Impact properties and microstructure evolution of 304L stainless steel

Abstract

The impact properties and microstructure evolution of 304L stainless steel have been studied systematically by means of a split Hopkinson bar and TEM metallographic techniques. Cylindrical specimens were deformed at room temperature under strain rates ranging from 10 2 to 5×10 3 s −1, with the true strains varying from 0.05 to 0.3. Results indicate that the mechanical properties and the microstructure largely depend on impact loading. Increasing the strain rate of impact loading increases both flow stress and strain rate sensitivity. However, the inverse tendency is observed for the activation volume. The effect of loading rate on mechanical response and impacted substructure of 304L stainless steel is found to be directly related to the amount of dislocation and the amount of transformed α′ martensite. By using the proposed constitutive equation with the experimentally determined specific material parameters, the flow behaviour of tested material can be described successfully for the range of test conditions. Microstructural observations reveal that the morphologies and characteristics of both dislocation and α′ martensite are sensitive to changes of loading conditions. At high strain rates and large deformation, greater dislocation density, more shear bands and more α′ martensite transformation are observed. Significant strengthening was found to result from dislocation multiplication and α′ martensite transformation, but the latter's effect is more evident as high rate loading is imposed. Correlation between dislocation density, fraction of α′ martensite, flow stress and strain rate is confirmed and is discussed in terms of the observed microstructure.