This specification is intended to cover fasteners for critical applications where a high degree of reliability is required. Commercial fastener standards should be specified for general purpose fasteners.

Coating of grade 8 steel fasteners and socket head cap screws (grade 574 and 4340) for corrosion protection increases their susceptability to embrittlement. The uncoated fastener grades are subject to accelerated corrosion in seawater environments. Therefore, grade 8 steel fasteners and alloy steel socket head capscrews should not be coated due to their susceptability to embrittlement and are not recommended for use in seawater environments. The above requirements on coating and use in seawater environments do not apply to fasteners used as replacements for FF-S-86 fasteners.

Grades 410, 416, and 431 corrosion resistant steels should not be used for seawater environment shipboard applications. These fastener grades are susceptible to both stress corrosion cracking and hydrogen embrittlement due to the hydrogen released in the corrosion reaction when galvanically coupled with steel.

The recommended use temperatures for steel fastener grades are contained in table XIV. TABLE XIV. Recommended use temperatures of steel fasteners.1/

Steel grades Use temperature ---------------------------------------------------------------------- Carbon steel grades SAE 2 and 5 General use to 650 degrees Fahrenheit (°F) Grade B7 externally threaded used For use to 775°F fasteners with grade 2H, 4, or 7 nuts Grade B16 externally threaded fasteners For use to 1000°F used with grade 4 or 7 nut Grade L7 and L43 For low temperature use from minus 60°F to 775°F 1/ The temperatures apply to the fastener materials only and are subject to coating material limitations of table XVI.

The recommended materials for fasteners exposed to salt water spray are contained in table XV.

Grades 400 and 500 nickel alloy fasteners should be used for connections of material submerged in bilge or seawater and in bilge regions.

Corrosion resistant steel bolts and nuts (300 series) should be used for mounting radio and radar antennas to their foundations and where corrosion resistance and low magnetic properties are desired.

The characteristics of the various coatings are contained in the applicable coating specifications. Suitable applications are summarized in table XVI. TABLE XVI. Coatings for various applications.

Upper Required Coating type Base material temperature1/ salt spray Environment limit test (min) ----------------------------------------------------------------------------------------- Metallic-ceramic Carbon and alloy 1000°F 1000 hours High temperature MIL-C-81751, steels2/ ⅝ inch and severe corrosion type I, class 4 and larger IVD aluminum Carbon and alloy 925°F 336 hours Moderate temperature MIL-C-83488, steels2/. No size and corrosion type II, class 3 limitation Immersion zinc/ Carbon and alloy 650°F 336 hours Moderate temperature flake MIL-C- steels2/. No size and corrosion 87115, class 3 limitation Zinc coating Carbon and alloy 300°F 96 hours Ambient temperature ASTM B 695, steels2/. No size and mild class 12 or limitation corrosion ASTM B 633, type II, class Fe/Zn 13 1/ Maximum service temperature applies to coating alone and not to the base metal (see table XIV). 2/ Grade 8 alloy steel shall not be coated as all coatings are anodic to steel and may produce hydrogen embrittlement in high strength steels.

Users are advised that for level I applications (as defined in NAVSEA 0948-LP-045-7010, appendices A and B) lot definition "a" shall be used when ordering. Lots procured to definition "b" may be inhomogeneous with regard to material chemistry and processing except for heat treatment. Therefore, the sampling plan defined in table XII may provide significantly less assurance than lot definition "a".