Transporting substances with high radioactivity used in nuclear medicine is strategic. Lean duplex stainless steel UNS S32304 is a candidate for external construction of a cask for transporting radioactive material. Thin sheet lean duplex stainless steel specimens were welded by autogenous TIG processing (tungsten inert gas) in order to avoid neutron activation of the filler metal. Post-welding heat treatments were studied considering a possible scenario of fire or crash during transportation. The metallographic analysis indicated that post-welding heat treatments promoted a change in austenitic phase content in the fusion zone. An analysis of the stress-strain curves using work hardening models revealed that only the Voce and Hockett???Sherby models presented goodness-of-fit to the experimental data. It was demonstrated that the parameters of both models are correlated to the volumetric austenitic phase content present in the fusion zone, being equivalent to the quantification of the bulk phase. This work proposes a new direct evaluation methodology for volumetrically quantifying phases using both mathematical work hardening models fitted to the experimental data from stress-strain curves.