The effects of strain rate and welding current mode on the dynamic impact behavior of plasma-arc-welded 304L stainless steel weldments

Abstract

This article uses a split-Hopkinson pressure bar to investigate the effects of strain rate in the range of 103 s−1 to 8 × 103 s−1 and welding current mode upon the dynamic impact behavior of plasma-arc-welded (PAW) 304L stainless steel (SS) weldments. Stress-strain curves are plotted for different strain rates and welding parameters, and optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques are used to analyze the microstructure and fracture characteristics of the weldments. The results confirm that the strain rate and the welding current mode have a significant influence upon the dynamic impact behavior and microstructure evolution of 304L SS weldments. It is shown that for a constant strain, the flow stress increases with strain rate for both welding current modes, and that the pulsed current (PC) mode results in a higher weldment strength than the continuous current (CC) mode. Weldments fabricated using the PC mode exhibit an improved resistance to thermal softening, a greater strain-rate sensitivity, and a lower activation volume. The OM and SEM observations reveals that an adiabatic shear band dominates the fracture characteristics of both weldment types under impact loading. Microstructural analysis reveals that for both welding current modes, the dislocation density and volume fraction of α′ martensite increase with an increasing strain rate, while the twin formations reduce under the same conditions. Comparing the evolution of the microstructure in the base metal and the fusion zone, it is found that for both welding current modes, a higher dislocation density exists in the fusion zone, and that a larger volume fraction of α′ martensite and a greater twin density are present in the base metal. Furthermore, the dislocation density and volume fraction of α′ martensite is greater in PC weldments than in their CC counterparts. Finally, the present results indicate that the PC welding mode produces a weldment with superior dynamic impact response and improved weldment fracture characteristics.