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National Advisory Committee for Aeronautics, Technical Notes - Fundamental Effects of Cold Work on Some Cobalt Chromium Nickel Iron Base Creep Resistant Alloys

The influence of cold—working on the creep properties of an alloy
containing 20 percent cobalt, 20 percent chromium, 20 percent nickel,
and the balance iron and on the same alloy modified by small additions
of tungsten alone or tungsten, molybdenum, and columbium in combination
was studied. It was concluded that the effects of cold—working on creep
resistance were the same for all the alloys studied. This was from the
standpoint of the temperature range over which cold-working could be
expected to be beneficial (temperatures up to 16000 F) and also the
maximum amount of cold-working which could be used to improve creep
properties at about 12000 F (between 15 and no percent). These conclu-
sions were reached in part by studying the creep properties and also in
part by studying internal stress relaxation at the test temperatures
which previous wprk had shown to be the controlling factor in the
response of such alloys to cold-working.

This report is the fifth of a series concerned with the fUndamental
factors influencing the creep properties of creep—resistant alloys for
use in aircraft propulsion systems. Previous articles have dealt with
the influence of chemical composition, precipitation, and cold-working
on high-temperature properties of such alloys (see references 1, 2, 3,
and h.

One Of the previous investigations (see reference 1) showed fairly
conclusively that the improvement in creep properties to be had by cold-
working lOtharbon N—l55 alloy was the result of the introduction of
internal stresses - such stresses acting to broaden X—ray diffraction
lines. Further it was found that increasing degrees of cold reduction
resulted in improved creep resistance up to that amount which caused
significant X—ray line sharpening, which is interpreted as internal
stress relaxationl, to occur at the creep test temperature. From this
it was also reasonable to expect that the maximum temperature at which
cold—working can be expected to improve creep resistance will be the
maximum temperature at which appreciable internal stresses (X-ray line
breadths) are retained during test or service. It also leads to a
redefinition of "cold—working" as working at any temperature for which
no appreciable internal stress relaxation occurs during the working
operation or during the cool down after such working. The temperature
dependence of the influence of "cold-working" on creep resistance was
checked and found to agree with this view.