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Many times we are asked the question about grain flow orientation for embrittlement samples - longitudinal or transverse? At the steel mill during the hot and cold rolling process, the steel, containing impurities known as inclusions, will assume a primary orientation or grain direction. Secondary grain flow directions (2) occur at right angles to the primary direction. The primary direction is usually referred to as the longitudinal and the secondary directions are called transverse (long and short). These impurities/inclusions align themselves during hot and cold rolling, producing preferred orientations within the steel. They can cause differences in the mechanical properties of the steel, depending on how the steel is put under stress. The presence of these inclusions and their orientation within the steel can also cause differences in the hydrogen embrittlement behavior of a steel component.
In the late 1950's and early 1960's, hydrogen
embrittlement problems were occurring with more frequency in the U.S.
aircraft industry. During a technical forum in Los Angeles in September
1961 a consensus was reached with several aircraft primes and representatives
of the metal finishing industry to begin embrittlement testing. The embrittlement
samples developed then (and still the world standard today) are manufactured
from 4340 low alloy air melt steel, heat treated to the highest range
(260-280 KSI) with notch placement the last manufacturing step. The notch
is required to possess a Kt (stress concentration) factor of 2.9 - 3.3
which results from the 60 deg. included angle, 0.010" notch radius, and
a notched cross sectional area one half that of the gauge shank area.
Soon after this 1961 Technical Forum, specifications QQ-P-416 (Cadmium)
QQ-N-290 (Nickel) and QQ-C-320 (Chrome) were revised to require embrittlement
testing on a periodic basis. Over the next 7 years efforts at accomplishing
the intent of these new tests began to falter as some suppliers found
ways to jade the testing in their favor This was accomplished via selecting
4340 steel melt chemistries that tended to be less sensitive to embrittlement.
In the late 1960's, numerous plating specifications were revised again
to require test samples to be made from steel with the loading axis perpendicular
to the short transverse grain direction of the steel.
Manufacturing productivity concerns influenced
all major aircraft primes in recent years, and now the majority of the
metal finishing specifications existing in the U.S. aircraft and aerospace
industry specify longitudinal grain hydrogen embrittlement test samples
in conjunction with sensitivity testing of each manufacturing lot. Some
exceptions do exist and the reader is advised to refer to their procuring
agency contractual requirements.
Approximately 90% of all hydrogen embrittlement
test samples used for sustained load tensile testing today are longitudinal
direction samples. The requirement of an embrittlement sensitivity test
assures that each lot of samples produced is sensitive to hydrogen embrittlement.
This in turn assures satisfactory performance of the test samples in monitoring
potential embrittling processes.
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