Abstract
The effects of various thermomechanical treatments (TMT) on the Charpy impact ductile to brittle fracture transition curve of 4340 alloy steel were examined. The TMT included austenitization at 950°C for 30 minutes followed by forging to 60% reduction of thickness at high temperature and then oil quenched to room temperature. The quenched specimens were then tempered at various temperatures for four hours. Best improvement on impact strength and ductile to brittle transition temperature was obtained by the TMT with a tempering temperature of 550°C. The notch toughness was increased by more than 80%, and the transition temperature is lowered by about 40°C when compared to specimens tempered at 450°C, 500°C or 550°C without forging at austenite region. The specimens tempered at 600°C or 650°C also possessed good toughness, however, the strength was at least 15% lower than the specimen tempered at 550°C. The specimen tempered at 550°C possessed best combination of strength and toughness. The mechanisms of the improvement on notch toughness and ductile to brittle transition temperature are explained in terms of SEM fractograph and TEM microstructure.
Published Version
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