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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.

Air melt low alloy steels many years ago tended to have more pronounced grain orientation differences in mechanical properties than now, and it was felt then that transverse embrittlement samples would provide more sensitivity in catching embrittlement. One specification ASTM F519 was revised in the early 1970's to require a plating sensitivity test with each manufacturing lot of test samples, a requirement that still stands today. The existence of this sensitivity test requirement has helped greatly and gives assurance that each lot of embrittlement samples will catch hydrogen embrittlement if it occurs during a metal finishing operation.

Current Industry Status:

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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