Scientia Sinica

INTERNAL FRICTION PEAKS ASSOCIATED WITH THE TEMPERING OF MARTENSITE IN STEELS*

K& T’inc-Sur (T. S. Ké, #288) and Ma Yrnc-Liane (REE )

(Institute of Metal Research, Academia Sinica)

ABSTRACT

Internal friction in hardened carbon steels was measured with a torsion pendulum and an internal friction peak was observed around 130°C when measurements were taken from room temperature upwards. This peak disappeared completely after the temperature of the specimen once reached 170°C. This phenomenon was observed in carbon steels containing carbon ranging from 0.29% to 1.4%, and also in an alloy steel. The appearance of this internal friction peak seems to indicate that the transformation product (€-carbide) formed in the first-stage tempering of martensite is coherent with its parent phase, and the origin of internal friction is the stress-induced rmaovement of the plane of coherence.

The above-mentioned internal friction peak was not observed in 0.25% carbon steel specimens having a martensitic structure. However, after such a specimen had been tempered at a temperature around 300°C, an internal friction peak was observed’ around 150°C, This indicates that the transformation product formed in martensite containing 0.259% carbon in the third-stage tempering is coherent with its parent phase. Since the internal friction peak associated with this transformation product behaves differently from that associated with €-carbide, it is concluded that the third-stage transformation product is not €-carbide.

I. INTRODUCTION

In the tempering of hardened carbon steels, the first-stage transformation occurs in the temperature range of 80—160°C. Important information as to the nature of the transformation product formed during this stage was given by Kurjumov and Lysak based on the results of X-ray study on single crystal martensite. They found that a low-carbon martensite containing 0.257-C was formed as an intermediate phase during the first-stage of tempering". This shows that the change of tetragonality (axis ratio) of martensite is not continuous in the process of decomposition, but changes abruptly to a low-carbon martensite with a definite lower axis ratio corresponding to a carbon content of 0.25%, C. This experimental observation was later confirmed by other workers”).

The first stage of tempering consists, thus, of the formation of a low carbon martensite containing about 0.25% C and a transitional phase of carbide

*First published in Chinese in Acta Physica Sinica, Vol. XI, No. 6, pp. 479—492, 1955.