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National Advisory Committee for Aeronautics, Technical Notes - Abnormal Grain Growth in S-816 Alloy

This report presents the results obtained to date of an investiga-
tion to establish the fundamental causes of abnormal grain growth in
S-816 alloy under conditions encountered during the forging of blades
for the gas turbine of jet engines, The data reported are confined to
the experiments conducted to learn how to produce abnormally largegrains
in bar stock under controlled conditions. These are being reported
because they may be useful to those concerned with the problem.

Abnormal grain growth was induced by temperature cycling alone.
Water-quenching of the bar stock from 21500 or 23000 F rendered the
samples susceptible to abnormal grain growth upon reheating to 23000 F.
Air—cooling Was sufficient to induce grain growth during subsequent
solution treatment for some of the prior treatments of the stock. Cer—
tain of the prior structures were susceptible to grain growth by tem—
perature cycling only to 21500 F.

Studies of critical deformation for abnormal grain growth on sub-
sequent solution treatment indICated that the critical deformation by
rolling lies between 0 and 2 percent. The critical deformation was
independent of the temperature at which the deformation was carried out.
Repeated critical deformations between reheats to 21500 F resulted in
much larger grains after solution—treating at 23000 F than a single
critical deformation.

To varying extents the conditions for abnormal grain growth are
sensitive to the prior history of the material. Several‘other possible
variables have not been investigated. For this reason the generality
of the results remains to be established. This factor, however, will
probably be of no great practical significance for solution treatment
at 23000 F while it will be important to the occasional cases of abnormal
grain growth when temperatures are restricted to 21500 F. Apparently
avoiding abnormal grain growth will require deformations in excess of
2 percent and relatively slow cooling from.the forging temperature.