The plastic deformation behaviour of AISI 4340 alloy steel subjected to high temperature and high strain rate loading conditions

The purpose of this study is to investigate the deformation behaviour of AISI 4340 alloy steel under high strain rates ranging from 500 to 3300 s^-1 and at constant temperatures in the range of 25-1100°C by means of a split Hopkinson bar. The tested temperatures are obtained by enclosing the specimen in a clam-shell radiant-furnace. The S.E.M. and T.E.M. techniques are also used to analyze the fracture and microstructure characteristics of the deformed specimens. With the macroscopic and microscopic results, a constitutive equation incorporating the effects of temperature, strain rate and work-hardening rate is proposed to describe the plastic deformation behaviour of the material. The experimental results indicate that the flow stress of AISI 4340 alloy steel increases with strain rate, but decreases with the augmentation of temperature. Also, the work-hardening coefficient, strain rate and temperature sensitivities change with the variation of strain rate, strain and temperature levels. Using the parameter values of the materials obtained from mechanical testing in the Johnson-Cook constitutive equation, the simulated results show that the proposed constitutive equation predicts an acceptable flow stress value with a reasonable extent of error. The adiabatic shear failure mode predominates in the fracture behaviour of the material over the whole range of strain rates and temperatures studied. Microstructural observations reveal that the features of dislocations and the precipitation of particles change in accordance with the variation of the strain rates and the loading temperatures.