bardster
11-07-07, 03:35 PM
What Does the Crystal Structure Look Like?
As the iron is heated up to above the lower transformation temperature of 1,350xF the crystal structure transforms to a face-centered cubic structure. Therefore, we can say that in order to make the phase of austenite, we need to apply sufficient heat to create the phase.
In addition to this, there is a growth that will occur due to the atomic structural change. Therefore, the size of the mold will change as it is heated. This is called growth and should not be confused with distortion.
So by definition, the lower transformation temperature (or the magnetic change line) is the temperature that the ferrite phase (a body-centered cubic structure) begins to change to austenite (a face-centered cubic structure).
What Then Happens to Steel?
Steel is simply an alloy of iron and carbon. If we look at Figure 2, it can be seen that the diagram only addresses iron. We are now considering steel, therefore a horizontal carbon line is now drawn and shown in Figure 3.
3101
Iron carbon equilibrium diagram.
The line at 0.77 percent carbon is known as the eutectoid line. To the left of the line, the steels are known as hypo eutectoid steels (ferrite condition) and to the right of the line, the steels are known as hyper eutectoid steels (cementite condition). In order to establish what the upper change temperature would be to ensure a complete phase change form ferrite to austenite, one would need to know the carbon content of the steel. In other words, if we consider a steel at 0.40 percent carbon, we would look for a 0.40 percent on the horizontal carbon line and extend the line vertically. At the point where the line intersects the upper change line, the intersect point would extend a line horizontally to intersect the vertical temperature line. This would be the temperature where the ferrite has changed fully to austenite. Once the austenitizing temperature has been established, then approximately 50xF is added to that temperature to ensure that the steel is in the austenite region for full transformation. If the steel is left at a temperature that is in the transformation area of austenite + ferrite, then a mixed phase will exist and will not fully transform. Both phases have different volumes.
Once the steel is in the austenite region, it is necessary to cool it down to create the particular phase that is necessary for the steel to function, either for machining or for performance. The rate at which the steel is cooled will determine the phase or microstructure. The cool down can be slow or fast, depending on what is to be accomplished.
By controlling the soak temperature and the cool down rate of the steel, we can determine the process to be accomplished. Those processes include annealing, normalizing, stress relieving, hardening and tempering.
As the iron is heated up to above the lower transformation temperature of 1,350xF the crystal structure transforms to a face-centered cubic structure. Therefore, we can say that in order to make the phase of austenite, we need to apply sufficient heat to create the phase.
In addition to this, there is a growth that will occur due to the atomic structural change. Therefore, the size of the mold will change as it is heated. This is called growth and should not be confused with distortion.
So by definition, the lower transformation temperature (or the magnetic change line) is the temperature that the ferrite phase (a body-centered cubic structure) begins to change to austenite (a face-centered cubic structure).
What Then Happens to Steel?
Steel is simply an alloy of iron and carbon. If we look at Figure 2, it can be seen that the diagram only addresses iron. We are now considering steel, therefore a horizontal carbon line is now drawn and shown in Figure 3.
3101
Iron carbon equilibrium diagram.
The line at 0.77 percent carbon is known as the eutectoid line. To the left of the line, the steels are known as hypo eutectoid steels (ferrite condition) and to the right of the line, the steels are known as hyper eutectoid steels (cementite condition). In order to establish what the upper change temperature would be to ensure a complete phase change form ferrite to austenite, one would need to know the carbon content of the steel. In other words, if we consider a steel at 0.40 percent carbon, we would look for a 0.40 percent on the horizontal carbon line and extend the line vertically. At the point where the line intersects the upper change line, the intersect point would extend a line horizontally to intersect the vertical temperature line. This would be the temperature where the ferrite has changed fully to austenite. Once the austenitizing temperature has been established, then approximately 50xF is added to that temperature to ensure that the steel is in the austenite region for full transformation. If the steel is left at a temperature that is in the transformation area of austenite + ferrite, then a mixed phase will exist and will not fully transform. Both phases have different volumes.
Once the steel is in the austenite region, it is necessary to cool it down to create the particular phase that is necessary for the steel to function, either for machining or for performance. The rate at which the steel is cooled will determine the phase or microstructure. The cool down can be slow or fast, depending on what is to be accomplished.
By controlling the soak temperature and the cool down rate of the steel, we can determine the process to be accomplished. Those processes include annealing, normalizing, stress relieving, hardening and tempering.