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The effect of various zinc phosphating treatments on the hydrogen embrittlement susceptibility of high-strength steel artillery submunitions and the effect of subsequent thermal relief treatments were evaluated in this study.
The submunitions were two types of grenade bodies fabricated from AISI 4140 steel and AISI 15B41 steel. The first type of grenade body contained an embossed inside wall and was heat treated to a hardness Rockwell “C” (HRC) range of 38 to 45 HRC. The second type of grenade body contained a smooth inside wall and was heat treated to a hardness range of 44 to 50 HRC.
Test specimens, machined from the heat treated and zinc phosphate-coated components, were in the form of rings that were stressed to 65% of the nominal breaking load for 200 h. Specimens that failed the 200-h test were considered embrittled; specimens that did not fail were considered nonembrittled. Failed specimens were examined with a scanning electron microscope for the evidence of intergranular fracture, which substantiated that hydrogen embrittlement had occurred.
It was shown that the zinc-phosphated material was susceptible to hydrogen embrittlement. The susceptibility increased when the exposure time and the acid concentration of the zinc phosphate process increased. Thermal bake-out treatments at ambient and at elevated temperatures were effective in reducing the embrittlement effect. Thirty days at ambient temperature or 3 h at 93°C (200°F) were found to be adequate in eliminating hydrogen embrittlement. These relief treatments are not effective when the acid concentrations in the zinc phosphate-coating process are abnormally high.
hydrogen embrittlement, high-strength steels, zinc phosphate coatings, thermal relief treatments, bakeouts, delayed failure testing
Voorhis, Gerard P.
Materials engineer, U.S. Army Armament Research, Development, and Engineering Center, Dover, NJ