Scientia Sinica

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SCIENTIA SINICA Vol, V

Attention is drawn to the fact that there is no 130°C peak on curve I (Fig. 7). The absence of this peak was also found to be the case in a lowcarbon steel specimen. This specimen was carburized to a carbon content of 0.25% and was quenched from 930°C into 10% brine after a thoroughly homogenization treatment. It has a martensitic structure as revealed by metallographic examinations. The internal friction curve for this specimen is shown by curve I of Fig. 8, and the absence of the 130°C peak is obvious.

The absence of the 130°C peak can be regarded as an indication of that a martensite of 0.25% carbon composition does not transform into €-carbide during the first-stage tempering. Since we have concluded from internal friction measurements that €-carbide is formed in specimens containing 0.291.47% carbon, we can consider the absence of €-carbide in the first-stage transformation of the 0.257 carbon specimens as in agreement with Kurjumov’s X-ray observation that martensite of 0.25% C is an integral transformationproduct in the tempering of high carbon martensite.

After the completion of curve I of Fig. 7, the specimen was cooled down in the measuring furnace from 280°C. Internal friction measurements were taken again from room temperature upwards, and it was found that the internal friction began to rise at 120°C (cure II). The “235°C peak” became lower than that of curve I. When the temperature was lowered after measurements had been taken up to 300°C, the “235°C peak” was found to be further reduced, but a pronounced internal friction peak was observed unexpectedly around 150°C as shown by curve III. When the temperature was raised again and internal friction measurements were taken, a lower internal friction peak was observed around 140°C as shown by curve IV (a frequency of vibration of 0.46 cycle per second was used in this measurement, which was lower than that used in previous runs). The internal friction peak was further lowered (curve V) when measurements were taken with the temperature lowered from 180°C. Finally, this internal friction peak disappeared completely after the measurements were repeated several times with the temperature changing up and down.

Measurements on the 0.25% carbon steel specimen were proceeded as shown in Fig. 8. After the completion of curve I, the specimen was furnacecooled from 280°C and measurements were taken again from room temperature upwards. Curve II shows that the internal friction value rises gradually from 120°C up. As the temperature was lowered from 260°C, a pronounced internal friction peak was also observed around 150°C as shown by curve III. Hereafter, the variation of this internal friction peak with the raising or lowering of temperature was similar to the case of the first specimen as shown in Fig. 7.

From the experimental results described above, we may consider that the low-carbon martensite (containing 0.25%C) transforms at temperatures