Investigation on Abnormal Brittleness of High Density Sintered Iron

Abstract

It has been reported by H. Kuroki and Y. Tokunaga that most of the commercial iron powders represent an abnormal brittleness, mainly grain boundary rupture and partly cleavage, in the density range of 6.8-7.3 g/cc. In the present work some attempts were made to clarify the cause of these brittle ruptures.It was concluded that the abnormal brittleness is caused by a remarkable grain growth in the density range around 7g/cc. Based on the experimental observations, two different mechanisms of embrittlement are considered.First, the embrittlement can occur through the enhancement of the ductile-brittle transition temperature caused by grain growth, as was observed in the case of electrolytic and millscale iron powders, which is well known on ferritic steel.The second mechanism is due to a grain boundary rupture and observed on reduced ore powder. This type of rupture was observed even at high temperatures (>150°C). Although a large number of fine nonmetallic inclusions are contained in reduced ore iron powder, the volume fraction and the notch effect of these inclusions seem to be relatively smaller than those of the residual pores in the sintered iron structure. Therefore, the brittleness of the coarse grain structure of the reduced ore iron powder compact could be of the same character as low temperature brittleness, but abnormally expanded to high temperatures with a largely diffused transition temperature.