Originally published September 1997
(Whether youíre building an SR-71 or a homebuilt with a titanium gear spring, this will be important...)
Use passivated stainless steel bolts in the AN and NAS series when bolting a titanium part. Examples are AN4C10 and NAS6304U-10. AN4C10 is a 1/4-inch diameter bolt, 1-5/64 (nominally 1 inch) long, 7/16-inch grip, made of a corrosion resistant steel in the 90 ksi tensile range. NAS6304U-10 is a 1/4-inch diameter bolt, ??? long, 10/16 grip, made of A286 corrosion resistant steel and unplated.
Note: The NAS63xx series is A286 cres (cres--corrosion resistant steel, i.e. stainless steel) in the 160 ksi tensile range. The NAS62xx series is alloy steel and the NAS64xx series is 6AL-4V titanium in 160 ksi tensile range at normal temperatures. At 450 deg F it is good for 95 ksi. (Bolt selection is another long subject.)
Even the preparers of the NAS specifications had to learn about titanium embrittlement. When the spec was initially prepared in 1968 there was a cadmium plated titanium bolt defined. The galvanic table indicates that it would act sacrifically as on a steel bolt. It was declared "Inactive for New Design" after December 1991.
Thanks again to Steve Mitchell and Ron Yarborough for sharing their knowledge.
Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 8 April 1998