Abstract
The relationships between strength indices (yield strength, tensile strength and hardness) and parameters of microstructure in two commercial plain carbon martensitic steels (0.7% and 1. 1% C) have been studied after varying isothermal tempering treatments at 690°C.The size-frequency distributions of cementite particle diameter on a polished cross-sections of the specimens which tempered at 690°C for various periods of time (6mn. to 400h) and the number of ferrite grains per unit area for the specimens after times in excess of 10h have been measured by the use of an optical and an electron-microscope, and from these results, the mean diameter of cementite particles, the mean free ferrite paths, and the ferrite grain size of the specimens after 10h or longer tempering.The various relationship which have been proposed between parameters of the microstructure and strength indices are confirmed with the present data. The results which were confirmed the linearity of the relationship derived by Gensamer and co-workers are showed a fairly large experimental scatter and the reverse-(Japanese He) type curve. The strength indices as a function of the mean diameter of cementite particles have been shown the discontinuity at the corresponding values for the tempering time between 1h and 3h. From the present results, it is suggested that the strength indices of the tempered steel containing non-coherent particles of cementite depend upon the development of minute ferrite grains which are bound by the low energy boundary, the transition from the construction of low energy boundary to that of ordinary grain boundary, and the growth of ferrite grains whichare bounded by the ordinary boundary.
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