What is it that makes an alloy steel aircraft quality?
There are several differences as compared to the normal or commercial quality alloy.
In comparing the chemical analysis we see that aircraft alloys require lower levels of both
phosphorus and sulfur to reduce the potential for harmful inclusions.
The melt cycle requires that the material is vacuum degassed or melted under vacuum to
remove oxygen and other dissolved gasses resulting in a further reduction of harmful inclusions.
Aircraft alloys also require testing to assure that the above requirements have resulted in the
expected cleanliness and integrity of the alloy steel.
Tests are performed to assure the material is essentially free of internal cracks, voids, or
excessive segregation.
Other tests of the microstructure categorize any remaining inclusions as to their size and
frequency. Standards limit both the size and frequency. The larger the inclusion, fewer are
allowed.