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National Advisory Committee for Aeronautics, Technical Notes - Abnormal Grain Growth in M-252 and S-816 Alloys

A laboratory study was carried out to establish the basic causes
of abnormal grain growth in air— and vacuumpmelted Mr252 and Sw8l6 alloys.
The results were in general agreement with a previous study of Waspaloy,
Inconel X-550, and Nimonic 80A alloys. Results of tests on the five alloys
indicated that small reductions of essentially strain-free metal were the
basic cause of abnormal grain growth. In most cases, there was a narrow
range of reductions responsible for abnormal growth between reductions
of O.h and 5.0 percent. In a few special cases the responsible reduc-
tions 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 hot—working operations is prob-
ably 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 cause susceptibility to abnormal
grain growth. Removal of strain by recrystallization during working fol-
lowed 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 has a relatively minor effect, pro—
vided 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 of the alloy.