| Effects of austenitizing and tempering temperatures on the Charily impact toughness of AISI 4340 steel have been investigated. The ductile to brittle transition temperature of AISI 4340 steel is lowered with increasing austenitizing temperature from 790℃, showing a trough at 900℃, and then is raised with increasing austenitizing temperature beyond 900℃. The room temperature impact toughness of AISI 4340 steel is also superior when austenitizing treatment is done at 900℃, while the toughness is inferior when the austenitizing temperature is deviated from the 900℃. A trough in room temperature Charpy impact energy, indicative of temper embrittlement, is observed at 350℃ tempering. This temper embrittlement phenomenon is also observed in ductile to brittle transition temperature and -196℃ impact energy. The prior austenite grain size is gradually increased with increasing austenitizing temperature from 790˚to 900℃, however, the grain coarsening proceeds rapidly with increasing austenitizing temperature above 900℃. |
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