Scientia Sinica

No..1. KE, MA: INTERNAL FRICTION ASSOCIATED WITH TEMPERING OF MARTENSITE 29

around 300°C into a product which is coherent with its parent phase and thus gives rise to internal friction. This corresponds to the transformation occurred in the third-stage tempering. To obtain a pronounced internal friction peak, specimens should be tempered at a sufficiently high temperature for a sufficient length of time so that an appreciable amount of the transformation product was formed. In completing curve I of Fig. 7 or Fig. 8, the specimen reached a temperature of 280°C only for a brief period and therefore the internal friction showed only a gradual rise from 120°C upwards (curve IJ). Only when the specimen was tempered again at a higher temperature for a longer period of time that a pronounced internal friction peak was observed around 150°C (curve III). But, on the other hand, the internal friction became lower when the tempering temperature was too high or the tempering time was too long. This is probably because of that the coherency of the transformation product was destroyed by these treatments.

The transformation product of the third-stage tempering of low-carbon martensite has been investigated by electron microscopic observations'!. With a specimen: containing 0.15% C, the formation of carbide was observed after a tempering for one hour at 205-315°C. According to the internal friction experiments described above, we can consider that the transformation product is coherent with its parent phase, but this product is not €-carbide. The 130°C internal friction peak associated with €-carbide disappeared completely after a tempering at 170°C. This indicates that the coherency which gives rise to the internal friction peak was completely destroyed by tempering at 170°C. However, the internal friction peak associated with the transformation product of the low-carbon martensite was observed with temperature lowered from 300°C. This indicates that the coherency was not destroyed by a tempering at 300°C.

One may also conclude that no ferrite was formed when 0.25% carbon martensite was tempered around 300°C, as the carbon-diffusion peak around 40°C was not observed after such a tempering treatment (see Figs. 7, 8).

Further research is in progress on the internal friction peak associated with the transformation of low-carbon martensite in the third-stage tempering.

IV. Discussions

1. On the mechanism of the internal friction peak.

Further experiments are necessary for an understanding of the exact mechanism of the 130°C internal friction peak. Following is a preliminary account based on the assumption that the observed internal friction peak 1s associated with the stress-induced movement of the surface of coherency between the €-carbide and its parent phase. While internal friction measurements are taken from room temperature upwards to about 80°C, the