Mechanical properties not only depend on the chemical composition of an alloy, but it also depends on strukturmikronya. An alloy with the chemical composition samadapat have different microstructure and mechanical properties will be different. Microstructure depends on the process experienced, especially the behavior of the process-heat received during the process.
Process-heat behavior is a combination of heating and cooling operation at a certain speed is made to the metal or alloy in the solid state, as an attempt to obtain certain properties. Sell-heat process basically consists of several stages, starting with a warm-up to a certain temperature, followed by detention for a few moments, and then do the cooling at a certain speed.
In general, the heat treatment (heat treatment) are classified into two types:
1. Near Equilibrium (Approaching Equilibrium)
The purpose of the heat treatment Near Equilibrium is to:
a. Soften the crystal structure
b. Smooth the grain
c. Eliminating stress in
d. Improve machineability.
Near types of heat treatment Equibrium, for example:
- Full Annealing (annealing)
- Stress relief annealing
- Process annealing
2. Non Equilirium (not equilibrium)
Non Equilibrium is a hot destination for getting violent and higher strength.
Type of Non Equibrium heat treatment, for example:
- Surface Hardening (Carburizing, nitriding, cyaniding, Flame hardening, Induction hardening)
In the manufacturing process, the required chemical composition is obtained when the steel in the form of a liquid phase at elevated temperatures.
At the time of the cooling process of melting temperature, the steel begins to change into a solid phase at a temperature of 13500, in this phase was the ongoing changes in the microstructure. Changes in the microstructure can also be done by the heat treatment.
When the cooling process is done slowly, it will be achieved for each type of microstructure that is balanced according to the chemical composition and temperature of the steel. Changes in microstructure at various temperatures and levels of carbon can be seen in the Phase Diagram Balance (Equilibrium Phase Diagram).
Diagram Figure EquilibriumFerrite Near-Cementid (Fe-Fe3C)
From the diagram above we can see that the cooling process changes changes in crystal structure and microstructure are very dependent on the chemical composition.
On the carbon content reached 6.67% formed microstructure called cementite Fe3C (can be seen on the far right vertical line).
Properties cementitte properties: extremely hard and very brittle
On the left side of the diagram where the carbon content is very low, at room temperature to form ferrite microstructure.
In the steel with a carbon content of 0.83%, the microstructure formed is perlite, conditions of temperature and carbon content is called a point Eutectoid.
In the steel with low carbon content up to eutectoid point, the microstructure formed is a mixture of ferrite and pearlite.
At the point eutectoid steel containing up to 6.67%, the microstructure formed is a mixture of perlite and cementite.
At the time of cooling from the melting temperature of steel with low carbon content, will form the Delta ferrite microstructure into austenite microstructure.
In the steel with a higher carbon content, the melting temperature drops with rising levels of carbon, straight shape of the melting transition into austenite.
The emphasis lies on the micro structure, lines and Carbon Content.
a. The content of Carbon
0.008% C = maximum solubility limit of carbon in ferrite at room temperature
0.025% C = maximum solubility limit of carbon in ferrite at temperatures of 723
b. Degrees Celsius
0.83% C = Point Eutectoid
2% C = Carbon solubility limit at Gamma iron at temperatures of 1130 degrees Celsius
4.3% C = Point Eutectic
0.1% C = Carbon solubility limit at Delta iron at temperatures of 1493 degrees Celsius
Liquidus line is a line that indicates the beginning of the cooling process (freezing).
Solidus line is a line that shows the end of the process of freezing (cooling).
Solvus line is a line that shows the boundary between solid phase premises solid phase or solid solution with a solid solution.
Acm line = line of the solubility of carbon in iron Gamma (Austenite)
A3 line = line temperature where ferrite changes into Autenite (Gamma) on heating.
A1 line = line where there is a change of temperature Austenite (Gamma) into the ferrite on cooling.
Lines A0 = Line magnetic transformation temperature which occur in Cementid.
Line A2 = Line temperature which occurs in the Ferrite magnetic transformation.
Ferrite is a metal composition that has a maximum limit of 0.025% C Carbon solubility at temperatures of 723 degrees Celsius, crystal structure BCC (Body Center Cubic) and at room temperature has a solubility limit of 0.008% Carbon C.
Austenite is a solid solution that has the maximum limit of 2% C Carbon solubility at temperatures of 1130 degrees Celsius, the crystal structure of FCC (Face Center Cubic).
Cementid is a compound consisting of the elements Fe and C with a certain ratio (has the empirical formula) and the crystal structure Orthohombic.
Lediburite Eutectic mixture is between the Gamma iron Cementid formed at temperatures of 1130 degrees Celsius with carbon content of 4.3% C.
Pearlite is a mix between Ferrite with Cementid Eutectoid formed at temperatures of 723 degrees Celsius with carbon content of 0.83% C.
In general, heat treatment with Near Equilibrium condition that can be called by annealing.
Annealing is a behavior process heat (heat treatment) that is often made to the metal or alloy in the manufacturing process of a product. Stages of this annealing process begins by heating the metal (alloy) to a certain temperature, holding at a certain temperature for a certain time before in order to achieve the desired changes and then cooling the metal or alloy cooling was at a rate slow enough. Type annealing was varied, depending on the type or condition of the workpiece, the heating temperature, duration of detention, the rate of cooling (cooling rate), etc.
1. Full annealing (annealing)
A heat treatment process to produce a coarse perlite (coarse pearlite) but soft by heating to austenitizing and cooled with a kitchen, fixing grain size and in some cases also improve machinibility.
