Scientia Sinica

20 : SCIENTIA SINICA Vol, V

haying a hexagonal close-packed structure, and is called €-carbide because its resemblance in structure with the €-phase in Fe-N system™!, On the basis of X-ray observations, Jack considered that this €-carbide was coherent with the primary martensite, with the (101) plane parallel to and coherent with the (101) plane of martensite. According to him, the interplanar spacing of the (101) planes of €-carbide is almost identical with that of the (101) planes of tetragonal martensite, so that periodicity is maintained along the (101) direction. Although Jack’s suggestion on the existence of €-carbide was confirmed later by many workers, yet the evidence he pointed out as to the coherency has been considered to be inconclusive.

We observed recently an internal friction peak associated with the tempering of martensite in steels. This observation leads to the belief that internal friction measurements may be applicable to the study of mechanism of transformation in the tempering of martensite.

In a two-phase structure having a coherency relationship, the application of a stress (no matter how small it is) often gives rise to a stress-induced movement of the coherency plane and leads to the appearance of internal friction. Worrell has observed an internal friction peak associated with the stress-induced movement of the twin-boundaries in a Cu-Mn alloy (containing about 90% Mn)"). Internal friction measurements may thus be utilized as a usefui tool for judging the existence of a state of coherency.

In the following is described the occurrence of the internal friction peaks in the tempering of martensite in steels. Further experimental results will be reported later.

I]. ExprerImENTAL ARRANGEMENT AND SPECIMENS

A torsion pendulum”! with a frequency of vibration of about 2 cycles per second was used for internal friction measurements. Specimens were prepared mostly from low carbon steel and II X15 steel.

The low carbon steel contains 0.22% C, 0.44% Mn, 0.018% Si, 0.015% P and 0.04% S. Steel rods were swaged to a final diameter of about 15 mm with several times of intermediate vacuum annealing. Specimens thus obtained were annealed at 650°C in vacuum and then furnace-cooled.

Carburizing treatment of the specimens was proceeded inside a high temperature furnace (920°C) in an atmosphere of circulating dry hydrogen and benzene vapour. After carburizing, the specimen was homogenized by heating in vacuum for 2-4 hours at a temperature 20—30°C higher than the carburizing temperature.

Steel UIXi5 contains 0.96% C, 1.5% Cr, 0.29% Mn, 0.1% Ni, 0.27% P and 0.01% S. Specimens of this steel were also prepared by swaging to a final