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National Advisory Committee for Aeronautics, Technical Notes - Abnormal Grain Growth in Nickel-Base Heat-Resistant Alloys

A laboratory study was carried-out to establish the basic causes
of abnormal grain growth in air- and vacuumsmelted.Waspaloy, Inconel
X-550, and Nimonic 80A alloys. All of the results indicated that small
reductions of essentially strain-free metal were the basic cause of
abnormal grain growth. Between reductions of 0.h and 5.0 percent, in
most cases, there was a narrow range of reductions responsible for
abnormal growth. In a few special cases the responsible reductions
were as low as 0.1 percent and as high as 9.7 percent.

The prevention of abnormal grain growth clearly requires avoidance
of small critical reductions. The main problem is to anticipate and to
avoid conditions leading to critical deformation. Insuring that all
parts of a metal piece receive more than 5- to lO-percent reduction will
prevent it. Nonuniform metal flow during hotsworking operations is
probably the major source of abnormal grain growth. Any small reduction,
particularly if it includes a strain gradient so that the critical reduc—
tion will definitely be present, is a common source. Strains arising
from thermal stresses during rapid cooling can develop susceptibility.
Removal of strain by recrystallization during working followed by a small
further reduction can, in certain cases, induce abnormal grain growth in
the presence of large reductions.

The phenomenon of abnormal grain growth is remarkably independent
of temperature of working and of heating temperatures. If the heating
temperature and time are sufficient for abnormal grain growth, higher
temperatures increase the grain size only slightly. Prior history of
the alloys before critical straining also has relatively little effect,
provided the prior treatment reduces strain below the critical amount.
Certain conditions of working or heating seemed to minimize abnormal
grain growth. These, however, do not appear dependable for controlling
abnormal grain growth because of the probability that their effectiveness
is dependent on prior history.