Electron Microscopic Study of Precipitates in the 50°C Aging of Iron and Steel

Abstract

With an electron microscope, the present authours have studied the behaviour of precipitates in the case of aging of 0.018%C iron and 0.08%C steel at 50°C. In the age-hardening curve, two steps were observed. The first step appeared between 3 hr and 4 hr aging, and the second step (peak) was observed in the specimen aged for 20 hr. From this experimental results and electron microscopic observations, the authours have concluded as follows: (1) The precipitation phenomenon began from the first step and the rise of hardness before 3 hr aging might be due to the G.P. zone. (2) The precipitation periods in which the disk-like precipitates of about 130∼200 Å were precipitated alone, were continued untill 14 hr aging. (3) In the second step (20 hr aging), the precipitates were not precipitated in largest quantity in each crystal grain. (4) In the 50 hr aging, the low concentration zone of precipitates were grown to about 1000∼1700 Å in breadth along grain boundary as Photo. 11 on some occasion, and in the slip bands, the precipitates of about 150∼350 Å were precipitated perferentially upon the slip lines. (5) The tendency of the site of precipitation generally took the following sequence: The grain boundaries, the vicinity of boundaries, the slip bands and then the whole surfaces in each crystal grains. (6) The disk-like precipitates of about 130 Å were grown to the extent of about 200 Å as the disk-like shape by the process by which the solute atoms are coagulated to precipitates. (7) The disk-like precipitates were thought to solidify again in matrix with the lapse of aging time, and then the spherical precipitates were problably precipitated anew from its matrix. (8) The spherical precipitates when grown to about 300 Å, coalesed with each other and then grew into the large spherical precipitates. (9) In the crystal grain, the large plate-like precipitates formed by coalescence of spherical precipitates were almost never observed, but were observed slightly in the grain boundaries and its vicinity. (10) In the 8760 hr(1 year) aging, precipitates were observed abundantly on the whole surface of every crystal grain, and the specimen’s hardness was rose to a value higher than the highest value in 70°C aging. (11) The size of the precipitates was not taken constant size in the specimen which was aged for some aging time, and there was coexistence in the precipitates of various size. (12) In the grain boundaries which appeared the low concentration zones of precipitates, were observed the disk-like precipitates. (13) The precipitates were thought to consist of iron carbide.