Scientia Sinica

No. 1 KE, MA: INTERNAL FRICTION ASSOCIATED WITH TEMPERING OF MARTENSITE 21

diameter of 2 mm with several times of intermediate annealing. The final annealing temperature was 870°C.

The carbon content in all specimens was determined by combustion method after the final treatment.

Ill. ExprertmeNTaL REsULTs

1. Discovery .of the internal friction peak

A specimen from low carbon steel was carburized at 920°C for about two hours. It was then homogenized at 950°C for four hours and quenched into coid water. Carbon analysis showed that the specimen contains 1.36% C. Metallographic examination revealed a structure of martensite and retained austenite. Internal friction measurements were made on the specimen immediately after quenching with a frequency of vibration of 2 cycles per second. As the temperature was raised, a pronounced internal friction peak appeared around 130°C. This peak rose again beginning at about 160°C as shown by curve I of Fig. 1. The specimen was cooled in the measuring furnace to room temperature from 220°C.

Internal friction measurements were again taken by raising the temperature. Curve II of Fig. 1 shows that the internal friction peak around 130°C observed in the first experiment now disappeared completely. It is seen from curve II that when the temperature was raised above 220°C, another internal friction peak appeared near 235°C. This peak was shown to be associated with the interaction of carbon atoms with the quenching stress from the results of previous experiments").

Attention was drawn toward the appearance of the 130°C peak, which was observed for the first time. In order to remove the interference effect of the 235°C peak, internal friction values of curve II were subtracted from those of curve I. The dotted curve III thus obtained shows that this new internal friction peak extends from 70°C to 170°C which coincides with the temperature range of the first stage tempering of martensite. This suggests that the observed internal friction peak may be connected with the transformation of martensite in the first-stage tempering. With this in mind, another specimen was similarly treated as the first one only that the specimen was refrigerated at -78°C immediately after it was quenched from 950°C. Internal friction measurements were taken with this refrigerated specimen beginning at room temperature and curve IV was obtained as shown in Fig. 1. The 130°C peak is seen to be higher after this low-temperature treatment. It is well known that no structural change could occur in the specimen after the refrigeration except the transformation of part of the retained austenite into martensite. Accordingly, the increase in height of the internal friction peak is related to the increase of the amount of martensite in the specimen.