Ynoshin Imai and Tetur Ishizaki
The present investigation has been carried out to ascertain the effect of
nitrogen on blue brittleness in steels. In the 1st and 2nd report of ``Nitrogen
as the alloying element in steel, it has been shown that the phenomenon of the
temper brittleness is due the presence of the nitrides separated along the solubility
line below the AN point. Another type of bittleness, usually observed
in C-steels is ``blue brittleness,'' which manifests itself during working or
stressing at the temperature from 100° to 300°C, or by cold-worked steel becoming
brittle if heated to this temperature. To as certain the effect of nitrogen
on the blue brittleness, the short time tensile properties at the temperature
from 0° to 320°C for the steels showed in Table 1~Table 4 were determined. All
specimens employed for this investigation were air cooled from 950°C, and then
slowly cooled after tempered for 3 hours at 550°C. The tensile values of each
steels are plotted in Fig. 2~Fig. 29, and the increased percent of tensile strengh
and decreased percent of elongation or reduction of are at blue brittleness
range are plotted in Fig. 8, 14, 23, and 28.
From the present investigation anthers concluded the effect of the nitrogen on the blue brittleness in steels is as follows :
(1) There is a clear correlation between the blue-brittleness and nitrogen as shown in Fig. 8, 14, 23, and 28.
(2) The blue-brittleness did not occur in steels free from the nitrogen and the more nitrogen content is in steels, the more blue-brittleness clearly appears.
(3) In pure iron the blue drittleness increased severely at the nitrogen content rang from 0° to 0.007%, and with more nitrogen content, the blue brittleness increased slowly.
(4) By the existence of carbon, the severely increased range of blue brittleness is reduced.
(5) The blue brittleness appeared at about 70°C and the maximum point of tensile strength moved to high temperature with the increase of nitrogen.
(6) The blue-brittleness is decreased by the existence of manganese, but according to increasing of nitrogen content, the blue-brittleness clearly appears and the beginning point of blue-brittleness moved to low temperature.
(7) In the steels containing less nitrogen, the blue brittleness is recovered slowly by heating, and in the steels containing more nitrogen the recovey is rapid.
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