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Subject on This Issue:
* Steels & Properties
* Heat Treatment


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A Carburizing Study
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This experiment was carried out to obtain the relationship between the temperature profile during the carburizing process and the resulting hardness in various positions in a cylinder carburizing furnace.

Experimental

Nine specimens were used to monitor the carburizing results in nine different locations in the furnace. The specimens were placed on a cage that is always used to place parts for carburizing. Four specimens were put on the top layer, one on the middle layer, and the rest were put on the bottom layer.

The four specimens were arranged such that it could represent the temperature uniformity and carburizing results of the whole layer of parts to be carburized. One specimen was put in the center, while the other three were put on three locations with the same distance from the center (and consequently from the furnace wall too), forming an equilateral triangle. Fig.1 illustrates the placement of the specimens on the top layer.

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Fig.1 Arrangement on the top layer (four samples)

The distance from specimen 4 to the other three specimens are the same. Hence, it can be concluded that the three specimens are exposed in the same way to the furnace wall, but at three different directions. Same placement also applies for the bottom layer. In the middle layer, one specimen is placed on the center.

Carburizing cycle

The carburizing cycle is a typical one for production parts, as shown in Fig.2.

 

Fig.2 A carburizing process

Hardness Test

After the specimens were carburized, a sample was prepared from each of them and the surface hardness and depth hardness were examined. The hardness results are given in Figs.3-7.

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Fig.3 Surface hardness of six samples


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Fig.4 Hardness of six samples at 0.05mm depth

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Fig.5 Hardness of six samples at 0.10mm depth

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Fig.6 Hardness of six samples at 0.20mm depth

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Fig.7 Hardness of six samples at 0.30mm depth

Remarks

There are not much differences in the hardness of all specimens on the surface and up to a certain case depth. The case depth measurements are taken at 0.05mm, 0.1mm, 0.2mm, and 0.3mm, and the deviations of the hardness values are 2.8 HRC, 1.1 HRC, 1.4 HRC, 4.6 HRC, and 17.5 HRC for the surface, 0.05mm, 0.1mm, 0.2mm, and 0.3mm respectively. The hardness values up to 0.20mm do not show much difference among one another, but at 0.3mm depth, the hardness difference is evident.

The difference in hardness is caused by the variation in specimen temperature during carburizing. The temperature uniformity of the test specimens was confirmed to be within the tolerance range of ±5 deg C by putting a thermocouple into each of them. But it was observed that the temperature of specimens 6-9 were always above those of specimens 1-5. Higher temperature at the carburizing range enables more carbon to diffuse into the specimens. It is understandable that specimen 6-9 have higher hardness at deeper case depths.

 
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