Mechanical properties and microstructural features of AISI 4340 high-strength alloy steel under quenched and tempered conditions

In this work: the mechanical properties and microstructures of AISI 4340 high strength alloy steel under different tempering conditions are investigated. The specimens are quenched and tempered to a martensite structure and loaded to fracture at a constant strain-rate of 3.3 x 10(-4) s(-1) by means of a dynamic material testing machine (MTS 810). The mechanical properties and strain-hardening exponent are considered as function of the tempering conditions. The morphological features of the as-quenched martensite and their evolution during tempering are described. Fractographs of the specimens are also made in order to analyse their fracture and embrittlement mechanisms. The results indicate that the mechanical properties and microstructural features are affected significantly by tempering temperature and holding time. The strength and hardness of tempered martensite drop as the tempering temperature and holding time are increased. However, the ductility increases with increasing tempering temperature and holding time, except when tempered martensite embrittlement occurs. Microstructural observations reveal that the carbide precipitates have a plate-like structure at low temperatures, but are spheroid-like at high temperatures. Under the tested conditions. the fracture appearances show that the material failed in a ductile manner except for the case of tempering at 300 degrees C, where tempering martensite embrittlement occurs due to the existence of retained interlath austenite. (C) 1999 Elsevier Science S.A. All rights reserved.