The effects of austenitizing and tempering temperatures on the mechanical properties and fracture surface characteristics of AISI 4340 steel were investigated. The microstructure was also examined by scanning and transmission electron microscopy. The results indicated that with increasing the austenitizing temperature from 850°C to 1200°C, the yield strength, ultimate tensile strength, and impact energy were decreased by 23%, 24%, and 35%, respectively. Moreover, the increase of tempering temperature from 200 to 350°C decreased these mechanical properties by 15%, 7.5%, and 35%, respectively. The optimal combination of strength and ductility was obtained at austenitizing temperature of 900°C and tempering temperature of 200°C. The variation of total elongation and strength-elongation balance with the austenitizing and tempering temperatures were similar to that of the average dimple area. Likewise, the variation of the yeild strength with the tempering temperatures was similar to that of the average dimple area and opposite to that of the dimple area density.
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