抄録
In order to clarify the effect of cyclic loading during the tempering on the temper brittleness of Ni-Cr steel, the transition temperatures of specimens which had been tempered under various loading and cooling conditions were examined, and the precipitation and the grain boundary etching in each tempered specimen were observed by the electron microscope.
The summarized results are as follows:
(1) In all cases of the cooling methods, such as water cooling, furnace cooling A (cooling rate: 4°C/min), furnace cooling B (cooling rate: 1°C/min) and furnace cooling A with loading, the effect of cyclic loading during the tempering on the transition temperature clearly exists and the transition temperature rises with the increase of stress amplitude. And the effect of cyclic loading on the transition temperature is more remarkable at higher cooling rate. The transition temperatures in both conditions of the furnace cooling A and the furnace cooling A with loading are nearly the same value at all stress amplitudes.
(2) The grain size and the hardness in the tempered specimens under the various conditions are almost constant, that is, the grain size is about 40μm in diameter and the hardness is about 20HRC. There are no effects of cyclic loading during the tempering on the grain size and the hardness in the tempered specimens.
(3) There is a slight correlation between the volume of precipitation and the transition temperature. The transition temperature has a tendency to rise with the increase of the volume fraction of precipitation, but this tendency is not so clear. There is a clear correlation between the extent of boundary etching and the transition temperature. When the cyclic load during tempering increases, the extent of boundary etching increases and the transition temperature move to the higher side in temperature. In other words, when the cyclic load during tempering increases, the diffusion of some component or the micro precipitation on the grain boundary might be facilitated and the temper brittleness might be promoted.
