annealed
  • Normalizing annealed
  • Tempering annealed
  • Queching annealed
  • Cementation
• Annealed
• Softening Annealed
• Normalization Annealed
• Globalization Annealed
• Stress Relieving Annealed ve Range Annealed
• Quenching hardening
• Heat removal during quenching mechanism
• Environments quenching
• Quenching cool down the effect of environment temperature and the quenching method
• Factors affecting quenching hardening
  • 1. Surface Condition Of The Fragment
  • 2. Size and Mass
• Tempering
• Part Size or mass effect of quenchinghardening
  • 1. Martemperleme
  • 2. Ostemperleme
• Applied Surface hardening steels Transactions
• 1. Cementation(Carburizing)
• 2. Nitrüzasyon (Nitridation)
• 3. Flame Surface Hardening
• 4. Induction Hardening

HEAT PROCESS OF GENERAL PRACTICE

Heat treatment applied to two different groups according to the ratio of carbon contained in steel can be grouped;

1-Eutectoid sub- steels (%C <0,8),

2-Eutectoid over steels (%C > 0,8)

Şekil 1- Iron - Carbon equilibrium diagram

Heat process of steel ostenitleştirme (ostenizasyon) will begin with. Ostenitleştirme; steel is slowly heated up to a suitable temperature, fully austenitic structure to be transformed into (ie, the internal structure of each region to show a similar structure) refers to annealing. Ostenitleştirme for the steel material, sub-critical temperature line (AC1) is heated to a temperature above.

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

Şekil 2- Smoothing applied to non-alloy steels, normalizing, annealing temperature ranges for globalization and hardening processes.

1-STRESS RELIEVING ANNEALED

Stress relief annealed forming, casting or welding internal stresses arising from transactions to reduce steel pieces, typically between 550-650 ° C after heating and slow cooling process.

2-NORMALIZING ANNEALED

Normalized crystal structure of annealed steel material is more homogeneous, and a subsequent heat treatment to make a finer carbide steel to ensure proper distribution of the critical temperature (re-crystallization temperature) tavlanıp 40-60 ° C above the air cooling (Figure 2).

3-QUENCHING

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

4-TEMPERING

Tempering, the steel is hardened by heat treatment of cooling water at the end to resolve tensions and leading its martensitic steel özlülüğünü and to increase the resistance is usually heated to between 150-450 ° C and rapidly cooled to a suitable gevrekliğini removal process. Cracks to minimize the tempering process must be done immediately after quenching.

5-Cementation (SURFACE HARDENING)

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

Steel parts, case hardening the surface to core temperature enough time to progress to a depth of carbon diffusion are desired. This time period is called case hardening. In the meantime, diffuse inward from the surface of the carbon steel part is called the depth of case hardening depth of progress.

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

Figure 3 - Schematic illustration of the annealing process

Normalizing (Normalization) annealed:

Normalization typically annealed grain, scaled 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 up to temperatures above about 40-50oC ısıtıp, tavlandıktan calm in the air outside the furnace cooling process (Fig. 2 ).

The main objectives of normalization annealed;

a)  

Annealed Globalization:

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

Globalization annealed with one of the following methods are performed.

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, low critical temperature line (AC1) heated and cooled from temperatures immediately above and below, ie, the annealed release.

c) Material AC1 annealed 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-temperature annealing process, pieces of steel within the pearlitic structure with a cementite network will cause a distraction. Annealed as a result of globalization, a ferritic matrix with scattered cases in which an internal structure consisting of spherical carbides are obtained. Globalization at the end of annealed hardness of the steel is reduced, whereas increases sünekliliği. As a result of this transaction, eutectoid steels are suitable for handling becomes higher.

Annealed globalization, people are very high carbon steels. Low carbon steels are subject rarely tavına globalization. Because this kind of globalization annealed steels soften and eventually leads to some difficulties during the machining of this excessive softening. Medium-carbon steels for the acquisition of sufficient ductility before plastic forming, sometimes tavına are subject to globalization. Annealed during the annealing period of globalization must be set up well. If steel, cementite particles tavlanırsa longer than necessary period of time, and this show combined growth of the steel processing adversely affects the ability.

Softening, cuttings globalization and normalization processes are applied in order to streamline processing. However, according to the rate of carbon steel in heat treatment applied is selected.

Gerilim Giderme Annealed And Range Annealed

Stress relief annealed, casting, welding and internal stresses caused by cold forming operations to reduce the conversion of metallic materials under temperatures of up to a suitable temperature after heating and slow cooling process. This process, sometimes referred to as the conversion temperature or annealed below the critical temperature. Steel materials at temperatures between 630oC to 540oC tavına are subject to stress reduction.

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

Quenching Hardening:

After annealing, a slow or moderate pace cool steels, the carbon atoms are dissolved in austenite austenite structure are separated by diffusion. Cooling rate is raised, with diffusion of carbon atoms in solid solution can not find enough time to leave. Iron atoms, even if they move a little, due to being locked up in a different structure consists of carbon atoms in solution. 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. Volumetric growth of a quantity of steel materials during martensitic transformation occurs. The volume growth, creating a very high level of local stresses in the structure of steels to undergo changes in shape will cause excessive distortion or plastic. distortion cage structure, water hardness and strength of the steel increases.

After quenching martensite formed under a microscope and sometimes appears in the form of a needle or a thorn in bunch of straw-like appearance exhibitions. Martensitic structure of steel in most vague and faint, so it can not be easily distinguished. Residual austenite in the back of the fund creates a high carbon steels, martensite needle or spike-shaped structure looks more prominent.

Martensitic transformation occurs during cooling alone. Therefore, the conversion is independent of time, namely a decrease in temperature alone is cool. The most important feature of the martensite, a phase is very hard. In steels, martensite sementitten comes after the hardest phase. High hardness values, but sufficient to obtain the carbon-containing steels.