Effect of Transient Combustion Species on 4340 Steel.
SRI INTERNATIONAL MENLO PARK CA
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The erosion of gun barrels results from combined chemical-thermal-mechanical action of propellant gases on the gun bore. The rate of the erosion is too great for corrosion by normal gaseous species. It is likely that transient, energetic species including free radicals, such as atomic hydrogen, atomic nitrogen and carbon- or oxygen containing radicals, are the active species that chemically alter the gun bore. Partially dissociated by a microwave discharge hydrogen, nitrogen, and methane were allowed to react with 4340 steel strips heated to 400-900 C. While undissociated gases at low pressures approx. 1 torr were found to have negligible effect other than those due to heat treatment, atomic free radicals were highly reactive at steel temperatures ranging from 600 to 900 C. At 600 C atomic hydrogen removed carbon from surface layers and at 900 C bulk decarburization was rapid. Surface hardness decreased with carbon removal and the microstructure was observed to change from martensite to ferrite with increasing decarburization. Microwave dissociation of a 3 CH4-helium mixture was found to case-carburize the steel strips. Similarly, dissociated nitrogen nitrided the steel. The hardness after exposure to dissociated methane or nitrogen were substantially higher than that of the original material.
- Metallurgy and Metallography