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Application of the isothermal time-temperature diagrams
The isothermal time-temperature diagrams make it possible to determine the transformation processes which take place at constant temperatures above the temperature of martensite. The diagrams are important for isothermal heat treatments e.g. for austempering, patenting, isothermal transformation tempering (isothermal OB annealing), delayed martensitic hardening and soft annealing. (figure 7)
Application of the time-temperature diagrams for continuous cooling
In practical heat treatment of steels, most of the transformation processes take place during continuous cooling (see figure 6). This applies, for example. to hardening. During hardening, the steel is supercritically cooled from a temperature above the GOSK line in the iron-carbon diagram in order to allow martensite to form.
The cooling curve, which corresponds to this minimum cooling speed, must not cross the line for the start of the transformation in the perlite and bainite stage. It can be taken straight from the time-temperature diagram. The cooling speed which can be achieved depends on the quench intensity of the hardening medium and the dimensions of the workpiece. For cooling round rods, the cooling speeds can be determined relatively precisely.
In relation to the core zone of a round rod, the following table gives the cooling parameters or cooling speeds for hardening with water, oil and air, depending on the diameter.
Cooling parameters λ1)
or cooling speed v for the core of workpieces cooled in water, oil and air.
|
λ
a/o. V |
Diameter in mm when cooled in: |
| Water |
Oil |
Air |
| 0,01 |
10 |
|
|
| 0,03 |
18 |
|
|
| 0,06 |
25 |
10 |
|
| 0,1 |
35 |
18 |
|
| 0,2 |
50 |
30 |
|
| 0,35 |
70 |
45 |
|
| 0,5 |
85 |
60 |
|
| 0,7 |
105 |
70 |
|
| 1 |
130 |
90 |
10 |
| 2 |
180 |
130 |
18 |
| 3 |
230 |
170 |
25 |
| 5 |
300 |
220 |
40 |
| 7 |
360 |
280 |
55 |
| 9 at 20°C/min |
420 |
320 |
65 |
| 18 at 10°C/min |
600 |
500 |
130 |
| 26 at 5°C/min |
900 |
750 |
220 |
| 72 at 2,5°C/min |
1300 |
1150 |
400 |
| 144 at 1,25°C/min |
1900 |
1700 |
700 |
| 255 at 0,8°C/min |
2400 |
2200 |
1000 |
| 450 at 0,4°C/min |
3500 |
3300 |
1900 |
Workpiece moved moderately.
1) λ = Cooling time from 800 – 500°C in sec. x 10-2
Figure 7 (To enlarge, please click on the picture in question )
Isothermal time-temperature diagram and types of heat treatment
The information given in figures 8 to 12 can be used to refer to any points in the cross section. The continuous curve on the ordinate gives the cooling parameters and speeds for a diameter of between 50 and 800 mm depending on the distance from the surface. The dotted curve in each diagram corresponds to cooling in the core of round rods. Figures 8 and 9 refer to quenching in water. The ratios for quenching in oil are presented in figures 10 and 11. Figure 12 only applies to cooling in air.
The given distance to the face in the Jominy hardenability test is also a clear cooling parameter and therefore the distance from the face of the face quenching specimen can be directly drawn in on the right ordinate in figures 8 and 10 as a second scale.
The corresponding cooling parameters and cooling speeds can be calculated from figures 8 and 12 and, in the time-temperature diagram for continuous cooling, these make it possible to predict, relatively precisely, the microstructure after transformation and the grade of hardness achieved though the cross-section, depending on the quailty of steel, dimensions and hardening medium used. Hence, using the continuous time-temperature diagram and the cooling curves, it is possible to calculate the maximum cross section for each grade of steel and with which hardening medium, perfect full hardening is achieved. It is also possible to calculate what improvements in the development of the microstructure can be achieved by selecting a hardening medium which provides a greater thermal shock.
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| Figure 8
Quenching in water |
Figure. 9
Quenching in water |
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(To enlarge, please click on the picture in question) |
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| Figure 10
Quenchingin oil |
Figure. 11
Quenchingin oil |
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(To enlarge, please click on the picture in question ) |
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Figure 12
Cooling in air
The following examples make this clearer:
Example 1
With which cross-section is full hardening achieved in steel 58 CrV 4 (figure 6 prev. page) when quenched in oil?
Solution:
The cooling curve where transformation into perlite and bainite has as yet been avoided is taken from the continuous time-temperature diagram in 58 CrV 4 (figure 6 previous page) . In our case, this is not quite curve 4 with a cooling parameter of 0.31. With a parameter of approx. 0.25, the bainite area would not have been crossed.
If this value is tranferred to the table, with cooling in oil and a cooling parameter of 0.25, full hardening is still achieved in a cross-section of approx. 35mm.
Example 2
A shaft of 100mm in diameter made of 36 CrNiMo 4 is hardened in oil. What hardness and what microstructure are to be expected at the edge and in the core?
Solution:
According to the curve for 100mm diameter in figure 10, the core cools with a cooling parameter of approx. 1.3 (Absciss value 50) and the edge with a cooling parameter of 0.25 (absciss value = 0). The cooling parameter 1.3 for the core corresponds to a curve between the fourth and fifth cooling curve from the left in the time-temperature diagram (figure 13). According to this curve, there is approx.1% ferrite and 85% bainite in the core with residue of martensite and austenite and a hardness of 340 HV. The edge has a microstructure of approx. 10% bainite as well as martensite and small amounts of residual austenite and a hardness of approx 500 HV.
Figure 13 (To enlarge, please click on the picture in question )
Time-temperature diagram for continuous cooling
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