At full annealing process is usually done by heating the metal up above the critical temperature (for steel hypoeutectoid, 25 Degrees to 50 Degrees Celsius above the line A3 being for steel hypereutectoid 25 Degrees to 50 Degrees Celsius above the line A1). Then proceed with slow enough cooling (usually the kitchen or in the materials that have good heat insulation properties).
Please note that during the warming below the critical temperature, the A1 line has not been a change in microstructure. New changes began to occur when the heating temperature or temperature reaches the A1 (grains of pearlite is transformed into austenite crystals are fine). At hypoeutectoid steel when the heating was continued to a higher temperature, the crystal grains begin to transform into a number of austenite crystals are smooth, medium grain existing austenite crystals (derived from pearlite) barely grew. This change was completed after touching a line A3 (A3 critical temperature). At this temperature austenitic crystal grains are still very smooth and homogeneous. By raising the temperature slightly above the critical temperature A3 (A3 line) and give holding time (holding time) accordingly it will obtain a more homogeneous austenite crystalline grains are also still smooth so that when later cooled slowly will produce crystal grains of ferrite and pearlite smooth.
Steel is in the process of warming up process is experiencing temperatures that are too high or the holding time (holding time) is too long usually austenitenya crystal grains will be too rough and when cooled slowly will produce coarse ferrite or pearlite so that its mechanical properties are also not good (to be more brittle ). For hypereutectoid steel, annealing is a preparation for the next process and not an end of the process.
A heat treatment process that produces fine perlite, cooling using air media, harder and stronger from the anneal.
Technically, the process is almost the same as the annealing, which is usually done by heating the metal up above the critical temperature (for steel hypoeutectoid, 50 Degrees Celsius above the line for steel hypereutectoid A3 was 50 degrees Celsius above the Acm line). Then followed by cooling in air. This cooling faster than the cooling in the annealing.
A heat treatment process to produce spherical structure carbida (spheroid) in the ferrite matrix. At this Spheroidizing process will improve machinibility on high alloy steels Carbon levels. It can simply be described as follows: that the steel hypereutectoid the dianneal it has a structure consisting of pearlite is encased by tissue cemented. The existence of a network cemented (cemented network) This led to steel (hypereutectoid) has a low machinibility. To fix the cemented network must be destroyed by spheroidizing process.
Spheroidizing was conducted by warming up to around the critical temperature A1 below or a little below it and left at that temperature for a long time (about 24 hours) and then cooled. Because it is at a high temperature for a long time then cemented that had been shaped plate or plates would crumble into small balls (spheres) called Spheroidite dispersed in ferrite matrix.
4. Annealing Process
A heat treatment process that is intended to soften and increase the return tenacity workpiece to be deformed further. Basically, the process Annealing and Stress relief annealing that it has in common that both processes do still below the A1 (critical temperature A1) so basically all that happened was recrystallized alone.
5. Stress relief annealing
A heat treatment process to relieve residual stress as a result of the previous process. Keep in mind that steel with a carbon content below 0.3% C can not be hardened by making the form martensite microstructure. Well, how can the hardness increases but no martensite microstructure? Yes, it can be done by cold working (cold working) but keep in mind that the effects of cold working will Timbu voltage in the name or to eliminate the residual stress and residual stress is necessary to Stress relief annealing process.
Heat Treatment with cooling
A. Heat Treatment with no continuous cooling
If the steel is cooled from a higher temperature and then detained at a lower temperature for a certain time, it will produce a different microstructure. This can be seen in the diagram: Isothermal ransformation Diagram.
Figure 6.4 Isothermal transformation diagram for 0.2 C. 0.9% Mn steel
Explanation of the diagram:
Diagrams depends on the chemical composition mainly carbon content in steel.
For steel with a carbon content of less than 0.83% retained certain temperature dititik located at the top of the curve C, will produce pearlite and ferrite structures.
When arrested the temperature at a certain point the bottom of the curve C but still hand side above the horizontal line, it will get the microstructure Bainite (harder than pearlite).
When arrested the temperature at a certain point below the horizontal line, it will receive martensite structure (extremely hard and brittle).
The higher the carbon content, the two pieces of the curve C will be shifted to right.
The grain size is strongly influenced by the high heating temperature, the longer the duration of heating and heating will arise larger granules. The faster the cooling will produce a smaller grain size.
B. HEAT TREATMENT WITH CONTINUOUS COOLING
In practice the cooling process in the manufacture of steel material to be continuous from a higher temperature to lower temperature.
Effect of cooling rate manerus to microstructure formed can be seen from the diagram continuos Cooling Transformation Diagrams.
Explanation of the diagram:
In the process of slowly cooling as in line (a) will produce pearlite microstructure and ferlit.
In the cooling process was, like, on the line (b) will produce pearlite and bainite microstructure.
In the rapid cooling process, such as a line (c) will produce martensite microstructure.
In practice there are three heat treatment in steel making:
Softening (Annealing): heating intermediate product at a temperature 850-9500 C in a certain time, and then cooled slowly (such as a line-diagram above). This process takes place in the kitchen (furnace). The resulting granules are generally large / coarse.
Normalizing: heating intermediate product at a temperature of 875 9800C followed by open air cooling (such as line-b diagram above). The resulting granules are generally coincides with the implementation of the grinding heat conditions (rolling).
Quenching: system rapidly cooling the steel product by spraying water on immersion and immersion into the product is still hot water or oil medium. This cooling system such as line-c above diagram.
Aside from the aforementioned third heat treatment system there is also a second stage heat treatment in a temperature range below the austenite is called Tempering. Reheating steel products is usually done to products that were previously in quenching. Once in tempering, it is expected that the product will be more resilient and tough.